Tuatara: Volume 22, Issue 1, February 1976
The Natural History and Behaviour of the New Zealand Fur Seal (Arctocephalus Forsteri)
The Natural History and Behaviour of the New Zealand Fur Seal (Arctocephalus Forsteri)
The New Zealand fur seal, Arctocephalus forsteri (Lesson, 1828), lives and breeds on the rocky shores of southern New Zealand and its subantarctic islands. Although brought close to extinction by commercial sealers during the nineteenth century, the species has recovered well under protection and is increasing steadily in numbers and expanding its range (Csordas and Ingham, 1965; Falla, 1965; Stonehouse, 1965; Stirling, 1968). The fur seal was first given legal protection in 1875, and from then until 1916 seasons were of limited length and permits were required for seal killing. Since 1916 no fur seals have been legally killed (except for research purposes) apart from on Campbell Island in 1924 and 1926, and in parts of southern New Zealand in 1946 (Sorensen, 1969b).
By comparison with most other species of otariid seal the biology of A. forsteri is poorly known, but there have been rapid advances in knowledge in recent years. In particular, since 1969 there have been intensive studies of distribution, abundance, population structure, breeding biology and behaviour, and some results have already been published (Stirling, 1970; Crawley and Brown, 1971; Miller, 1971, 1974; Crawley, 1972, in press; McNab and Crawley, in press; Wilson 1974a, b). The new information renders existing general accounts of the biology of A. forsteri unsatisfactory in some respects (Scheffer, 1958; King, 1964; Sorensen, 1969a, b; Gaskin, 1972), so the aim here is to present a more up to date account of the natural history and behaviour of the New Zealand fur seal. In this paper, the emphasis is on presenting the most recent information on the topics covered, rather than on reviewing the literature, but we hope that no important material has been omitted. The account relies a great deal on our own work on distribution, breeding biology and social behaviour, the full results of which will be published separately. The observational page 2 work was carried out from 1970-74 on the Open Bay Islands, Westland, the Snares Islands and in numerous locations throughout New Zealand. In addition to serving as a useful summary of present knowledge of the biology of the New Zealand fur seal, this paper should also indicate where priorities for future research should lie.
Classification and Physical Characteristics
Taxonomic confusion within the genus Arctocephalus has been clarified by King (1968, 1969), Shaughnessy (1970), Repenning et al. (1971) and Stirling and Warneke (1971). The current view is that there are eight species of Arctocephalus, defined chiefly by characteristics of the skull. The decision by Repenning et al. to synonymise the New Zealand fur seal, A. forsteri, and the South Australian fur seal, formerly A. doriferus (Wood Jones), is of particular relevance to this paper. Work on the South Australian fur seal by Stirling (1971a, b) and Stirling and Warneke (1971) adds considerably to our knowledge of the social behaviour of A. forsteri, but because of the unknown effects of geographic separation and climatic differences between New Zealand and South Australia, only New Zealand populations are discussed here unless otherwise stated.
The work of Repenning et al. (1971) shows that there are three sizes of fur seals: a large species, A. pusillus; a small species, A. galapagoensis; and a group of six medium-sized species, including A. forsteri. A. forsteri has been described by Webb (1871), Clark (1875), Thomson (1921) and Sivertson (1954), and fuller details of appearance and morphology can be found in these papers. The coat colour of adults merges from a dark grey-brown dorsally to a lighter grey-brown ventrally. The thick underfur is a rich chestnut colour; the guard hairs are coarse and dark grey, often with white tips which impart a silvery sheen to the dry fur. Bulls have thick manes composed of long, coarse guard hairs. Pups are black at birth but moult to a silvery-grey after about five months. The fur appears darker when wet. Adults and pups are illustrated in Figs. 1-3.
Adult males (bulls) reach 2 m and 200 kg and adult females (cows) about 1.5 m and 90 kg. Bulls are much more massive than cows, particularly around the neck and shoulders, and during the breeding season the average male: female weight ratio is 3.5: 1.
Fig. 1: Adult male (bull) New Zealand fur seal in an alert posture. Note the massive neck and chest.
Fig. 2: Adult female (cow) New Zealand fur seal. Note the slender neck and generally svelte appearance.
The New Zealand fur seal, like the South African fur seal, A. p. pusillus (Rand, 1967), is a coastal species with a rather limited range. Distant migrations are not performed and it is probable that the bulk of the population remains near land.
The present distribution of New Zealand A. forsteri, as determined from published information, questionnaires and personal observations, is shown in Figs. 4 and 5. Fur seals penetrate as far north as the Three Kings Islands (34°10′ S., 172°08′ E.; Singleton, 1972) and as far south as Macquarie Island (54°30′ S., 157°E.; Csordas and Ingham, 1965). Within these latitudes, fur seal distribution is discontinuous and seasonally variable. Throughout the year, seals are most plentiful on the rocky coasts of the South Island, Stewart Island, the Chatham Islands and the New Zealand subantarctic islands, but in winter large (> 500) colonies form in the south of the North Island, and small groups occupy suitable coastal terrain as far north as Three Kings Islands.
Fig. 4: Map of New Zealand showing the distribution of New Zealand fur seal rookeries and hauling-grounds in all areas except Fiordland and Stewart Island. Inset: New Zealand and the off-lying and subantarctic islands; fur seals breed on all the named islands except Antipodes Island.
The breeding season is from late October to early February for the territorial bulls, but cows and pups remain on the rookeries until August-September. Only in winter and early spring are seals ashore in significant numbers on the hauling grounds north of the breeding range; from May onwards adult and subadult males come ashore in increasing numbers to reach peak density in July-August. These essentially bachelor bull colonies diminish rapidly in size in September-October. On most winter haul-out sites there are always a few pre-breeder or post-reproductive males ashore, but in the far north the hauling grounds may be deserted during the summer. During the breeding season the fur seal distribution is more restricted than at other times of the year, but obviously not all the seals are involved in breeding. Non-breeders congregate in large numbers on rocky beaches adjacent to the rookeries or on nearby islets. On many islands, including the Chatham Islands, Big Solander Island, Campbell Island (Bailey and Sorensen, 1962), Auckland Islands (Wilson, 1974a) and the Snares Islands (Crawley, 1972), both non-breeding colonies and rookeries occur. The rookeries are usually on the exposed west coasts of the islands, while the hauling grounds are on the more sheltered east coasts. Campbell Island seals show a reversal of this tendency, due probably to an absence of suitable habitats. In addition to the discrete rookeries and main hauling grounds, there are concentrations of non-breeders, mainly large subadult males, in the vicinity of rookeries.
Characteristically, the fur seal is an inhabitant of exposed rocky coastline, but there are records of groups of seals in sheltered waterways. There is a small, but well-established, bachelor bull colony in Paterson Inlet, Stewart Island, and seals frequently haul out on islands in Dusky Sound and in Milford Sound. Seals have also been recorded from most of the fiords and sounds, from rivers in Otago, Canterbury, Marlborough and Nelson, and from Otago and Hokianga Harbours.
On the east coast of New Zealand repetitive counts are available from Bunkers Islets (46° 50′ S.), Tia Island (47° 05′ S.), Cape Saunders (45° 50′ S.), Motunau Island (43° 05′ S.), Kaikoura (42° 20′ S.) and Turakirae Head (41° 25′ S.). Around Stewart Island, maximum numbers of seals are ashore at breeding colonies in autumn. Fur seal numbers at Cape Saunders reach a peak in January, decline until mid-winter, increase again in August-September, and finally decline to a minimum in November. At Motunau Island (Cox et al., 1970) seals have been seen ashore in January, April, May-September and October, being most numerous in September. At Kaikoura, maximum numbers occur in May and June and numbers remain high until October, after which there is a rapid decline to a summer minimum (November-January), followed by a steady increase again in February and March (Stonehouse, 1965; Stirling, 1970; Miller, 1971). At Turakirae Head, monthly counts made by Whitaker (unpublished) from May, 1967, to August, 1969, showed maximum numbers ashore in June-July; very few were seen in October-January.
The above information suggests that there is a movement of seals northwards from Stewart Island beginning in January, with many reaching Cape Saunders in January-February, Kaikoura in May-June, and the Wellington area in July. Beginning in August, a southward movement occurs and by September many are again in the Otago area. In November, numbers ashore are at a minimum everywhere.
On the west coast the situation is different. Seasonal counts are available only for Gillespie's Point (43° 25′ S.), where maximum numbers are ashore in August-September, about two months later than at the same latitude on the east coast. This time-lag is unexplained at present.
Some seals move south from the breeding grounds in late summer. At Macquarie Island (54° 30′ S.) there is an influx of non-breeders in February-March, but most leave again before mid-winter (Csordas and Ingham, 1965). Movements of seals between the other subantarctic islands are unknown.
Fur seals of the genus Arctocephalus are nearly always found on exposed rocky coastline. Falla (in Sorensen. 1969b) recognised three major types of habitat used by New Zealand fur seals: tumbledown (talus) beaches, terraced rocky ledges and small islets. He suggested that these are used by breeding animals, non-breeding males and immatures respectively. Our work suggests that this is an oversimplification.
Breeding colonies are found mainly on exposed west or north-west coasts and characteristically have three main features:
|1.||Some form of shelter from heavy seas, e.g. off-lying reefs, stacks or rocks, or large rocks on the seaward fringe of the rookery.|
|2.||Topographical irregularities, usually in the form of large, jumbled, angular rocks at the base of cliffs, or eroded rock surfaces with sharp ridges, cracks, guts, etc. (Fig. 6).|
|3.||Retreat areas above the storm splash zone for cows and pups. In colonies where landforms protect the beach from heavy seas retreat areas are unnecessary and may not be present.|
Stirling (1971a) suggests that for South Australian A. forsteri the sea, or rock pools, must be readily available for cooling in warm weather. This is less important in the cooler climate of southern New Zealand and the subantarctic islands. However, warmer localities such as Open Bay Islands do have pools which are used by seals in warm weather.
Habitat requirements for non-breeding colonies are less stringent than for rookeries and a wide variety of rocky coast is used. The only critical factor appears to be easy access. Most preferred are shelving rocky ledges and boulder beaches (Fig. 7); rounded or small boulders and stones are generally avoided. Hauling grounds occur on suitable points, spurs, headlands, reefs and islets, and less often in bays. They are all regular stations and only wandering individuals come ashore elsewhere.
The use of different types of terrain must be interpreted with caution, since it is often uncertain as to whether habitat use is due to availability or to terrain preference. The present population is still much smaller than that which existed prior to the sealing days (McNab, 1907), and we can only guess what use a larger population would make of what is presently less-preferred terrain. Also, the extent to which the present use of coastal areas depends on the seals' return to ancestral hauling grounds is conjectural. Burrows (1971) suggested, using evidence from soil studies and gastrolith finds, that seals once spread over the Open Bay Islands, whereas now they are confined to the coastal rocks and the fringes of the vegetation. Similar changes in habitat use probably also occurred elsewhere.
In his book on marine mammals of the New Zealand region, Gaskin (1972) noted that there was no reliable estimate of the numbers of the New Zealand population of A. forsteri because no complete census work had been carried out. Since 1970, we have been trying to rectify this omission, and G. J. Wilson has made the study of fur seal distribution and abundance his major task. However, it is page 11 unlikely that a complete census will ever be achieved, if only because of the physical difficulty of gaining access to many of the remote rookeries and hauling grounds. In many cases access is possible only by boat in fine conditions, and good weather is rare in most parts of the fur seal's range. Significant progress has been made, however, and we have recent data from most of the New Zealand mainland and offshore islands, Stewart Island, and the Snares, Auckland and Chatham Islands.
The counts were made during the period January 1970-February 1973, mainly during the summer, and in all kinds of weather at various times of days, as opportunity permitted. Wilson (1974b) has prepared a detailed record of counts at each colony visited, including correction factors for time of day, weather conditions, etc., but for the present purpose only the broad over-all picture is given. The reliability of the counts varies greatly, as some visits involved an extended period ashore, allowing a detailed study of the colony; others permitted only counts from a clifftop; while some precluded a count ashore altogether, and only a rough idea of numbers could be gained from a rocking boat offshore. Our estimates for each region are given in Table 1, together with data for Macquarie Island (Johnstone, 1972), Antipodes Island (Taylor, in Sorensen, 1969b), Campbell Island (Bailey and Sorensen, 1962) and the Bounty Islands (Falla, in Sorensen, 1969b).
|Solander Island||5,000||Present study|
|Stewart Island||3,150||Present study|
|Snares Islands||1,150||Crawley (1972)|
|Auckland Islands||1,000||Present study|
|Campbell Island||2,000||Bailey and Sorensen (1962)|
|Bounty Islands||5,500||Falla, in Sorensen (1969b)|
|Macquarie Island||650||Johnstone (1972)|
|Chatham Islands||2,100||Present study|
We estimate that the number of fur seals within the breeding range is about 36,000, while the figure for the total New Zealand population could be as high as 40,000, if the non-breeding animals which remain on the hauling grounds north of the breeding range during the summer are included. This estimate of 40,000 is to be compared with that of about 20,000 by Falla (in Sorensen, 1969b), which was based mainly on counts in only part of the range in 1947-48, and 20,600 by Gaskin (1972), who used the best evidence available from his own counts and those of Falla, Sorensen and Taylor (in Sorensen, 1969b). Our estimate is about 100% greater than the previous estimates, but this is page 12 probably largely due to the more intensive nature of the studies, rather than to a massive increase in the population.
No attempt is made here, in this general account of fur seal biology, to analyse in detail the results of the counts or to present data on age structure and sex ratio of the colonies. This information will be published separately by G. J. Wilson.
Foods and Feeding
There has been only one detailed investigation of New Zealand fur seal foods (Street, 1964), but observations by Falla (in Sorensen, 1969b), Rapson (in Sorensen, 1969b), Bailey and Sorensen (1962) and Csordas and Ingham (1965) have provided useful information. Our knowledge of the diet of A. forsteri remains sketchy, however, and further study is needed. A list of possible food items, based on published data and personal observations, is given in Table 2.
|Common Name||Scientific Name||Authority|
|1.||Squid (Arrow)||Notodarus sloanii||Falla, in Sorensen (1969b)|
|2.||Squid (Broad)||Sepioteuthis bilineata||Street (1964)|
|3.||Octopus||Octopus maorum||Street (1964)|
|4.||Barracouta||Thyrsites atun||Street (1964)|
|5.||Red cod||Physiculus bachus||Street (1964)|
|6.||Whiptail||Macruronus novaezealandiae||Street (1964)|
|7.||Kahawai||Arripis trutta||Street (1964)|
|8.||Horse mackerel||Trachurua sp.||Street (1964)|
|9.||Butterfish (Greenbone)||Coridodax pullus||Street, Rapson, in Sorensen (1969b)|
|10.||Lamprey||Geotria australis||Street (1964)|
|11.||Blind eel||Eptratretus cirrhatus||Street 1964)|
|12.||Ling||Genypterus blacodes||Street 1964)|
|13.||Blue cod||Perapercis colias||Street 1964)|
|14.||Flounder||Rhambosolea plebeia||Rapson, in Sorensen (1969b)|
|15.||Crayfish||Jasus sp.||Falla, in Sorensen (1969b)|
|16.||Crab||Not identified||Street (1964)|
|17.||Penguins||Eudyptes spp.||Csordas and Ingham (1965)|
Street's (1964) study of fur seal foods was carried out to check the validity of claims by fishermen that seals were detrimentally affecting stocks of commercially important fish. His findings indicated that there was little justification for this view. He examined the stomach contents of 70 seals taken from Kaikoura Peninsula, Banks Peninsula, Cape Saunders, Nugget Point and Bench Island in Foveaux Strait, and discovered that octopus (Octopus maorum), squid (Notodarus sloanii and Sepioteuthis bilineata) and barracouta (Thyrsites page 13 atun) made up 90.8% of the stomach contents by weight. Rapson (in Sorensen, 1969b) found food in only 25 of 91 stomachs examined; octopus or squid were present in 22 stomachs and fish (one barracouta, one greenbone (Coridodax pullus) and one perch-like fish) in only three. Observations by Falla (in Sorensen, 1969b), Bailey and Sorensen (1962), and Csordas and Ingham (1965) confirm that squid is a preferred food while fish are relatively unimportant in the diet.
Penguins are also commonly eaten by fur seals. Both Falla (in Sorensen, 1969b) and Csordas and Ingham (1965) recorded penguin feathers and other remains in stomachs, faeces or regurgitated material, while Bailey and Sorensen (1962) described how fur seals chased and devoured Rockhopper penguins (Eudyptes chrysocome) near the shores of Campbell Island. Seals resident on the subantarctic islands commonly include penguins in their diet, perhaps because penguins are more readily available than fish in those waters. Squid and octopus are still the major foods, however.
The work of Street (1964) indicates that fur seals feed principally in surface waters at night, when they take squid and surface fish. Octopus are collected from the sea floor, often by day. Seals have been reported by fishermen to bring large fish such as ling (Gerypterus blacodes) and blue cod (Parapercis colias) to the surface during the day and break them up for eating. It is probable that this very obvious activity over-emphasises the occurrence of such behaviour. Fur seals may even benefit the crayfishing industry by eating octopus, which are important predators of crayfish (Jasus sp.).
In summary, our present knowledge of the diet of the fur seal in New Zealand waters indicates that squid and octopus are the most important food items, and these are supplemented by penguins (particularly around subantarctic islands) and surface-feeding fish.
The Annual Cycle
This general account of the annual cycle of A. forsteri is based on our own observations and the work of Sorensen (1969a, b), Stirling (1970) and Miller (1971).
Seals are present in the rookeries and on the hauling grounds throughout the year, although the proportions of the various sex and age classes vary seasonally. Few bulls frequent the rookeries in the March-September period, the resident population comprising small subadults, yearlings and cows. Yearling numbers decrease steadily between June and November and few remain when the first pups are born in November. From mid-October to mid-November the number of bulls ashore increases steadily, while cows arrive in large numbers in late November and throughout December. From mid-November to late December the number of bulls increases slowly. The relative absence of cows ashore in late November may be because pregnant females feed heavily at sea in the weeks before giving birth. Most cows page 14 arrive about two weeks after the main influx of bulls, by which time territory establishment is already well advanced. The numbers of both bulls and cows in the rookeries decrease sharply in early January.
Pupping is from late November to mid-January with a peak in mid-December. Cows stay with their newborn pups for about ten days and during that period they mate with the nearest male, usually a territorial bull. Their first feeding trip is of three to five days and on their return they suckle the pups for two to four days. As the pups grow older the cows leave them alone for longer, and the pups congregate into unstable small groups called pods. Each pup leaves the pod to suckle its mother on her return from each feeding trip.
The decline in the numbers of bulls and cows ashore during January is a consequence of the gradual breakdown of the territorial system following the main birth and mating period. The bulls may have spent up to ten weeks ashore without food or water, and they depart for the feeding areas to regain their strength. The cows may have been ashore for two to three weeks, during which time they have given birth, mated, and suckled their young; they move away for their first feeding trip. In February there may be no bulls ashore, and possibly only half as many cows as in January. The feeding trips of the cows are longer than in December and January. As the adult population declines there is an influx of subadults and yearlings on to the main breeding rocks. Many of the subadults occupy offshore rocks during the breeding period, while some of the larger subadult males even infiltrate the edges of the rookery before being chased by the territorial bulls.
During the March-September non-breeding period the rookeries are occupied by pups, yearlings, small subadults of both sexes and females still suckling young. Many of the bulls and large numbers of subadult males move north for the winter. Cows probably spend long periods at sea feeding when they are not at the rookeries suckling their young.
Behavioural Postures and Calls
The postures and calls of New Zealand A. forsteri have been described in detail by Stirling (1970) and Miller (1971), and those of South Australian A. forsteri by Stirling (1971a, b) and Stirling and Warneke (1971), and this brief account is based largely upon their descriptions. Our studies indicate that Stirling's (1971a, b) descriptions of the postures and calls of the South Australian fur seal apply also to the New Zealand fur seal, and in the following account his descriptive terms are used. A list of postures and calls, together with their possible functions and use by various classes of seal, is given in Table 3.
The agonistic behaviour of A. forsteri appears to be highly ritualised. Most of the postures and calls described are used by page 15 territorial bulls during threat displays and serve to indicate territorial status, affirm territorial boundaries, and communicate readiness to fight. Some calls may allow individual recognition of neighbouring territory-holders.
|POSTURES||Territorial Bulls||Other Bulls||Cows||Subadult Males||Pups and Yearlings||PROBABLE FUNCTIONS OF POSTURES AND CALLS|
|1. Full neck display||*||†||‡||‡||‡||Threat. Demonstrates size and indicates dominance.|
|2. Neck-waving||‡||†||‡||†||‡||Prolongation of threat display|
|4. Oblique||*||‡||†||‡||High-intensity threat before attack|
|4. Alert||*||*||†||*||†||Indicates awareness|
|5. Open-mouth threat||*||*||*||*||†||May be aggressive or submissive|
|6. Facing away||†||*||*||*||‡||Appeasement|
|7. Horizontal neck-stretch||*||†||‡||‡||‡||Intense aggression|
|8. Submissive||‡||*||*||*||*||Avoidance of encounter with dominant|
|9. Normal||*||*||*||*||*||No specific function|
|10. Elimination||*||*||*||*||*||Defaecation and urination|
|11. Pup-calling||*||Adopted by female calling young|
|1. Male full-threat||*||†||‡||‡||‡||Indicates readiness to fight|
|2. Male low-intensity||*||†||†||†||‡||Threat. Does not precede attack.|
|3. Male guttural challenge||*||†||†||‡||High-intensity threat|
|4. Male and female submissive||‡||*||*||*||*||Indicates submission to dominant|
|5. Female threat||*||Brief threat|
|6. Male barking||*||†||‡||Sexual interest; territorial status; individual recognition|
|7. Pip-attraction||*||Attracts straying pup|
|8. Female attraction||*||To attract mother; response to pup-attraction call|
|9. Male moan||*||*||Uncertain function|
In the non-breeding season, the relationship between individuals seems to be determined by a size-based dominance hierarchy, but during the breeding season the social situation is dominated by the territorial behaviour of the bulls. Generally speaking, territories are held by the bulls with the largest necks, these bulls normally being the largest over-all as well. Territorial bulls appears to be of equal status, the earlier size-based dominance being non-operative throughout the breeding season. Encounters between territory-holders are essentially meetings between bulls of similar size and status, and fighting is usually avoided by use of ritualised displays. Territorial bulls interact also with non-territorial bulls, cows, subadults and pups and employ a variety of postures, calls and movements in these encounters. In addition, all other classes interact with one another to varying degrees, using appropriate behaviours as they do so. Interactions between cows and pups are special, and some calls are unique to these classes.
Most calls and postures are common to all classes, though predominantly used by some, while others are confined to one, or a few classes. Most of the postures and calls are concerned with threat or submission and are used mainly, but not exclusively, by territorial bulls. Postures used frequently by members of both sexes and all ages include the normal sitting posture, the elimination posture for defaecation and urination (Stirling and Gentry, 1972), the alert posture (general awareness) and the submissive posture (seldom by territorial bulls). Both adult and subadult males adopt the various threat postures, but these are most often used by territorial bulls. In increasing intensity of threat the postures are: full neck display, neck-waving, oblique stare and horizontal neck stretch. The full neck display probably demonstrates the size of a bull and indicates his dominance, thus reducing conflicts. If this display is insufficient to resolve a dispute, the belligerents become positioned chest to chest and wave their heads from side to side out of phase with each other. This neck waving prolongs the threat display and might provide an attacking position. The oblique position of the head (given mutually) normally follows milder threats and usually precedes attack. The horizontal neck stretch is also a high-intensity threat display. Facing away is characteristic of subordinate males when confronted by dominant males, either after threat displays or fighting. Subordinate animals of both sexes adopt a submissive posture, with lowered head and neck, when seeking to avoid confrontation with dominants.
The open-mouth threat is given by all classes but is especially characteristic of cows. It may truly be a threat or may function as an appeasement posture also, particularly in encounters with territorial bulls. The specific posture for calling pups is restricted to cows and is characterised by an extended neck.page 17
As with the postures described above, many of the calls uttered by A. forsteri are concerned with demonstrating status and communicating threat. The male full threat and low-intensity threat calls, the guttural challenge and the bark are given mainly by territorial bulls. All probably aid individual recognition and demonstrate territorial status, while the challenge and the bark may affirm territorial boundaries as well. The full threat call and the guttural challenge also indicate a readiness to fight, while the bark indicates sexual interest. Cows have a high-pitched guttural growl which serves as a threat to all other classes. Both sexes utter submissive squeals when threatened or defeated by dominant animals. The remaining calls are highly specific; cows whine (or occasionally moan) to attract their pups, while the pups wail in response to attract their mothers.
Subadult males are much quieter than adults; they give general moans, whimpers and growls, as well as submissive squeals when appropriate. The larger ones practise the various threat calls.
The ritualised threat and boundary displays given by territorial bulls serve to maintain and defend their territories without wastage of energy in fighting. Of course, fighting is common between bulls of similar size during territory establishment, and also between established males and challengers, but generally displays involving various postures and calls serve to maintain control of territories and subdue non-territorial bulls, cows and subadults.
Behaviour of Males
The rookeries of the New Zealand fur seal have a more distinct organisation during the breeding season (October to February) than at other times of the year. During spring, a few bulls arrive on the breedings grounds and establish large, rather ill-defined territories. A size-based hierarchy operates, with subordinates being chased by larger dominant bulls. Later in the season, when more bulls are ashore, territories are smaller, better defined and vigorously defended, and all territorial bulls appear to be of similar status.
Territories are established prior to the arrival of the pregnant cows; some territories may never contain cows while others have large numbers. Territorial bulls attempt to influence the movements of pregnant cows and endeavour to maintain ‘harems’ within their territories, but with little success (Miller, 1974). Cows pass freely between territories although bulls may block them for up to an hour. Throughout the season bulls try to extend their territories to include areas with cows and this often leads to conflict with neighbouring bulls. Once cows have given birth to their single pup they remain with it near the birth site for about ten days; this sedentariness makes it probable that they will mate with the territory owner, rather than with page 18 any other male, when they come into oestrous about eight days after giving birth. Cows leave on feeding trips every few days after the initial suckling period, and to re-establish contact with their pup they rely on locating it in the vicinity of the birth site. This fidelity to the birth site makes it appear that groups of cows constitute the ‘harem’ of a territory owner.
although bulls may block them for up to an hour. Throughout the season bulls try to extend their territories to include areas with cows an dthis often leads to conflict with neighbouring bulls. Once cows have give nbirth to their single pup they remain with it near the birth site for about ten days; this sedentariness makes it probable that they will mate with the territory owner, rather than with any other male, when they come into oustrous about eight days after giving birth. Cows leave on feeding trips every few days after the initial suckling period, and to re-establish contact with their pup they rely on locating it in the vicinity of the birth site. This fidelity to the birth site makes it appear that groups of cows constitute the ‘harem’ of a territory owner.
Bulls seldom voluntarily forsake their territories during the period between territory establishment (in late October or early November) and the termination of the period over which copulations take place (early January). They live in the confined space of the territories for up to 70 days, fighting, mating, sleeping and defaecating more or less in the same place. All other classes of seals except cows and pups are excluded from the territories until the end of the mating period. After the departure of many of the cows on their first feeding trips, strict territorial defence ceases and many bulls leave the rookery.
Formation of Territories
During winter and early spring the few bulls present on the breeding grounds are not territorial. However, on Open Bay Islands in August and September, individuals may occupy the same area of ground on several successive days. Miller (1971) divided bulls on the Open Bay Islands into three categories: early, intermediate and late arrivals. The first bulls appear ashore in mid-October and immediately establish large, rather ill-defined territories. Most of these early arrivals are quickly deposed, often without physical combat, by the intermediate arrivals, larger bulls who arrive later in October and in the first half of November. None of the early arrivals appears able to retain a territory long enough to gain access to cows, and it is possible that they are males attempting to breed for the first time. Generally, they successfully deter and chase large subadults but retreat from larger bulls. The intermediate arrivals establish territories which they do not leave, unless defeated in a fight, until the end of the mating period. The final category is that of the late arrivals, who must establish themselves by fighting. Some of these depose current page 19 territory holders; others manage to seize part of an existing territory and establish themselves after a battle or two; while others suffer several defeats, give up the idea of gaining a territory, and settle on the fringes of the breeding area.
After the mating period, in late summer, the hostility of territorial bulls decreases. Some territory-holders extend their movements to include abandoned territories. Others depart, and their territories are taken over for a short time by small, unestablished bulls or large subadults. In late December and early January, the hostility of territory holders who remain on station is so reduced that they may be deposed, after a mild struggle, even by large subadults.
Territory Shape and Size
The choice of territory site by bulls is influenced by the topography of the rookery, the proximity of other bulls and the presence of other territories. Topography limits the size of a territory but the physical prowess of the bull fixes its boundaries. At the height of the season few territories exceed 100 m2; large territories present at the beginning of the season are whittled down as the density of bulls increases.
Territory boundaries usually follow natural irregularities of the terrain, and such natural boundaries seldom need demarcation and defence by the territory owners. Generally, boundary regions which are topographically well-demarcated change little, while poorly-defined borders between territories may shift in position. Trespasses by neighbours over poorly-defined boundaries are often tolerated by territory owners, but where a topographical irregularity defines the border, trespass is readily recognised and quickly dealt with.
Territories change in size and shape as areas are gained by fighting or opportunistic expansion, and lost by defeat or readjustment of boundaries following changes of neighbours. Where there are no obvious topographical boundaries to territories, bulls appear to respect invisible boundaries beyond lunging distance. These boundaries conform to pre-established limits, the outcome of fighting or threatening, and change if the territory owners change.
Types of Territories
The New Zealand fur seal does not appear to hold aquatic territories, although many are adjacent to the sea. There appear to be five main types of territories, not all being present at the same time.
|1.||The first territories are large and are formed early in the season by small bulls. They are defended mainly by threat displays, with dominant animals chasing subordinates. These territories do not last long as their owners are soon deposed by larger bulls arriving later, and the pattern of territories changes.page 20|
|2.||During the first three weeks of November the successful fighters divide up the area among themselves. The favoured territories are near the sea and include areas subsequently favoured by cows. Defence of these areas is continual at first as bulls continue to arrive and contest the areas. These territories are occupied for the duration of the breeding period, although boundary changes may occur if successful challengers annex areas, or if new owners rearrange boundaries with neighbours. Most of these territories contain cows later.|
|3.||Some territories are not formed until the cows have arrived. The presence of cows may stimulate some smaller adults to challenge resident bulls, and occasionally one may occupy a small corner of a previously large territory. Also, late arrival of large challengers may cause re-arrangement of territories.|
|4.||Small adults and large subadults may occupy and defend territories on the margins of the main breeding areas at any time during the summer. The large bulls often ignore them completely if there are no oestrous cows contained within the territories.|
|5.||Finally, at the end of the summer, the resident bulls may depart, and small adults and large subadults move in and occupy territories temporarily.|
Maintenance of Territories by Fighting
Bulls defend their territories for six to ten weeks without eating or drinking. Their stamina, strength, fighting ability, and probably also their experience and temperament, determine how successful they are in territory maintenance. Although border defence involves mainly ritualised displays and threats, serious challenges must be met by physical contact.
The successful bulls who are resident during the main breeding period often acquire their territories early in the season without need for much fighting, but heavy fighting is often necessary for them to maintain their position long enough to mate. Detailed descriptions of pre-fight behaviour and fighting are given by Miller (1971) and Stirling (1971a), but the main points are summarised here.
New arrivals which do not intend to dispute ownership of territories, but merely wish to make their way to unoccupied land. arrive discreetly, keeping their heads down, and move ashore cautiously, maintaining a flattened profile. They may lie down several times to escape attention. If challenged, they either flee, giving submissive calls, or, if cornered, give submissive whines, attempt to defend themselves and seek an escape route. Large challenger bulls behave quite differently: they haul out on rocks opposite the site to be contested and give threat calls while sitting alert. They then proceed quickly to the specific area chosen, making a vertical image readily page 21 recognised as a challenge by the resident bull. Confrontation by the territory owner immediately leads to chest-to-chest positioning and a fight.
Fighting seems to be ritualised. Bulls meet chest-to-chest and attempt to grip their opponent on the face, neck, upper back or shoulder, and shake. They push with their chests in an attempt to unbalance one another, and alternately wave their necks from side to side. They also manoeuvre for position and make sudden lunges at the head and neck, sometimes inflicting severe wounds. The neck and shoulders are well protected by thick skin and fur, but the face, back, sides and tail region are relatively unprotected and easily torn open.
Bulls use the advantage of height whenever possible. In evenly-matched encounters, the bulls hang on to one another and push steadily, and stamina seems to decide the outcome. Fights seldom last longer than a few minutes. When one bull has had enough it ceases any aggressive behaviour and gives a submissive screech while backing away. This behaviour often, but not invariably, prevents further attacks by the winner. Victorious bulls often bite the hind flippers of retreating bulls, who receive further harsh treatment from the occupants of neighbouring territories.
Victory is accomplished by damaging an opponent, pushing him into an adjacent territory or outlasting him in strength. Infliction of damage is essential if challengers are to be deterred from further attacks; it is not enough for a territory-holder to be able to tolerate punishment. Challengers need to be exceptionally gifted to win early battles, as possession of a territory seems to give bulls confidence.
Fighting diminishes as territories are mutually recognised by neighbours, and threats and displays are normally sufficient to defend borders. However, although there is a decrease in the number of encounters once the females have arrived and settled down, there is an increase in the proportion of encounters involving fighting, due to the efforts of bulls in territories without females to oust more fortunate neighbours. However, over-all only about 30% of encounters need to be resolved by fighting.
Activity Budgets of Territorial Bulls
Fur seals, like most other animals, are involved in various activities throughout the day, and generally they divide their time between such activities in a fairly consistent way. So far, only the time budgets of territorial bull A. forsteri have been studied, mainly because this class of seal is easy to observe, being restricted in its movement, conspicuous, and uninvolved in care of the young or feeding. Stirling (1971a) studied activity budgets of Australian A. forsteri bulls and Miller (1971) did the same for New Zealand A. forsteri; their findings agree quite closely and the following account applies to A. forsteri territorial bulls generally.page 22
Although the exact partitioning of time between activities varies between individuals, from day to day, and seasonally, there are nonetheless several general statements which may be made about male activities. Territorial bulls are least active during the middle of the day, and over-all they spend about 70% of their time lying down, either asleep or awake. When not sprawled out on the rocks they spend most of the rest of their time (up to 20%) in an upright posture, which combines general alertness with a normal sitting posture. The adoption of this posture is most prevalent during the peak reproductive period.
The only other activities which are important enough to single out as significant components of the activity regime are intra-sexual behaviour (displays and fighting) and inter-sexual behaviour (herding and copulating), which take up about 5% of the time. These are particularly subject to variation throughout the breeding period; in particular, bulls interact most with cows when the latter are in oestrous. Over-all, Miller noted that on Open Bay Islands, between December 6, 1970, and January 9, 1971, 87% of the interactions of territorial bulls were with cows; the remaining interactions were with other territorial bulls (9.6%), subadult males (1.8%) and pups (1.6%). Clearly, although pups are the dominant class numerically for much of the season, territorial bulls virtually ignore them.
There is still a great deal to learn about the activity budgets of territorial bulls, and the budgets of cows, subadults and pups are virtually unknown. The apparently continual activity of a seal rookery is obviously not largely caused by the actions of territorial bulls, which seek to conserve energy by eliminating unnecessary movements, so the other classes of seal present, particularly pups, must be the main contributors.
Copulations may occur between mid-November and mid-January, but most are in December because of the post-partum oestrous exhibited by cows. Mating early in the season probably involves virgin cows or some who failed to rear young the previous year. Cows with pups come into oestrous about eight days after giving birth and normally, but not invariably, mate with the owner of the territory.
Bulls continually monitor the sexual state of the cows in their territory by sniffing their perineal region (Miller, 1974). Receptive cows are playful and non-aggressive towards bulls, but there is no true courtship. Bulls whimper and seek body contact on recognising a receptive cow. Mounting is dorsoventral and is often repeated. Bulls frequently bite cows during copulation, the resulting activation of the cow apparently exciting them. Cows show increased aggression after a time and bulls have to work hard to keep them in position until ejaculation has occurred. Copulations take from five to 30 minutes.
Behaviour of Females
Dispersion of Breeding Cows
The dispersion of breeding cows affects that of territorial bulls. Cows arrive on the rookeries in greatest numbers in late November and early December. They prefer certain locations, usually within the seaward territories, and tolerate a certain amount of crowding rather than move on to flat, relatively featureless areas. There is some avoidance by newcomers of very crowded areas where ownership of resting ledges is disputed.
Most births take place near some sort of topographical irregularity such as a boulder, cliffside or washed-up log. Certain areas, particularly rock-filled guts, are preferred for pupping, and are used by a succession of cows. As post-parturient cows and their pups remain close to the birth spot for about ten days, cow and pup numbers increase in the preferred areas. Some competition for space probably results in the gradual use of pupping sites further inland by cows pupping later, but the complete lack of use of some apparently suitable areas suggests that considerable tolerance of crowding does exist. In summary, the shore line is a preferred pupping area, but the avoidance of overcrowded conditions induces some cows to move inland. Both habitat preference and competition for space determine the dispersion of cows; herding of cows by bulls is not an important factor.
Cows sometimes appear on the rookery weeks before giving birth. Near the time of pupping they choose a site which they occupy for up to five days before giving birth.
Interactions between cows must influence their distribution. They threaten one another for the slightest disturbance and fight for favoured rest spots such as ledges, or shady places on hot days. Cows do not bite, but confine themselves to pushing and uttering threat calls. Territorial bulls usually investigate any altercation between cows in their territories and threaten one or the other until one retreats.
Cows become restless shortly before giving birth, often circling repeatedly and sniffing the rocks which their hindquarters touch. They become highly aggressive towards other cows and pups. At the commencement of labour, straining begins and the hindquarters are lifted clear of the ground; circling increases in frequency and intensity. Once the foetus starts to pass, contractions become stronger. Cows often move around when the foetus is half out, as though trying to drag the pup the rest of the way.page 24
The time for the birth process varies from a few minutes to several hours. About 60% of births were breech presentations on the Snares Islands in 1970 and births were of similar frequency by day (mainly mornings and evenings) and night. Placentas were passed at birth in about 20% of instances, but usually came away within a few (< 6) hours.
A detailed description of the birth process and early mother-pup behaviour is given by McNab and Crawley (in press).
Post-parturient Behaviour of Mothers
Immediately after giving birth most cows are placid and tolerant of neighbours, perhaps because of fatigue. Shortly afterwards, cows with newborn pups are more aggressive than those with older pups.
Cows pick up and smell the pup repeatedly during the first 30 minutes after birth but make no attempt to remove foetal membranes where these are still enclosing the pup. Mouthing, raising and frequent rubbing of the body of the pup by the mother are indulged in for some time following birth. In many cases, cows deliberately lie on the pup and seek this type of physical contact for up to an hour. Mouthing and lifting may still occur days after birth.
Auditory communication between pup and mother is the most characteristic sound in rookeries. Pups start calling immediately after birth, but cows respond only after 30 minutes or so. Cows give a quiet lowing noise, while pups emit a ‘female-attraction call’.
Mothers retrieve their pups if they wander away. The cows try to lure pups in the appropriate direction but occasionally they pick them up by the scruff of the neck and carry them.
Schedule of Cows
Most cows spend up to five days in the birth area before giving birth, although they may have been ashore elsewhere for weeks. They come into oestrous about eight days after giving birth, and normally leave the rookery a day or two after copulation for their first feeding trip. They are normally away for three or four days at first, and on their return suckle their pups for a similar period. Thereafter, the feeding trips tend to be of longer duration than the suckling periods.
While on shore, cows spend about 60% of their time with their pups, about half of which is spent suckling. The remaining time is spent wandering around the rookery or lying on some favoured ledge away from the pup.
Newly-arrived cows are wet and apparently attractive to pups. On arrival, some cows immediately begin calling and move to where they left their pup, while others do not call but move directly to the correct locality and accept their pup after smelling it. Normally, both pups and cows shout on approach. Cows threaten pups until they page 25 establish which is their own by smelling it; foreign pups are rejected by adoption of a stiff, nose-up posture. Cows become less solicitous of their pup's welfare as it matures.
Threat and submissive behaviour are similar in cows and bulls, but cows are less likely to attack one another, and show greater flexibility in postures and sequences. Larger cows usually win any squabbles, but cows with newborn pups are the most aggressive. The disturbance caused by cows moving about the rookery leads to most interactions, but competition for cool places and favoured resting ledges is important.
Females and Other Classes
When bulls approach cows it is normally to herd or investigate them, and cows resist this attention with open-mouth threats, snorts and growls, and often make aggressive jabs at the face. They show submissive behaviour if a bull is aggressive. At all times other than during copulation, cows try to keep their hindquarters away from bulls. Large subadult males dominate cows but all others are subordinate to cows.
Cows threaten foreign pups by intention movements rather than by threat postures and vocalisations. They show most aggression towards foreign pups which they displace from resting spots.
Behaviour of Pups and Subadults
The black pups are born during the hot summer and form part of a densely packed herd. Each pup is protected by its mother for about ten days, but thereafter must fend for itself for long periods among a crowd of indifferent, even hostile, adults. Maternal solicitude is evident when the pup is very young but decreases rapidly. The pup is mainly responsible for maintaining the mother-pup relationship. The cow returns to the birthplace to locate her pup after an absence at sea and it is up to the pup to be there. Strong associations do not persist after weaning, which may take up to a year.
As pups become the dominant class numerically life becomes less hazardous for them. Early in the season, pups escape injury from trampling only by hiding in crannies or behind rocks, or by assembling in open spaces.
Pups form loosely-knit pods, usually of four or five animals, after a couple of weeks, when sufficient of the mothers are away on feeding trips. The aggregations allow mutual protection and companionship page 26 and tend to localise the movements of pups, perhaps facilitating the renewal of contact between mother and pup. As pups grow older they wander more widely, and soon extend their activities to the rock pools and channels. They swim, bite floating seaweed, and generally behave in an inquisitive manner.
Pups play with one another, indulging in mock battles. They tend to be fairly quiet as a group, but engage in constant movement. They sleep very soundly, with the hind flippers folded forward under the belly and the fore flippers folded back between the hind. The neck is stretched out and the head rested on the rocks. Some pups curl up alone while others prefer to snuggle together.
Pups moult at about two months, the moult taking up to four weeks, and are thereafter termed yearlings. They are still confined to the rookery and continue to suckle for several months.
Subadults are noisy and active. They are numerous at the rookery during the winter and also predominate in the winter hauling grounds around the coasts further north. During the breeding period some of the subadults occupy offshore stacks and rocky areas on the fringes of the main rookery. Females mature earlier than males and large subadults are therefore almost certainly males.
Adult Sex Ratio in Rookeries
The ratio of breeding bulls to cows in breeding colonies is difficult to determine. As pupping places coincide with territories previously formed by bulls, there is a grouping of cows around single bulls which gives the impression of organised groups or ‘harems’. However, until cows are restricted to one spot by having given birth they are able to pass relatively freely between territories despite all efforts of bulls to prevent them. Before the peak of pupping there is therefore continual change in the distribution of cows and new ones are continually arriving. Following the commencement of pupping, many cows are confined to a territory because they must remain with their pup at the birth place for the first ten days or so, but at any one time there is an unknown number of cows away on their first post-partum feeding trip.
Because of the continual shift in harem populations accurate counts of cows are difficult, and harem size is usually calculated from the number of pups eventually born in a particular territory. Clearly, the over-all number of cows using a particular territory cannot be determined from the number present on any one day, which may be quite small and is variable. Essentially, the territories cope with a passing parade of cows, and the tempo of pupping bears a direct relationship to the number of cows in territories at a particular time.page 27
While taking the above into account, it is possible to calculate some measure of the bull : cow ratio by relating the total number of territorial bulls and breeding females in the colony on several days. At the peak of the breeding period on the Snares Islands in 1970, the maximum recorded ratio of territorial bulls to cows was 1 : 7.4 and the mimimum was 1 : 2.5. However, as the cows were not distributed evenly the number of cows per territory was variable: an average grouping in a single territory was 4 and the maximum observed was 12 (Crawley, 1972). It is possible also to determine harem sizes from counts of pups; pups do not move away to sea, and each cow bears only one pup, allowing a direct conversion of pup to cow numbers. On the debit side is the fact that the number of pups present at any specific time is difficult to assess, because they lay under or behind rocks, in cracks, and behind adult seals. Using this method, the average ratio of bulls : pups on the Snares was 1 : 5 and on Open Bay Islands in 1970-71 was 1 : 6.1 (Miller, 1971).
This account of the natural history and behaviour of the New Zealand fur seal summarises present knowledge of the subject but also, and perhaps more importantly, reveals areas of ignorance. The topics covered most fully are distribution, abundance, habitat use, breeding biology and behaviour, while foods and movements are given only a sketchy treatment. Conspicuous by its absence is the important subject of population dynamics. The coverage described reflects the availability of reliable information on the topics concerned, and it is clear that the dynamics of the fur seal populations should now be given priority in allocating research time and money.
Knowledge of population dynamics is essential for the planning of programmes for the conservation, management or exploitation of the fur seal. To gain this knowledge, it will be necessary to obtain data on the sex and age structure of populations, natality and mortality rates, rates of growth and development of young, productivity, and numerous other population parameters. This will involve further development of techniques for marking animals and determining their age and reproductive condition, and improvement of existing census methods. Some of this work is already being undertaken by members of the Zoology Department, University of Canterbury, with the aid of finance from the Fisheries Research Division of the Ministry of Agriculture and Fisheries.
We wish to thank the many fishermen who so kindly provided transport to and from islands and remote parts of the New Zealand coast. Financial aid was provided by the Fisheries Research Division. page 28 Ministry of Agriculture and Fisheries, and the University Grants Committee. Dr. R. S. Bigelow kindly commented on the manuscript and Mr Gavin Robinson drew Figures 4 and 5.
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* —Habitual use
† —Frequent use
‡ —Occasional use