Density‐dependent pup mortality in the Antarctic fur seal Arctocephalus gazellu at South Georgia

Abstract

The extent and causes of pup mortality in the Antarctic fur seal, Arctocephalus gazella, were investigated at sites of high and low density at South Georgia. Mortality was greater at the high density site, (17—31% of annual pup production) than at the low density site (36%). The main causes of death, starvation and skull injury, occur more frequently at the high density site. Most starvation was caused by failure of the mother‐pup bond to form, often caused by disturbance induced by the activities of breeding bulls. In some seasons starvation of older pups may be influenced by reduced food availability but this was usually of minor importance. Injury to the skull resulted from bites inflicted by females either accidently during birth or when pups tried to suck from females other than their mother. Trampling of young pups by bulls was probably responsible for the appreciable incidence of ruptured livers. Infectious disease and drowning played minor roles in pup mortality. Pups born late in the season suffered disproportionately greater mortality which may relate to female age and condition. Food availability (both during and prior to the breeding season) and weather are likely to account for year to year variation in pup mortality rates but the basic rate is primarily determined by breeding density. Further population increase and colonization of new beaches is expected until food resources during the summer, or more probably the winter, become limiting.

Are Pinnipeds Functionally Different From Fissiped Carnivores? The Importance of Phylogenetic Comparative Analyses

“…Major causes of pup mortality include being trampled by the adult males or being fatally wounded by other aggressive females. Both factors are amplified at higher densities through accidental separation of the pup from its protective mother (Anderson et al 1979;McCann 1982;Doidge et al 1984;Bowen 1991;Harcourt 1992;Baldi et al 1996). We suggest that the three observed grade shifts are adaptations to decrease pup mortality.…”

Intra-sexual female agonistic behaviour of the South American sea lion (Otaria flavescens) in two colonies with different breeding substrates

“…femalefemale), which can jeopardise offspring survival (Le Boeuf and Campagna 1994;Bradshaw et al 2000). In pinniped species, pup mortality is known to be density dependent (Anderson et al 1977;Doidge et al 1984;Fowler 1987;Harcourt 1992;Le Boeuf and Briggs 1977; but see Twiss et al 2003). Deaths of pups occur mainly when social events separate them from their mothers, and consequently they starve, are crushed by males or suffer aggression from alien females (Le Boeuf and Briggs 1977;McCann 1982;Doidge et al 1984;Vilá and Cassini 1990;Bowen 1991;Campagna et al 1992;Harcourt 1992).…”

“…Female aggression in otariids has also been related to competition for space in breeding colonies, particularly in species breeding at high densities or in colonies with limited availability of high-quality substrates (e.g. tide pools; Doidge et al 1984;Francis 1987;Campagna and Le Boeuf 1988a;Carey 1992;Maestripieri 1992;Majluf 1992).…”

Early mortality and perinatal growth in the subantarctic fur seal (Arctocephalus tropicalis) on Amsterdam Island

“…However, at a given date, pups born early are also older and more agile so they may avoid encounters with aggressive adults. In Antarctic and South American fur seals, higher mortality in pups born late in the season compared with pups born earlier results from densitydependent mortality in these species (Doidge, Croxall & Baker, 1984;Harcourt, 1992). In South Georgia and Peru, beaches are mostly¯at and composed of pebbles (Doidge et al, 1984: plates 1 & 2; R. Harcourt & J. Francis, pers.…”

“…In northern fur seals, mortality mostly occurs while pups are left unattended for the ®rst time, with the greatest number of deaths occurring probably as a result of starvation just before the mother returns to nurse her pup (Trites, 1992b). Early mortality may also occur with increasing population density, as observed at several colonies of Antarctic and South American A. australis fur seals (Doidge, Croxall & Baker, 1984;Harcourt, 1992).…”