Le phoque de Weddell (Leptonychotes weddelli L.) à Pointe Géologie et sa plasticité sociale

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Abstract
The Weddell seal population was studied to clarify its social structure. The population was estimated by the capture recapture method at around 340 seals plus approximately a hundred pups each year. Counts taken under different climatic conditions showed that wind-chill plays the main role on the seals'' haul out. Light has only a slight influence. The total number of each age and sex grouping was collected for a complete cycle: During summer, when the sea is free from ice, we encountered as many males as females of which 20% were subadults; during winter, when the sea is iced over, there were as many males as females, and 20% were subadults, but the total numbers were much lower owing to their wide dispersal. In spring, during the breeding season, the males returned followed by the females who delivered young within 1-2 days. Females greatly outnumbered the males, and the subadults were not present. From birth to sexual maturity the sex-ratio is close to 1.2, followed by a drop to 0.9 when the sealsare fully grown. Death rate for males is higher than for females. The conditions for the establishment of a Traditional pupping area, and the important role played by cracks in the fast ice were discussed. An interpretive map was proposed which shows the distribution of the seals in spring during the breeding season. In the traditional pupping area dominant males defended the entry holes around which females gathered to give birth, so that harems formed. The sex-ratio for this area is 1 male to 4 or 5 females. In the peripheral males area, sex ratio was 3 or 4 males to 1 female. The biological significance of the sudden return of males, which leads to heavy fighting for the cracks. These rivalries continue throughout the breeding period and occur mainly on the traditional pupping area. By comparing the sex-ratio figures at Pointe Geologie with those at McMurdo Sound, Antarctica, the effects of local ice conditions on the social structure of the Weddell seal were established. A social lability occurs at Pointe Geologie even if a year of heavy fast-ice (1976) is compared with a year with many of cracks (1977). This would be due to fighting among males for the cracks during the breeding period. Individual recognition between a pup and its mother seems to be of general value for all the phocids, with some pecularities for the elephant seals. The synchronization of the births is high; 90% take place between Oct. 6-24. At the beginning of Nov. 1977 an early break-out of the fast ice killed > 45% of the pups. This break-out shows the importance of the cycle of fast ice for survival of this species. This is corroborated by the shift of the mean pupping date dependent on the latitude. The breeding strategy of the Weddell seal compared with that of the emperor penguin (Aptenodytes forsteri), shows convergent adaptations to the fast ice.