Factors affecting first‐year survival in grey seals and their implications for life history strategy

Abstract

In order to estimate the effect of weaning mass and body condition on the post‐weaning survival of grey seal pups from the Isle of May, Scotland in 1998 during their first year of life, a simultaneous analysis of live resighting and dead recovery mark–recapture data was used. A new type of tag was employed which allowed individuals to be identified when resighted alive ( Hall, Moss & McConnell 2000 ) as well as when found dead. The probability of post‐weaning survival to age 1 increased with body condition at weaning and differed between the sexes. Regardless of pup condition and time of year, the odds of survival for female pups over a 2‐month interval was estimated to be 3·37 (SE = 1·30) times higher than for males. Regardless of sex, a 1 standard deviation increase in pup condition was estimated to increase the odds of survival by a factor of 1·422 (SE = 0·226). For a male pup in average condition (0·41 kg cm −1 ) the estimated annual survival after adjusting for tag‐loss was 0·193 (SE = 0·084); for a female pup in average condition (0·39 kg cm −1 ) it was 0·617 (SE = 0·155). The effect of condition at weaning on survival was significantly greater for male pups than for females. This implies that high quality females should invest more heavily in their male pups because the marginal return, in terms of increased reproductive value, from any additional expenditure is twice that for females. Male pups in our sample were significantly heavier at weaning and in better condition than female pups. However, this does not provide conclusive support for our predictions, because we could not control for the effects of maternal size on weaned mass.