Factors affecting first-year survival in grey seals and their implicationsfor life history strategy

1. In order to estimate the effect of weaning mass and body condition on th e 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 resi ghting and dead recovery mark-recapture data was used. A new type of tag wa s employed which allowed individuals to be identified when resighted alive (Hall, Moss & McConnell 2000) as well as when found dead. 2, The probability of post-weaning survival to age 1 increased with body co ndition at weaning and differed between the sexes. Regardless of pup condit ion and time of year, the odds of survival for female pups over a 2-month i nterval was estimated to be 3.37 (SE = 1.30) times higher than for males. R egardless of sex, a 1 standard deviation increase in pup condition was esti mated to increase the odds of survival by a factor of 1.422 (SE = 0.226). F or a male pup in average condition (0.41 kg cm(-1)) the estimated annual su rvival after adjusting for tag-loss was 0.193 (SE = 0.084); for a female pu p in average condition (0.39 kg cm(-1)) it was 0.617 (SE = 0.155). 3, The effect of condition at weaning on survival was significantly greater for male pups than for females. This implies that high quality females sho uld invest more heavily in their male pups because the marginal return, in terms of increased reproductive value, from any additional expenditure is t wice that for females. Male pups in our sample were significantly heavier a t weaning and in better condition than female pups. However, this does not provide conclusive support for our predictions, because we could not contro l for the effects of maternal size on weaned mass.