Age at weaning in Steller sea lions

The duration of offspring care is critical to female fitness and
population resilience by allowing flexibility in life history strategies
in a variable environment.  Yet, for many mammals capable of
extended periods of maternal care, estimates of the duration of offspring
dependency are not available and the relative importance of flexibility of
this trait on fitness and population viability has rarely been
examined.  We used data from 4,447 Steller sea lions Eumetopias
jubatus from the Gulf of Alaska and multistate hidden Markov
mark-recapture models to estimate age-specific weaning probabilities.
Maternal care beyond age 1 was common: weaning was later for animals from
Southeast Alaska (SEAK) and Prince William Sound (PWS, weaning
probabilities: 0.538–0.648 / 0.784–0.873 by age 1 / 2) compared to animals
born to the west (0.714–0.855 / 0.798–0.938).  SEAK/PWS animals
were also smaller than those born farther west, suggesting a possible
link.  Females weaned slightly earlier (+0.080 at age 1 and 2)
compared with males in SEAK only.  Poor survival for weaned
versus unweaned yearlings occurred in southern SEAK (female survival
probabilities: 0.609 versus 0.792) and the central Gulf (0.667 versus
0.901), suggesting poor conditions for juveniles in these areas. 
First-year survival increased with neonatal body mass (NBM) linearly in
the Gulf and non-linearly in SEAK.  The probability of weaning at
age 1 increased linearly with NBM for SEAK animals only. Rookeries where
juveniles weaned at earlier ages had lower adult female survival, but age
at weaning was unrelated to population trends.  Our results
suggest the time to weaning may be optimized for different habitats based
on long-term average conditions (e.g., prey dynamics), that may also shape
body size, with limited short-term plasticity.  An apparent
trade-off of adult survival in favor of juvenile survival and large
offspring size in the endangered Gulf of Alaska population requires
further study.