Data from: Reproductive success is energetically linked to foraging efficiency in Antarctic fur seals

The efficiency with which individuals extract energy from their
environment defines their survival and reproductive success, and thus
their selective contribution to the population. Individuals that forage
more efficiently (i.e., when energy gained exceeds energy expended) are
likely to be more successful at raising viable offspring than individuals
that forage less efficiently. Our goal was to test this prediction in
large long-lived mammals under free-ranging conditions. To do so, we
equipped 20 lactating Antarctic fur seals (Arctocephalus gazella) breeding
on the Kerguelen Island in the Southern Ocean with tags that recorded GPS
locations, depth and tri-axial acceleration to determine at-sea behaviours
and detailed time-activity budgets during their foraging trips. We also
simultaneously measured energy spent at sea using the doubly-labeled water
(DLW) method, and estimated the energy acquired while foraging from 1)
type and energy content of prey species present in scat remains, and 2)
numbers of prey capture attempts determined from head acceleration.
Finally, we followed the growth of 36 pups from birth until weaning (of
which 20 were the offspring of our 20 tracked mothers), and used the
relative differences in body mass of pups at weaning as an index of first
year survival and thus the reproductive success of their mothers. Our
results show that females with greater foraging efficiencies produced
relatively bigger pups at weaning. These mothers achieved greater foraging
efficiency by extracting more energy per minute of diving rather than by
reducing energy expenditure. This strategy also resulted in the females
spending less time diving and less time overall at sea, which allowed them
to deliver higher quality milk to their pups, or allowed their pups to
suckle more frequently, or both. The linkage we demonstrate between
reproductive success and the quality of individuals as foragers provides
an individual-based quantitative framework to investigate how changes in
the availability and accessibility of prey can affect fitness of animals.