Variation in foraging effort by lactating Antarctic fur seals: response to simulated increased foraging costs |
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Authors: | I L Boyd D J McCafferty T R Walker |
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Institution: | (1) British Antarctic Survey, Natural Environment Research Council, Madingley Road, Cambridge CB3 OET, UK Tel.:01223 251610; e-mail: I.Boyd@British-Antarctic-Survey.ac.uk, GB |
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Abstract: | Seasonally breeding predators, which are limited in the time available for provisioning young at a central location, and
by the fasting abilities of the young, are likely to maximize energy delivery to the young by maximizing the rate of energy
delivery averaged over the whole period of investment. Reduction in food availability or increased foraging costs will alter
the optimal behavior of individuals. This study examined the behavioral adaptations of a diving predator, the Antarctic fur
seal, to increased foraging costs during lactation. One group of mothers (n=5, treatment) was fitted with additional drag to increase the cost of transport in comparison with a control group (n=8). At the scales of the individual dives, the treatment group made more shorter, shallower (< 30 m) dives. Compensation for
slower swimming speeds was achieved by diving at a steeper angle. Overall, diving behavior conformed to several specific theoretical
predictions but there were also departures from theory, particularly concerning swimming speed during diving. Diving behavior
appears to be adjusted to maximize the proportion of time spent at the bottom of dives. At the scale of diving bouts, no difference
was observed between the treatment and control groups in terms of the frequency and duration of bouts and there was also no
difference between the two groups in terms of the proportion of time spent diving. At the scale of complete foraging cycles,
time taken to return to the pup was significantly longer in the treatment group but there was no difference in the rate of
delivery of energy (measured from pup growth rate) to the pups in each group. Since mothers in the treatment group did not
use significantly more body reserves, we conclude that behavioral adjustments at the scale of individual dives allowed mothers
in the treatment group to compensate for the additional foraging costs. Pup growth rate appears to be less sensitive to the
foraging conditions experienced by mothers than foraging trip duration.
Received: 14 June 1996 / Accepted after revision: 16 November 1996 |
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Keywords: | Foraging Costs Fur seal Antarctic |
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