Abstract
In capital‐breeding marine mammals, prey acquisition during the foraging trip coinciding with gestation must provide energy to meet the immediate needs of the growing fetus and also a store to meet the subsequent demands of lactation. Weddell seals (Leptonychotes weddellii) that give birth following the gestational (winter) foraging period gain similar proportions of mass and lipid as compared to females that fail to give birth. Therefore, any changes in foraging behavior can be attributed to gestational costs. To investigate differences in foraging effort associated with successful reproduction, twenty‐three satellite tags were deployed on post‐molt female Weddell seals in the Ross Sea. Of the 20 females that returned to the area the following year, 12 females gave birth and eight did not. Females that gave birth the following year began the winter foraging period with significantly longer and deeper dives, as compared to non‐reproductive seals. Mid‐ to late winter, reproductive females spent a significantly greater proportion of the day diving, and either depressed their diving metabolic rates (DMR), or exceeded their calculated aerobic dive limit (cADL) more frequently than females that returned without a pup. Moreover, non‐reproductive females organized their dives into 2–3 short bouts per day on average (BOUTshort; 7.06 ± 1.29 hr; mean ± 95% CI), whereas reproductive females made 1–2 BOUTshort per day (10.9 ± 2.84 hr), comprising one long daily foraging bout without rest. The magnitude of the increase in dive activity budgets and depression in calculated DMR closely matched the estimated energetic requirements of supporting a fetus. This study is one of the first to identify increases in foraging effort that are associated with successful reproduction in a top predator and indicates that reproductive females must operate closer to their physiological limits to support gestational costs.
Highlights
The survival and reproductive output of wild animals depends on the individual’s ability to acquire sufficient resources and energy in the face of changing ambient conditions and ephemeral prey, and such challenges are accentuated in high‐latitude, polar environ‐ ments that experience dramatic seasonality (Bluhm & Gradinger, 2008; Bronson, 2009; Croxall, 1992; Walsh, 2008)
This study shows that there is significant variation in dive be‐ havior across the overwinter foraging period in fe‐ male Weddell seals
While seasonal differences may be driven by changes in prey‐fields, there is no evidence that reproduc‐ tive and non‐reproductive females forage in different areas and/ or on different species (Goetz, 2015; Goetz, Burns, Hückstӓdt, Shero, & Costa, 2017), so here we concentrate on the differences in behavior that are associated with putative pregnancy status
Summary
The survival and reproductive output of wild animals depends on the individual’s ability to acquire sufficient resources and energy in the face of changing ambient conditions and ephemeral prey, and such challenges are accentuated in high‐latitude, polar environ‐ ments that experience dramatic seasonality (Bluhm & Gradinger, 2008; Bronson, 2009; Croxall, 1992; Walsh, 2008). Southern elephant seals (Mirounga leonina) dive significantly longer over their post‐molt (ges‐ tational) foraging period as compared with the post‐breeding forag‐ ing trip, routinely exceeding aerobic thresholds (Hindell, Slip, Burton, & Bryden, 1992) It may be energetically beneficial for animals to exceed their ADL in order to exploit rich prey patches if acquisi‐ tion of these resources outweighs the costs of longer post‐dive re‐ covery times (Houston & Carbone, 1992). The Weddell seal (Leptonychotes weddellii) offers a unique model to assess whether dive behavior differs in measurable ways be‐ tween females that successfully give birth and those that do not This is because female Weddell seals weigh the same and have the same body composition (i.e., lipid stored for energetic capital) after their winter (gestational) foraging period, regardless of whether or not they produce a pup the following year. The utility of biologging devices to identify behavioral shifts that reliably predict successful reproduc‐ tive events in wild animals would have broad applicability
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