| Summary: | Weddell seals ( Leptonychotes weddellii ) are a model species for studying diving physiology. However, Weddell seal pups are not born with the elite diving capabilities of their adult counterparts and must develop their skills so they can survive into adulthood. Previous research has excluded an examination of pups' initial diving attempts. The aim of this study was to investigate the development of diving capabilities throughout ontogeny, beginning at 1 week of age, to create a more complete trajectory of development. To do this, we examined the dive behavior and blood oxygen stores of Weddell seal pups over the first seven weeks of life. Flipper‐mounted time‐depth recorders (Lotek LAT1800XLFP) were attached to 1‐week‐old Weddell seal pups (n=8) in Erebus Bay, Antarctica, and were recovered when the pups were 7 weeks of age. Time spent in the water, maximum dive depth, dive duration, bottom time, and post‐dive interval were calculated to characterize dive behavior. Blood samples were collected from three of these pups at 1 week, 3 weeks, 5 weeks, and 7 weeks of age. We measured hematocrit, hemoglobin, and red blood cell counts for all blood samples to assess age‐related changes in blood oxygen stores. Time spent in the water and number of dives per day both significantly increased as pups got older (P < 0.0001). The time pups spent in the water per day ranged between 1 min and 21 h. Although the maximum dive depths for pups ranged between 3 m and 14.78 m, the majority of “dives” were less than 2 m. Hematocrit, hemoglobin, and red blood cell counts also increased throughout early development. Correlations between dive behavior and hematology indicate dive behavior, rather than age, is a better predictor of oxygen storage development. Based on these data, we conclude early experience in the water is an important driver of physiological development in dependent Weddell seal pups. New swimmers may prioritize acclimatization to the water and strengthening their swimming muscles over attempting deeper foraging ...
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