Stephen J. Trumble

The diet of Weddell seals in McMurdo Sound, Antarctica as determined from scat collections and stable isotope analysis

Abstract The diet of adult and juvenile Weddell seals (Leptonychotes weddellii) in McMurdo Sound, Antarctica, was determined from both scat and stable isotope analyses, to ascertain if foraging behavior varied with age, season, or diving pattern. Scats were collected over 6 years and recovered hard parts identified. Stable carbon and nitrogen isotope values were determined for seal blood samples and potential prey items and used to identify primary prey species and assess trophic interactions. Pleuragramma antarcticum remains were recovered from between 70 and 100% of the scats, and there was little evidence for inter-annual or age-specific variation in foraging behavior. However, stable isotope and dive data analyses indicated that while most seals foraged predominantly on pelagic fish and squid, some juveniles concentrated on shallow benthic Trematomus spp. Combining these three methods permitted firm conclusions about diet and foraging behavior to be drawn.

The ontogeny of aerobic and diving capacity in the skeletal muscles of Weddell seals.

SUMMARY Our objective was to determine the ontogenetic changes in the skeletal muscles of Weddell seals that transform a non-diving pup into an elite diving adult. Muscle biopsies were collected from pups, juveniles and adults and analyzed for changes in fiber type, mitochondrial density, myoglobin concentrations and aerobic, lipolytic and anaerobic enzyme activities. The fiber type results demonstrated a decrease in slow-twitch oxidative (Type I) fibers and a significant increase in fast-twitch oxidative (Type IIA) fibers as the animals mature. In addition, the volume density of mitochondria and the activity of lipolytic enzymes significantly decreased as the seals matured. To our knowledge, this is the first quantitative account describing a decrease in aerobic fibers shifting towards an increase in fast-twitch oxidative fibers with a significant decrease in mitochondrial density as animals mature. These differences in the muscle physiology of Weddell seals are potentially due to their three very distinct stages of life history: non-diving pup, novice diving juvenile, and elite deep diving adult. During the first few weeks of life, pups are a non-diving terrestrial mammal that must rely on lanugo (natal fur) for thermoregulation in the harsh conditions of Antarctica. The increased aerobic capacity of pups, associated with increased mitochondrial volumes, acts to provide additional thermogenesis. As these future elite divers mature, their skeletal muscles transform to a more sedentary state in order to maintain the low levels of aerobic metabolism associated with long-duration diving.

Blood chemistry, hematology, and morphology of wild harbor seal pups in Alaska

Recent declines in harbor seal (Phoca vitulina) populations in Alaska have emphasized the need to obtain health index biomarkers for comparative purposes. This study was designed to compare blood chemistry and morphology reference range values between 2 harbor seal pup populations in Alaska, 1 population in continued decline within Prince William Sound, and another population in recent increase on Tugidak Island. Significant site-specific differences occurred in 5 of the 8 mean hematology values as well as in 11 plasma chemistry values. We also determined significant year-to-year variability in 8 (36%) mean plasma chemistries. These results form the largest available field-based blood reference database for harbor seal pups. They demonstrate that blood values can vary on the population scale and that health assessment or ecophysiological studies involving blood chemistry must utilize an appropriate set of reference values for valid comparisons.

Different Thermoregulatory Strategies in Nearly Weaned Pup, Yearling, and Adult Weddell Seals (Leptonychotes weddelli)

Mammals balance heat dissipation with heat production to maintain core body temperatures independent of their environment. Thermal balance is undoubtedly most challenging for mammals born in polar regions because small body size theoretically results in high surface‐area‐to‐volume ratios (SA:V), which facilitate heat loss (HL). Thus, we examined the ontogeny of thermoregulatory characteristics of an ice‐breeding seal (Weddell seal Leptonychotes weddelli). Morphology, blubber thickness, rectal temperature (Tr), muscle temperature (Tm), and skin temperatures on the trunk (Ts) and flipper (Tf) in 3–5‐wk‐old pups, yearlings, and adults were measured. Adults maintained the thickest blubber layers, while yearlings had the thinnest; Tr and Tm fell within a narrow range, yet Tr and Tm decreased significantly with body length. All seals maintained skin temperatures lower than Tr, our index of core body temperature. The Tss were positively correlated with environmental temperatures; conversely, Tfs were not. Although pups had the greatest proportion of blubber, their greater SA:V and limited ability to minimize body‐to‐environment temperature gradients led to the greatest calculated mass‐specific HL. This implies that pups relied on elevated metabolic heat production to counter HL. Heat production in pups and yearlings may have been aided by nonshivering thermogenesis in the skeletal muscle via the enhanced muscle mitochondrial densities that have been observed in these segments of this population.