Sex-Based Differences in Adélie Penguin (Pygoscelis adeliae) Chick Growth Rates and Diet

Abstract

Sexually size-dimorphic species must show some difference between the sexes in growth rate and/or length of growing period. Such differences in growth parameters can cause the sexes to be impacted by environmental variability in different ways, and understanding these differences allows a better understanding of patterns in productivity between individuals and populations. We investigated differences in growth rate and diet between male and female Adélie Penguin (Pygoscelis adeliae) chicks during two breeding seasons at Cape Crozier, Ross Island, Antarctica. Adélie Penguins are a slightly dimorphic species, with adult males averaging larger than adult females in mass (~11%) as well as bill (~8%) and flipper length (~3%). We measured mass and length of flipper, bill, tibiotarsus, and foot at 5-day intervals for 45 male and 40 female individually-marked chicks. Chick sex was molecularly determined from feathers. We used linear mixed effects models to estimate daily growth rate as a function of chick sex, while controlling for hatching order, brood size, year, and potential variation in breeding quality between pairs of parents. Accounting for season and hatching order, male chicks gained mass an average of 15.6 g d-1 faster than females. Similarly, growth in bill length was faster for males, and the calculated bill size difference at fledging was similar to that observed in adults. There was no evidence for sex-based differences in growth of other morphological features. Adélie diet at Ross Island is composed almost entirely of two species—one krill (Euphausia crystallorophias) and one fish (Pleuragramma antarctica), with fish having a higher caloric value. Using isotopic analyses of feather samples, we also determined that male chicks were fed a higher proportion of fish than female chicks. The related differences in provisioning and growth rates of male and female offspring provides a greater understanding of the ways in which ecological factors may impact the two sexes differently.

Sex allocation and sex‐specific parental investment in an endangered seabird

“…The difference in growth rate could be explained by African Penguin sexual size dimorphism, with males being slightly larger than females, and therefore having to grow faster to obtain their larger size, and is in line with sex-specific chick growth observed in other sexually size-dimorphic seabirds, e.g. Wandering Albatrosses , Black-legged Kittiwakes (Merkling et al 2012) and Ad elie Penguins Pygoscelis adeliae (Jennings et al 2016). In addition, in birds and mammals, male offspring are generally more sensitive to environmental conditions (Badyaev 2002), and African Penguin male chicks may have benefited more than female chicks from profitable prey availability during our study.…”

“…) and Adélie Penguins Pygoscelis adeliae (Jennings et al . ). In addition, in birds and mammals, male offspring are generally more sensitive to environmental conditions (Badyaev ), and African Penguin male chicks may have benefited more than female chicks from profitable prey availability during our study.…”

“…They are unlikely to be explained by male and female chicks being fed different diets (e.g. Jennings et al 2016), as African Penguin parents feed both chicks of their brood alternately for similar amounts of time (A. Spelt & L. Pichegru unpubl. data) and male chick growth rates were similar between mixed broods and male-only broods.…”

Sex‐biased survival contributes to population decline in a long‐lived seabird, the Magellanic Penguin

“…]; Adélie Penguins [Jennings et al. ]; African Penguin [Spelt and Pichegru ]). Because of their smaller size and less favorable surface area to volume ratio, female Magellanic Penguins may fledge at an energetic disadvantage to males, lowering their juvenile survival rates.…”

“…Studies have shown that male Magellanic Penguin chicks are fed at a higher trophic level (i.e., higher d 15 N) than female chicks (Forero et al 2002). The proportion of fish in the diet influences size at fledging in Ad elie Penguins (Whitehead et al 2015), and males of several species of penguin fledge at a larger size or in better body condition than do females (King Penguins in some years [Bordier et al 2014]; Ad elie Penguins [Jennings et al 2016]; African Penguin [Spelt and Pichegru 2017]). Because of their smaller size and less favorable surface area to volume ratio, female Magellanic Penguins may fledge at an energetic disadvantage to males, lowering their juvenile survival rates.…”