Feet first: Magellanic penguin chicks show adaptive growth

Growing animals should allocate their limited resources in ways that
maximize survival. Seabird chicks must balance the growth of features and
fat reserves needed to survive on land with those needed to successfully
fledge and survive at sea. We used a large, 34-year dataset to examine
energy allocation in Magellanic penguin chicks. Based on the temporal
trends in the selective pressures that chicks faced, we developed
predictions relating to the timing of skeletal feature growth (Prediction
1), variation in skeletal feature size and shape (Prediction 2), and
responses to periods of high energetic constraint (Prediction 3). We
tested our predictions using descriptive statistics, generalized additive
models, and principal component analysis. Nearly all of our predictions
were supported. Chicks grew their feet first, then their flippers. They
continued to grow their bill after fledging (Prediction 1). Variance in
feature size increased in young chicks but declined before fledging; this
variance was largely driven by overall size rather than by shape
(Prediction 2). Chicks that died grew slower and varied more in feature
size than those that fledged (Prediction 2). Skeletal features grew
rapidly prior to thermoregulation and feet and flippers were 90% grown
prior to juvenile feather growth; both thermoregulation and feather growth
are energetically expensive (Prediction 3). To avoid starvation, chicks
prioritized storing mass during the first 10 days after hatching, then the
body condition of chicks began to decline (Prediction 3). In contrast to
our prediction of mass prioritization in young chicks, chicks that were
relatively light for their age had high skeletal size to mass ratios.
Chicks did not show evidence of reaching physiological growth limits
(Prediction 3). By examining energy allocation patterns at fine temporal
scales and in the context of detailed natural history data, we provide
insight into the trade-offs faced by growing animals.