DOI: 10.5061/dryad.t76hdr80g
title: Feet first: Magellanic penguin chicks show adaptive growth

description: 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.
description: We measured four skeletal features (foot length, flipper length,
 bill length, bill depth) and mass of 9,491 Magellanic penguin chicks from
 hatching until death or fledging. Skeletal feature units are cm and mass
 units are kg. Our study took place at Punta Tombo, Argentina from 1983 to
 2017.
 Statistics_ByFeatureAge.txt
 We calculated descriptive statistics (average, standard
 deviation, coefficient of variation) for features and mass for each age of
 chick growth from ages 0 days to 90 days. We used the statistical program
 R to examine linear and non-linear allometric relationships between
 skeletal size and mass using linear, polynomial, and generalized additive
 models in the package mgcv (Version 1.8-31). We report parameters for
 these models at each age of chick growth for each skeletal feature and for
 overall body size (the first principal component of skeletal
 features).
 PCAOutputs_ByAge.txt We ran
 a principal component analysis on centered and scaled skeletal features
 using the prcomp function in R. We ran this analysis across all chicks and
 used it to calculate age-specific body condition by examining the
 residuals of the relationship between mass and overall body size
 (principal component analysis). We also ran a principal component analysis
 for each age of growth to examine how much of the variation in chick size
 at each age was due to overall size (first principal component) versus
 body shape (high order principal components).
description: Please see README file for additional details and for
 descriptions of each column.