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Feet first: Magellanic penguin chicks show adaptive growth

Citation

Gowaris, Natasha (2022), Feet first: Magellanic penguin chicks show adaptive growth, Dryad, Dataset, https://doi.org/10.5061/dryad.t76hdr80g

Abstract

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.

Methods

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).

Usage Notes

Please see README file for additional details and for descriptions of each column.