Rates of foot darkening with age in banded (Spheniscus) penguins suggest that foot pigments provide UV protection
Abstract
Spheniscus (banded) penguins have pale feet without spots at hatching. Individual penguins’ feet become spotted and darken with age. Rates of darkening among these four species is consistent with the hypothesis that the dark pigment acts in UV protection. The species that breeds at the lowest latitudes and experiences the highest UV radiation (Galápagos Penguins Spheniscus mendiculus) had feet that darkened fastest, and the species that breed at higher latitudes and experience less intense insolation (African S. demersus and Magellanic S. magellanicus Penguins) had feet that darkened slowest. Humboldt Penguins S. humboldti breed mostly at low latitudes and had intermediate rates of foot darkening. If the function of darkening is thermoregulation, feet should be darker at higher latitudes where penguins swim in colder water, the opposite of what we found. We also found that males’ feet darken somewhat faster than females’ feet, likely because females spend more time in their nests (burrows or under vegetation) than males and have less sun exposure. We found that feet darkened over years, but not within a breeding season. The color change is a life-long process, not a seasonal response to UV radiation. Finally, we showed that we can accurately predict the age structure of a colony of Magellanic Penguins based on a sample of foot colors. Penguins cannot be aged once they molt into adult plumage, at 6 to 24 months of age in Spheniscus penguins, and individuals can live more than 30 years. Age structure is important in conservation and management of populations. We propose foot darkening as a way to assess age structure in banded penguins. Foot color in a colony of Magellanic Penguins can provide a rapid, noninvasive method to estimate the age structure of the colony.