Acoustic allometry in begging calls of African penguin chicks

Acoustic allometry is the relationship between animal vocalisations and their body size. In 1995, Fitch and Hauser demonstrated that in mammals there is a correlation between body size and fundamental frequency and that the morphological characteristics of the phonatory organs directly influence the value of the fundamental frequency (F0). However, acoustic allometry has been little studied in birds. Vocalisations are one of the most important communication means in the African penguin (Spheniscus demersus). This study aimed to evaluate the presence of acoustic allometry in the begging calls of African penguin chicks. This species is endemic to South Africa, nowadays classified as endangered by the International Union for Conservation of Nature, IUCN. The study took place in the Chick rearing unit of the Southern African Foundation for the Conservation of Coastal Birds (SANCCOB), a conservation and rehabilitation centre for seabirds in Cape Town (South Africa). Chicks ranged from hatching to reaching 1,5 kg in weight (about one month old) and were recorded during weight and feeding sessions, with two-channel audio recorder (Zoom- F1-LP) and a shotgun microphone (Zoom SGH-6). In this analysis, 1514 vocalisations of 118 African penguins were selected and used. The following acoustic parameters: sound duration, maximum, minimum, mean, modulation, and time to maximum of the fundamental frequency (f0) were extracted for each vocalization using the software Praat. For the statistical analysis, a series of Generalised Linear Mixed Models (GLMMs) were run to study the relationship between acoustic parameters and the body size of the chicks. These analyses demonstrated the presence of a negative correlation between the increase in weight and the lowering of the fundamental frequency in banded penguins’ chicks during their first month of life. The GLMM showed a significant effect of the logarithm of the body weight on f0 modulation, maximum, mean, and minimum. My results suggest that increased fat mass is a determining factor in the lowering of the fundamental frequency. An effect of the penguins’ weight on the sound duration and on the maximum time of the fundamental frequency was not found, leading me to conclude that the duration of the single vocal unit of the begging calls does not reflect the air storage capacity in the air sacs. This work serves as a basis for future investigations about the use of individuals’ acoustic allometry as a tool for conservation in African penguins’ chicks, using the fundamental frequency growth curve in relation to weight gain as a parameter of body condition. This could be useful in penguins’ hand-rearing, but future studies are necessary to explore this aspect further.