Heart rate changes reveal that little blue penguin chicks (Eudyptula minor) can use vocal signatures to discriminate familiar from unfamiliar chicks

Researchers investigating social recognition typically measure only behavio ural responses during discrimination tests - physiological changes have bee n largely ignored. We examined whether little blue penguin chicks (Eudyptul a minor) could distinguish siblings from other chicks using auditory cues, by measuring behavioural and heart rate changes during playback experiments . Chicks were exposed to five treatments: the begging calls of siblings, ne ighbouring chicks and unfamiliar chicks, and two controls (heterospecific b egging calls and music). We also determined if chicks developed distinctive begging calls, by using F-ratios to quantify inter- versus intra-individua l variability in a range of acoustic parameters. and applying a discriminan t-function analysis. Inter-individual variation was greater for pitch than for temporal or amplitude parameters, suggesting that call pitch may be imp ortant for individual recognition. The discriminant-function analysis showe d each chick's calls were distinctive and could act as a vocal signature. T reatments did not instigate different behavioural responses. However, chick heart rates during playback of sibling calls were significantly higher tha n those recorded during stranger, but not neighbour, playback. A simple rec ognition system based on familiarity may allow this plesiomorphic and loose ly colonial penguin to gain at least some of the benefits associated with m ore advanced sibling recognition systems (some highly colonial seabirds dis criminate siblings from neighbouring chicks). Heart rate could be a useful measure of social recognition abilities, particularly in species when chang es in behaviour are not always evident or are difficult to observe.