Osmotic concentration of prey affects food discrimination behaviour in the A ustralian pelican

Abstract

The Australian pelican Pelecanus conspicillatus is the largest of all pelican species and can consume up to half their body weight per day, feeding predominantly on teleost fishes. Anecdotally, it has been suggested that pelicans preferentially avoid the consumption of small portions of elasmobranch fishes (e.g. sharks and rays), which prompted this investigation into their food discrimination behaviour. The large differences in the osmolarity and/or urea content between elasmobranch and teleost fishes are likely to underpin this behaviour. Osmoconformers such as elasmobranchs maintain an internal osmotic concentration similar to seawater, with this state being achieved primarily by the retention of the osmolyte urea, while other osmoconforming organisms such as squid likely conserve ions such as Na+ and Cl–. In contrast, osmoregulating teleosts maintain an osmolarity much lower than seawater and approximately the same as pelicans. Consequently, ingestion of teleost fishes results in minimal water movement; however, if a large bolus of osmoconformers are consumed this may lead to dehydration. It was hypothesized that pelicans would preferentially avoid the consumption of osmoconformers and accept osmoregulators. In addition, we investigated the underlying physiological basis for elasmobranch rejection, and which sense(s) are primarily utilized for such behaviour. We found that pelicans freely chose to accept offerings of osmoregulators at a significantly greater frequency than osmoconformers. Furthermore, the osmotic concentration (and not specifically urea) was considered to be the most likely cause of rejection, as squid, which do not conserve urea, were rejected equally as often as elasmobranchs. Finally, vision appears to be the sense utilized for this behaviour because when elasmobranchs were made to appear visibly ‘similar’ to teleost fishes they were consumed at equal frequencies. This study provides new insight into food discrimination in pelicans and might also be applicable to other seabirds.