Quantity discrimination in Port Jackson sharks incubated under elevated temperatures

Ocean warming can induce physiological and behavioural effects in marine predators that can cascade through ecosystems. A lack of understanding of the effects of elevated temperature on shark behaviour remains an impediment to forecasting ecosystem-wide impacts. Port Jackson shark eggs were incubated and reared at current and projected end-of-century temperatures (+ 3 °C). We tested juvenile’s learning ability with a quantity discrimination task. The mortality rate of sharks reared in warm water was 41.7% compared with no mortality in the present-day sharks. Contrary to expectations, our results suggest that surviving hatchlings from the elevated-temperature group took fewer days to reach learning criterion and had a higher proportion of correct choice compared with hatchlings reared under present conditions. Additionally, this is the first data suggesting that sharks can discriminate different quantities. Our results seem to indicate that learning and behaviour might play a role in allowing elasmobranchs to overcome some of the deleterious effects of climate warming, but further research is needed to fully comprehend these findings. SIGNIFICANCE STATEMENT: The world’s oceans are warming at an unprecedented rate, which will impair development and alter physiological and behavioural traits in marine predators. Learning may play a leading role in allowing apex and mesopredators to adapt to a rapidly changing environment; however, no studies have tested the impacts of ocean warming in their learning abilities. We incubated and reared Port Jackson shark eggs at current and projected end-of-century temperatures (+ 3 °C). Contrary to expectations, surviving juveniles from the elevated-temperature group showed better learning performance, potentially adding learning ability to a growing list of traits that incubation temperature can modify during early development in marine predators. Our results were not entirely negative; it is possible that increased learning performance might allow apex and mesopredators to increase foraging efficiency and match increased energetic demands caused by elevated temperature.