Evidence of behavioural thermoregulation by dugongs at the high latitude limit to their range in eastern Australia

Abstract

Many species of marine mammals have evolved behavioural adaptations to minimize heat loss to the surrounding water. We tracked 21 dugongs (Dugong dugon Müller 1776) using acoustic and satellite/GPS transmitters in 2012, 2013 and 2014 in Moreton Bay, Queensland at the high latitude limit of the species' winter range in eastern Australia to examine if there was a relationship between movements and environmental temperature that might suggest behavioural thermoregulation. Oceanic waters immediately outside the bay where the dugong's seagrass food is unavailable exhibited temperatures from 5.5 °C warmer to 3 °C cooler than the Eastern Banks, the major dugong habitat in the bay. All tracked dugongs made at least one (and up to 66) return trip(s) from the Eastern Banks to the adjacent oceanic waters. The probability of making an outgoing trip was highest in 2014 and lowest in 2013 when the water temperature inside the bay was higher than the other two years. The odds of making an outgoing trip were lower when temperature differences (outside minus inside) were small or negative but increased by a factor of up to 2.12 for each 1 °C positive difference. Individual dugongs were most likely to travel out of the bay between midnight and noon on an outgoing tide or at slack high water and return to the bay on an incoming tide or slack low water between noon and midnight. The amount of time a dugong spent outside the bay on each trip was relatively short with an overall median of 5.9 h. The dugongs' individual activity spaces generally declined as winter progressed suggesting a change in the cost-effectiveness of moving outside the bay. Our analysis adds to the evidence that dugongs undertake behavioural thermoregulation at least at the high latitude limits of their range.