Starvation threshold and fasting endurance of right whales

Baleen whales undertake long-distance migrations to oligotrophic calving/breeding grounds during winter, where they fast for extended periods. This remarkable ability to endure prolonged fasting highlights the crucial role of body size and fat reserves for survival, yet few studies have quantified fasting endurance using empirical data. We used drone-derived morphometric measurements together with respiration rate data to determine the fasting endurance of southern right whales Eubalaena australis (SRWs) as a function of body length, condition and activity level. We first identified the lower body condition threshold for survival (starvation threshold), using both theoretical (modelled relative blubber mass) and empirical data (morphometric measurements of a malnourished calf). We then determined the effect of body size and condition on energy expenditure and fasting capacity (blubber energy reserves). We identified the starvation threshold of SRWs at -0.40 body condition (40% below average), corresponding to ~3% relative blubber mass. Mass-specific energy expenditure decreased with body length, while fasting capacity increased with both size and condition, resulting in a significant increase in fasting endurance. However, higher activity levels reduced their fasting endurance. We used our model to predict the fasting endurance of the Critically Endangered North Atlantic right whale E. glacialis (NARW), showing that a severely entangled 5 yr old NARW can survive fasting for only ~1-2 mo compared to 6-14 mo for a healthy 5 yr old SRW. Our study highlights the importance of large body size in the fasting ability of baleen whales, a key factor in the evolution of extreme capital breeding.