Presumptive Photoperiodic Regulation of Food Intake and Body Mass in Killer Whales (Orcinus orca)

Estimates of energy intake are important for modelling population dynamics and assessing habitat quality of killer whales (Orcinus orca) and for husbandry under managed care. We estimated the energy requirements of four individual killer whales at Marineland Côte d’Azur in Antibes, France, using records of daily food intake, fish nutrient composition, and animal body mass over eight years for each animal. Relative metabolizable energy intake for maintenance (zero body mass change) and for average winter growth (0.07 to 0.09% body mass/d) were estimated at 667 to 892 and 1,054 to 1,237 MJ ME/kg0.75/d, respectively, representing two to four times the average mammalian basal metabolic rate. Seasonal fluctuations were observed in body mass as well as in food (kg) and energy (J) intake. Body mass, age, water temperature, season, and day length were all significantly correlated to energy intake, but models that accounted for day length fit the data distinctly better than models that accounted for water temperature, suggesting a photoperiodic control of intake. Intake generally increased from June to December and decreased from December to June. The turning points corresponded to winter and summer solstices, some two to three months prior to distinct seasonal changes in water temperature. We propose that changes in ambient temperature are not a strong stimulus for seasonal adjustment of food intake in killer whales. Given that turning points in body mass—presumably due to changes in blubber reserves—occur at a delay of two to three months after turning points in food intake, instantaneous adjustment of intake based on water temperature would often happen “too late” to adequately adjust the blubber layer for the thermoregulatory challenge. By using photoperiod to anticipate imminent temperature developments, killer whales and possibly other delphinid or even other marine mammal species adjust intake so that resulting extremes in body mass and blubber thickness parallel extremes in ambient temperature.