Oxygen stores plasticity linked to foraging behaviour and pregnancy in a diving predator, the Galapagos sea lion

Abstract

Summary 1. Survival of a population can be influenced by foraging success that is determined by ecological and physiological factors. Foraging in air‐breathing marine vertebrates is constrained by the duration of their dives, which is in part determined by their oxygen stores. Such organisms are excellent models to study how and when physiology limits behaviour and the associated role of ecological factors such as seasonal change. 2. The Galapagos sea lion (GSL) Zalophus wollebaeki is endangered and its population is declining. Previous work found that GSL exhibited three distinct foraging strategies during a warm season when resources are less abundant, possibly caused by intraspecific competition. 3. We examined adult female GSL diving behaviour during a cold, productive season compared to a warm season to test the hypothesis that they would converge onto one foraging strategy as a consequence of increased productivity. We investigated intra and inter‐seasonal changes in GSL oxygen stores during a warm (March 2005, n = 11) and cold season (August–September 2006, n = 12) at Caamaño Islet, Galapagos. 4. GSL exhibited persistence of three foraging strategies during the cold, productive season similar to those found during the warm season. This suggests that intraspecific competition associated with limited resources in this area of the Galapagos Islands is maintained throughout the year. 5. GSL exhibited contrasting seasonal changes in their oxygen stores. During the warm season haemoglobin (Hb) and blood volume (Bv) were significantly higher while myoglobin (Mb) was significantly lower compared to that in the cold season. Given that diving behaviour was similar between seasons, these changes are attributed to other causes such as pregnancy as most females were pregnant during the cold season. 6. GSL exhibited physiological plasticity within each season that was attributed to dive behaviour. Oxygen stores of the three foraging strategies, increased according to dive durations: haematocrit, Hb and Mb during the warm season and Hb, Mb and Bv during the cold season. 7. Knowledge of an organism’s physiological plasticity is important to assess its vulnerability to climate change. We document for the first time inter and intraseasonal physiological response coupled with diving behaviour in the same sex and age class of an air‐breathing diving vertebrate.