Short-term episodes of imposed fasting have a greater effect on young northern fur seals (Callorhinus ursinus) in summer than in winter

David A. S. Rosen 1 Beth L. Volpov 0 Andrew W. Trites 1 Steven Cooke 0 School of Life and Environmental Sciences, Deakin University , Burwood, Victoria 3125, Australia 1 Marine Mammal Research Unit, Fisheries Centre, University of British Columbia , AERL 247, 2202 Main Mall, Vancouver, BC , Canada V6T 1Z4 An unexpected shortage of food may affect wildlife in a different way depending on the time of year when it occurs. We imposed 48 h fasts on six female northern fur seals (Callorhinus ursinus; ages 6-24 months) to identify times of year when they might be particularly sensitive to interruptions in food supply. We monitored changes in their resting metabolic rates and their metabolic response to thermal challenges, and also examined potential bioenergetic causes for seasonal differences in body mass loss. The pre-fast metabolism of the fur seals while in ambient air or submerged in water at 4C was higher during summer (June to Sepember) than winter (November to March), and submergence did not significantly increase metabolism, indicating a lack of additional thermoregulatory costs. There was no evidence of metabolic depression following the fasting periods, nor did metabolism increase during the post-fast thermal challenge, suggesting that mass loss did not negatively impact thermoregulatory capacity. However, the fur seals lost mass at greater rates while fasting during the summer months, when metabolism is normally high to facilitate faster growth rates (which would ordinarily have been supported by higher food intake levels). Our findings suggest that summer is a more critical time of year than winter for young northern fur seals to obtain adequate nutrition. Introduction Wild animals rarely have the benefit of a steady supply of dependable food resources. Natural variation in prey availability and foraging success, digestive constraints and lifehistory considerations (such as periods required for nursing or mating) that may preclude timely foraging will result in intermittent nutritional intake. Animals have a number of adaptations, including down-regulation of resting metabolism, decreased activity, alteration of digestive processes and increased levels of subsequent food intake, that can be employed to compensate for inconsistent prey intake encountered on a normal basis to maintain an optimal nutritional plane that maximizes growth and survival. However, these adjustments are intrinsically limited in scope (Rosen et al., 2007). Furthermore, they may be over-taxed or not fully implemented during periods of unexpected food restriction experienced in less typical conditions (Rosen and Trites, 2002). The impact of unexpected periods of food restriction or fasting is likely to be greater for younger animals, given The Author 2014. Published by Oxford University Press and the Society for Experimental Biology. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0/), whichpermits unrestricted distribution and reproduction in any medium, provided the original work is properly cited. their relatively higher energetic requirements and their physical under-development and behavioural navet. Periods of unexpected short-term fasts may impact immediate survival due to negative energy balance or impact future reproductive success through decreases in body size (Wilson and Osbourn, 1960). Northern fur seals (Callorhinus ursinus) are an example of a species where young animals may be challenged to acquire sufficient prey even in normal circumstances. These fur seals become nutritionally independent in November at ~4 months of age, when they become largely pelagic entities (Ragen et al., 1995). During this initial period of independence, they have little experience with deep-water foraging and their physical diving capacity is under-developed (Baker and Donohue, 2000; Shero et al., 2012). It is likely that some northern fur seals have difficulty finding sufficient prey every day and, therefore, endure occasional fasts during their first months at sea. Mortality rates of northern fur seals are greatest during the at-sea phase of their first 2 years of their life (Lander, 1982; Trites, 1989). While predation rates may be substantial, the greatest source of natural mortality is likely to be an inability to catch sufficient prey to meet their energetic requirements. These normal patterns of periodic under-nutrition may be exaggerated due to changes in the fur seals biotic or abiotic environment; factors that have been implicated in the observed decline in fur seal populations (Trites, 1992; National Marine Fisheries Service, 2007). This population decrease is most notable on the Pribilof Islands in the Bering Sea, with a 6% yearly decline in the main breeding site at St Paul Island, Alaska (National Marine Fisheries Service, 2007). due to insufficient energy intake (Guppy and Withers, 1999). Changes in the resting metabolic rate of young pinnipeds have been examined over periods of natural fasting of otariid pups during the nursing period (Rea et al., 2000; Arnould et al., 2001; Beauplet et al., 2003; Verrier et al., 2009) or during the brief post-nursing moult period (Donohue et al., 2000), although the latter has been most intensely studied among phocid seal pups (Worthy and Lavigne, 1987; Nordy et al., 1990, 1993; Reilly, 1991; Rea and Costa, 1992; Lydersen et al., 1997; Houser and Costa, 2003). However, the metabolic response of mammals during life-history stages or seasons when food shortages normally occur may be different from the response during periods when food shortages are unexpected. Only one published study has examined changes in resting metabolism in response to episodes of experimental fasting in a young otariid outside of the normal nursing period (Rosen and Trites, 2002). It must also be noted that the metabolic depression response does not occur in physiological isolation and must be viewed in light of competing bioenergetic priorities. It is unclear, for example, how this strategy would be implemented during a life-history stage that is also characterized by an increased priority for growth (with its related concurrent increase in metabolism) and high environmental thermal challenges. Past studies have confirmed that changes in metabolism in response to under-nutrition in young Steller sea lions (Eumetopias jubatus) are seasonal in nature (Rosen and Trites, 2002), which is likely to reflect different energetic priorities formulated in response to natural changes in food supplies or other bioenergetic variables. Like most subpolar species, the energetic requirements of northern fur seals appear to differ with the time of year (Rosen and Trites, 2010). Therefore, metabolic depression might potentially be more effective or more likely to be invoked as a strategy to deal with periodic food shortages during different seasons. The impact of changes in metabolism or bo (...truncated)