Changes in the Body Composition of Fasting Polar Bears (Ursus maritimus): The Effect of Relative Fatness on Protein Conservation

103

SUMMARYMany species experience large fluctuations in food availability and depend on energy from fat and protein stores for survival, reproduction and growth. Body condition and, more specifically, energy stores thus constitute key variables in the life history of many species. Several indices exist to quantify body condition but none can provide the amount of stored energy. To estimate energy stores in mammals, we propose a body composition model that differentiates between structure and storage of an animal. We develop and parameterize the model specifically for polar bears (Ursus maritimus Phipps) but all concepts are general and the model could be easily adapted to other mammals. The model provides predictive equations to estimate structural mass, storage mass and storage energy from an appropriately chosen measure of body length and total body mass. The model also provides a means to estimate basal metabolic rates from body length and consecutive measurements of total body mass. Model estimates of body composition, structural mass, storage mass and energy density of 970 polar bears from Hudson Bay were consistent with the life history and physiology of polar bears. Metabolic rate estimates of fasting adult males derived from the body composition model corresponded closely to theoretically expected and experimentally measured metabolic rates. Our method is simple, noninvasive and provides considerably more information on the energetic status of individuals than currently available methods.

Section: Model Developmentsupporting

confidence: 59%

Section: Introductionmentioning

confidence: 99%

A body composition model to estimate mammalian energy stores and metabolic rates from body mass and body length, with application to polar bears

Molnár

Klanjšček

Derocher

et al. 2009

103

“…While in the den, they do not eat, urinate or defecate, and they preserve protein by recycling urea, while fat is combusted for energy (Nelson et al, 1973;Ramsay et al, 1991). Even though male polar bears do not spend extended periods in a den, they nevertheless accumulate large amounts of fat in preparation for periods of fasting, which, in some areas, may last for 7 months during the open-water season (Atkinson et al, 1996;Robbins et al, 2012).…”

Section: Hibernation and Starvationmentioning

confidence: 99%

Adaptations to polar life in mammals and birds

Blix

2016

138

This Review presents a broad overview of adaptations of truly Arctic and Antarctic mammals and birds to the challenges of polar life. The polar environment may be characterized by grisly cold, scarcity of food and darkness in winter, and lush conditions and continuous light in summer. Resident animals cope with these changes by behavioural, physical and physiological means. These include responses aimed at reducing exposure, such as 'balling up', huddling and shelter building; seasonal changes in insulation by fur, plumage and blubber; and circulatory adjustments aimed at preservation of core temperature, to which end the periphery and extremities are cooled to increase insulation. Newborn altricial animals have profound tolerance to hypothermia, but depend on parental care for warmth, whereas precocial mammals are well insulated and respond to cold with non-shivering thermogenesis in brown adipose tissue, and precocial birds shiver to produce heat. Most polar animals prepare themselves for shortness of food during winter by the deposition of large amounts of fat in times of plenty during autumn. These deposits are governed by a sliding set-point for body fatness throughout winter so that they last until the sun reappears in spring. Polar animals are, like most others, primarily active during the light part of the day, but when the sun never sets in summer and darkness prevails during winter, high-latitude animals become intermittently active around the clock, allowing opportunistic feeding at all times. The importance of understanding the needs of the individuals of a species to understand the responses of populations in times of climate change is emphasized.

“…Fasting humans (Dulloo and Jacquet, 1999), rats (Cherel et al, 1992;Dunn et al, 1982;Goodman et al, 1980), brown bears (Ursus arctos) (Hilderbrand et al, 2000), Gentoo (Pycoscelis papua) and king penguins (Aptenodytes patagonicus) (Cherel et al, 1993) and Svalbard ptarmigan (Lagopus mutus hyperboreaus) (Lindgard et al, 1992) allocate fat and protein to meet energetic costs based on their available reserves; fatter animals show a greater proportional contribution of fat to energy expenditure and spare protein more effectively. Fasting marine mammals, including elephant seal pups, subantarctic fur seal pups (A. tropicalis) and polar bears (U. maritimus), also show an increase in energy expenditure, contribution of fat to energy costs and the loss of fat tissue as a function of the size of initial fat reserves (Atkinson et al, 1996;Beauplet et al, 2003;Biuw, 2003;Carlini et al, 2001;Noren et al, 2003;Noren and Mangel, 2004). The proportional contribution of fat to energetic costs in grey seal pups is not a simple function of body fatness, and may depend on other factors such as activity levels and the requirements for fat and protein in developmental processes.…”

Section: Impact Of Body Composition On Postweaning Fuel Usementioning

confidence: 99%

“…While some protein use is unavoidable, protein sparing delays depletion of protein to critical levels, when tissue structure and function are irreversibly compromised and terminal starvation begins (Caloin, 2004;Garrow, 1959;Garrow et al, 1965). In other mammals, differences in size and body composition lead to substantial variability in the proportional contribution of protein and lipid to energy demands while fasting, and an inverse relationship exists between adiposity and the contribution of protein to energy expenditure (Atkinson et al, 1996;Cherel et al, 1992;Caloin, 2004;Dulloo and Jacquet, 1999;Dunn et al, 1982;Goodman et al, 1980). Inter-individual variability in the proportional contribution of fat and protein to energy expenditure has been demonstrated in some phocids (Biuw, 2003;Crocker et al, 1998;Muelbert et al, 2003;Noren and Mangel, 2004).…”

Section: Introductionmentioning

confidence: 99%

Effects of mass and body composition on fasting fuel utilisation in grey seal pups (Halichoerus grypusFabricius): an experimental study using supplementary feeding

Bennett

Speakman

Moss

et al. 2007

This study used supplementary feeding to test the hypothesis that fuel partitioning during the postweaning fast in grey seal pups is affected by size and composition of energy reserves at weaning, and by extra provisioning. Mass and body composition changes were measured during suckling and fasting to investigate the effect of natural differences in energy reserves at weaning on subsequent allocation of fat and protein to energy use. We fed seven pups for 5 days after weaning, to investigate the effect of increased fuel availability, and particularly protein, on fuel utilisation. After correcting for protein used during the moult, the proportional contribution of fat was 86-99% of total energy use. Pups with greater energy reserves, i.e. those that were heavier and fatter at weaning, had higher rates of fat and energy use. There was no significant relationship between adiposity at weaning and proportional contribution of fat to energy use, perhaps due to a limited sample size or range of body masses and adiposity. Supplemented individuals used energy, specifically fat, much faster and utilised proportionally less of their endogenous protein by departure than non-supplemented individuals. Fat metabolism contributed a similar percentage to daily energy use in both groups. These findings show that pups spare protein, even when energy use is dramatically increased. Pups that receive greater maternal provisioning and lay down more protein may have increased survival chances at sea. This study highlights the importance of protein reserves in first year survival of grey seal pups.