Differential Loss of Fat and Protein by Mule Deer during Winter

Abstract

Body composition of hand-reared mule deer (Odocoileus hemionus) was studied by use of tritiated water (HTO) over a 120-day period. Energy intake was controlled at different levels for three treatment groups. All treatment groups received submaintenance rations to ensure catabolism of energy reserves. Changes in total body water indicated catabolism of both fat and protein reserves. High-intake animals lost less fat (38%) than either medium(46%) or low(67%) intake deer. High and medium groups lost similar amounts of protein (16% and 18%, respectively), whereas low-intake animals lost 31% of their body protein. Significant loss of protein occurred in all groups within the first 40 days. Fat reserves were depleted at similar rates among treatments. Rates of protein loss between high-and medium-intake groups were similar, but the low-intake group differed significantly. Analysis of the amount of energy contributed to the energy pool by catabolism of fat or protein suggested three phases of protein catabolism, similar to those observed in starvation studies with other mammals. Consequences of fat and protein catabolism in wintering mule deer are discussed. J. WILDL. MANAGE. 49(1):80-85 During winter, wild North American ruminants may face a shortage of forage (Moen 1976, Wallmo et al. 1977). Even if an adequate quantity of forage is available, its quality may be so low that it cannot be effectively utilized (Short et al. 1966, Dietz 1967); because low quality forages are not readily digestible, they may not support large rumen microflora populations. As a result, wintering animals may extract energy so slowly that they enter negative energy balance despite presence of adequate quantities of available forage. Additionally, the rate at which ungulates are able to extract protein from such forages may be insufficient to meet their metabolic requirements. Thus, even when the quantity of winter forage is not limiting, forage quality may still impede the rate of energy acquisition by wild ungulates. During periods of prolonged negative energy balance, an animal draws upon fat and protein to meet its energy requirements. Differential use of these two reserves may affect the animal's survival probability or production of viable offspring. Catabolism of fat for maintenance appears to be a normal, nondetrimental part of the animal's annual cycle. Both synthesis and catabolism of fat proceed at high levels of energetic efficiency (van Es 1977). Protein synthesis and catabolism for energy use is less efficient than that of adipose tissue (Thorbek 1977); therefore, it is not energetically advantageous to rely heavily upon protein reserves to meet an energy deficit. Cahill (1970) suggested that mammals could not withstand a loss of onethird of total protein stores. Little is known about differential use of these two energy reserves in wild ruminants. de Calesta et al. (1975) suggested that protein and fat catabolism occurred simultaneously in starving mule deer. Panaretto (1964) showed both fat and protein catabolism had occurred in two groups of undernourished domestic sheep. Conditions that trigger substantial protein catabolism are not known, nor is it understood how the magnitude or duration of nutritional deprivation affects priorities associated with use of adipose and lean body tissue. Accordingly, we investigated in vivo catabolism of protein and fat reserves of mule deer during winter. Financial support was provided by Fed. Aid Proj. W-126-R, Colo., and Natl. Sci. Found. grant #DEB 7917293. We wish to thank P. H. Neil, D. L. Baker, E. S. Williams, P. A. Smith, and K. N. Callahan for their assistance in all phases of this investigation. MATERIALS AND METHODS