The influence of seasonality and quality of diet on the metabolism of the aardwolf, Proteles cristatus (Sparrman 1783)

Abstract

The aardwolf Proteles cristatus was studied for two years from December 1988 to January 1991 on Benfontein Game Farm, in the northern Cape Province, South Africa. The aardwolf s principle food source in this area, snouted-harvester termites Trinervitermes trinervoides, are largely inactive and hence unavailable during winter. Consequently, this seasonal food restriction imposes severe nutritional stresses on P. cristatus. Unlike the other typical "anteaters", aardwolves do not dig for their prey but lick foraging termites off the soil surface. Consequently, aardwolves do not possess the physical features which are characteristic of typical ant- and termite-eating mammals. Nevertheless, aardwolves do have a number of anatomical features which are clearly adaptations to their diet and feeding style. The most obvious of these are a broad spatulate tongue covered with large, hardened papillae, a broad palate, large mandibular salivary glands, and a muscular pyloric region in the stomach. The aardwolf is committed to a diet of surface foraging termites and due to reduced dentition is unable to feed on alternative, larger prey items. The nutritional quality of T. trinervoides, like other invertebrates, is poor. Aardwolves compensate for this by consuming vast quantities of termites. As many as 300 000 may be consumed per night during the warm summer months, thus enabling subcutaneous fat to be deposited. The body masses of aardwolves decreased by 25% from approximately 10 kg to 7,5 kg during the winter months (April - August). Subcutaneous fat reserves (measured at six body localities) showed a significant decline during winter. Blood samples analyzed during summer and winter for 16 blood chemistry and eight blood hematology parameters also showed significant sex, age-class and seasonal differences. Triglyceride and cholesterol values were the most reliable indicators of nutritional status. When inactive, aardwolves sought refuge in underground dens, which were usually modified springhare Pedetes capensis burrows. Daily denning times during winter were approximately 20 h in comparison to a shorter 15 h 20 min during summer. The den environment is thermally stable, representing a favourable micro-habitat to escape the thermal extremes of the northern Cape. In comparison to the extreme variation in ambient temperatures on the soil surface outside the dens, the den temperatures were constant, with temperatures of approximately 12°c and 27°C during winter and summer, respectively. In the den body posture and social huddling are hypothesized to be important energy saving mechanisms. A significant increase in insulation, with a concomitant decrease in thermal conductance, during winter were important in limiting heat loss. Water conservation through a limited use of evaporative cooling is important in maintaining body water homeostasis during winter. Active aardwolves maintained relatively constant body temperatures (Tb) of 37,0- 37,50C. While inactive, however, and particularly during winter, significant decreases in Tb were observed. Tbs as low as 31,2°C were recorded during the middle of the inactive period during winter. The observed drop in Tb results in energy savings of up to 17, 7%, and contributes to the aardwolf s ability to cope with the periodic limitations in energy supplies during winter. The basal metabolic rate (BMR) of captive aardwolves averaged 1869,5 ml 02 h-1 and 2194,3 ml 02 h-1 during winter and summer, respectively. These BMRs are significantly lower (58,9% - winter; 67,1 % - summer) than is predicted by allometric equations. A low BMR, which is further reduced during winter, decreases the overall energy requirements of the aardwolf. The aardwolf feeds on a food source which is largely avoided by other mammals. T. trinervoides are unavailable during winter and due to a number of physical specializations the aardwolf is unable to exploit alternative food sources. Survival during winter, however, is achieved through a combination of behavioural and physiological adaptations. As a consequence of the aardwolfs specialization on this unique diet, and the ability to cope with the absence of termites during winter, it has invaded a trophic niche which is almost entirely devoid of competitors.