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Soc. 80, 551–554 (1985). 20. Salstein, D. A. Monitoring atmospheric winds and pressures for Earth orientation studies. Adv. Space Res. 13, 11175–11184 (1993). 21. Salstein, D. A. & Rosen, R. D. in Proc. 7th Conf. on Climate Variations 344–348 (Am. Meteorol. Soc., Boston, 1997). 22. Rosen, R. D. The axial momentum balance of Earth and its fluid envelope. Surv. Geophys. 14, 1–29 (1993). Acknowledgements. We thank D. Spiegel for help with the computation, and F. Bryan, B. Chao, R. Rosen and D. Salstein for comments. This work was supported by NASA’s Mission to Planet Earth. Correspondence and requestsfor materials should be addressed to R.M.P. (e-mail: ponte@aer.com). High hunting costs make African wild dogs vulnerable to kleptoparasitism by hyaenas Martyn L. Gorman*, Michael G. Mills†, Jacobus P. Raath† & John R. Speakman* * Department of Zoology, University of Aberdeen, Tillydrone Avenue, Aberdeen AB24 2TZ, UK † National Parks Board, Kruger National Park, Box X402, Skukuza 1350, Republic of South Africa ......................................................................................................................... The African wild dog Lycaon pictus is critically endangered, with only about 5,000 animals remaining in the wild 1 . Across a range of habitats, there is a negative relationship between the densities of wild dogs and of the spotted hyaena Crocuta crocuta 2 . It has been suggested that this is because hyaenas act as ‘kleptoparasites’ and steal food from dogs. We have now measured the daily energy expenditure of free-ranging dogs to model the impact of klepto- parasitism on energy balance. The daily energy expenditures of six dogs, measured by the doubly labelled water technique, averaged 15.3 megajoules per day. We estimated that the instantaneous cost of hunting was twenty-five times basal metabolic rate. As hunting is energetically costly, a small loss of food to kleptoparasites has a large impact on the amount of time that dogs must hunt to achieve energy balance. They normally hunt for around 3.5 hours per day but need to increase this to 12 hours if they lose 25% of their food. This would increase their sustained metabolic scope to a physio- logically unfeasible twelve times the basal metabolic rate. This may explain why there are low populations of wild dogs in regions where the risk of kleptoparasitism is high. African wild dogs are medium-sized carnivores (weighing 25 kg) that live in packs of 4–20 adults and their dependent young. They feed predominantly on ungulates weighing 15–100 kg which they hunt and kill cooperatively. The members of a pack are normally nomadic within a large (500 km 2 ) home range. Wild dogs were formerly widespread south of the Sahara, but fewer than 5,000 individuals now survive 1 . This serious decline in numbers has been blamed on several factors including habitat loss, persecution by humans 3 and the transfer from domestic dogs of diseases such as rabies 4 . African wild dogs, however, live at low densities even in large areas of relatively undisturbed habitat, and their biomass is gen- erally one to two orders of magnitude lower than that of competing spotted hyaenas 2 . Across a range of habitats, there is a negative relationship between the density of dogs and hyaenas. Where the population density of hyaenas is high, and where visibility is good, for example on the Serengeti plains, these animals accumulate at the kills of dogs and reduce the dogs’ rate of food intake 5,6 . Hyaenas rarely take food from dogs in heavily wooded areas such as the Selous or Kruger Park, however 7,8 . It is claimed that kleptoparasitism by hyaenas has been part responsible for the decline, or even demise, of wild-dog populations in open habitats 9 . We used the doubly labelled water (DLW) technique 10 to measure the daily energy expenditures (DEEs) of six fully grown dogs (three males and three females) from a pack living in the southwest of the Kruger National Park, Republic of South Africa. The pack consisted of 5 adults, 16 yearlings aged 16 months, and 27 pups aged 3–4 months. In the Kruger National Park, dogs generally hunt early in the morning and again towards dusk. At the time of the study, the dogs were away from the den and hunting for a total of 207  15:1 min per day (mean  95% confidence interval (CI); n ¼ 59 days). They spent the remainder of the day at rest. Daily energy expenditure, measured by DLW, averaged 15.3 MJ (Table 1). This is equivalent to a food intake of 3.5 kg of ungulate meat per day, assuming an energy content of 5.2 MJ per kg wet weight, a digestive efficiency of 93% and a 10% loss of energy in the urine. This figure agrees well with field determinations of rates of food intake, which range from 2.5 to 3.5 kg per dog 7,8 . The high variability in DEEs among the six dogs probably reflects day-to-day variation in the involvement of individuals in the group hunt. As the study pack consisted of only 5 adults, together with 27 dependent pups and 16 yearlings who had just reached the age at which wild dogs hunt effectively, the pack might have been hunting rather more intensely than packs with a more favourable ratio of adults to young. For comparison, allometric equations 11 predict a DEE of 6.0 MJ per day for a moderately active 25-kg domestic dog and 6.7 MJ per day for a highly active individual. The field metabolic rate of a 25-kg eutherian mammal has been predicted to be 12.6 MJ per day 12 . Measurements of food intake by working border collies 13 give a value of 8.2 MJ per day for a 25-kg dog active for 6 hours per day. Alaskan sledge dogs weighing 25 kg used 47 MJ per day, as measured by both the DLW technique and the rate of food intake, on a 70- hour, 490-km sledge race across Arctic Canada 14 . By these com- parisons, African wild dogs appear to be working extremely hard, despite the fact that they are active for only 3.5 hours per day. Moreover, the predicted basal metabolic rate (BMR) for a 25-kg dog Table 1 Daily energy expenditures of six African wild dogs measured using the doubly labelled water technique Dog Sex Mass (kg) Plateau estimate (kJ per day) Intercept estimate (kJ per day) ............................................................................................................................................................................. 1 F 24 8,043 7,223 ............................................................................................................................................................................. 2 F 25 8,729 7,732 ............................................................................................................................................................................. 3 F 23 16,686 15,716 ............................................................................................................................................................................. 4 M 27 18,252 17,910 ............................................................................................................................................................................. 5 M 27 20,281 19,362 ............................................................................................................................................................................. 6 M 25 19,806 18,729 .............................................................................................................................................................................