Tuesday, 26 January 2016
African Wild Dogs: an energetics spat
As we were watching a Tasmanian Devil working its way through piles of chicken meat placed along the length of the verandah of our cabin at Loongana Mountain Retreat, I could not help thinking of a spat I had read about on the nutritional requirements of African Wild Dogs, Lycaon pictus (or Cape Hunting Dogs, as they were commonly known) and the effects of kleptoparasitism, i.e. big boys like hyenas stealing their food.
The reason why I thought of that was that this particular Devil was so full of food that it looked as if it might burst. The skin was stretched tight but it kept eating and even carried off a chicken breast and sternum as it waddled down the steps and disappeared into the night. But more posts on Tasmanian Devils later.
The spat on African Wild Dogs began with a paper in Nature in 1998 by Gorbman, Mills, Raab and Speakman2. They used isotope turnover methodology to determine energy demands in the wild from energy expenditure. Using those data, they then showed that an increase in kleptoparasitism to 25% of kills would require more than 12 hours of hunting per day—an unsustainable burden that could make the dogs susceptible to local extinction in areas where kleptoparasites are abundant. That calculation has been quoted widely (but not in the IUCN account of this species) in considering the survival of this, classified as Endangered, species. There is known to be an inverse relation between wild dog and hyena numbers.
In 2014, that view was challenged by Jongeling and Koetsier, writing in the African Journal of Ecology3.One of their arguments to support their view that kleptoparasitism is of no importance to survival was that packs of wild dogs often have attendant pups and, therefore, the amount of food that needs to be captured is greater than that needed to match the energy requirements of the adults. In response5, the authors of the 1998 paper concede this point but then calculate that their curve relating hours per day hunting required to percentage of food lost by kleptoparasitism would be shifted to the right such that for the unsustainable 12 hours of hunting, the level of kleptoparasitism would be 32.5 rather than 25%.
Jongeling and Koetsier’s second argument was that the dogs consume much more energy than the measured energy demand, and modelled mathematically a scenario in which the dogs eat one and a half to three times more than they need. If such a situation obtained the required hunting time at 25% kleptoparasitism drops to 6 hours per day. However, these authors in making such a claim, scored what can only be described as an own goal. In their rejoinder John Speakman and his colleagues soon pointed out that the first law of thermodynamics demands that the excess energy would have to go somewhere and that in two months a dog would put on 50 kg of weight!
They also made the case that it is very difficult to estimate intake in the field in observational studies.
Jongeling and Koetsier also used different values for the assumed energy content of prey. They quote the figures used by the authors of a book, The African Wild Dog1, to estimate the energy content of meat from Impala and Wildebeest which were those reported by the U.S. Department of Agriculture for low-grade beef carcasses (11.6 MJ per kg of flesh; 6.19 MJ per kg for viscera and skin); the value used by the authors of the 1998 paper was 5.2 MJ (which Jongeling and Koetsier’s claim was introduced ‘out of the blue’) but In the rejoinder there is no comment on this difference. I have to admit that I allowed myself a wry smile when I saw the figures that Creel and Creel are said to have used. The USDA figures matches that for dressed beef carcasses given in the food tables of McCance and Widdowson4. However, dressed beef contains much more fat than wild animal carcasses and the figure for lean beef is the same as that used by Gorbman et al, 5.2 MJ per kg.
So, apart from the point about hunting to feed attendant pups, Jongeling and Koetsier do not appear to have dented the case made for the putative effects of kleptoparasitism. There appears to me to be a lack of distinction between the weight of prey killed, the amount consumed by the dogs, the average daily energy expenditure and the amount of food eaten in a particular day. It is entirely possible for an animal to eat three times its daily energy requirement in one sitting without getting fat in the long term—if it does not then eat for three days. So observational intakes after a kill could be a fair estimate but unless the frequency of feeding is also known, the information is worthless in considering the energetics.
Having seen lions in Africa and India—and Tasmanian Devils—with stomachs so full that they could hardly move, it seems to me that the stomach in carnivores is used as a portable cache. The strategy seems to be: eat as much as you can as fast as you can, digest slowly, replenish reserves and then hunt again when hungry, i.e. when digestion is complete and the reserves are being drawn on. And, I ask, is that the pattern in the African Wild Dog?
Finally, I cannot help but point out that what happens to the lactating alpha female must be crucial for the survival of a pack and its individual members. With her increased energy requirement during lactation, a shortfall in food intake could be catastrophic for a local population.
Gorbman et al laid the foundations in their 1998 study on energetics in the wild. The time could be ripe for some simple observations and measurements in the now successful captive populations. That approach might now be more successful at shedding further light on energetics, food intake and the importance of kleptoparasitism in the survival of African Wild Dogs than any number of indirect studies and mathematical models of what might happen in the wild. Ecological physiology needs more physiology.
1 Creel S, Creel NM, 2002. The African Wild Dog: Behavior, Ecology, and Conservation. Princeton University Press.
2 Gorman ML, Mills MGL, Raath JP, Speakman JR, 1998. High hunting costs make African wild dogs vulnerable to kleptoparasitism by hyena. Nature 391, 479-481.
3 Jongeling TB, Koetsier T. 2014. The predicament of the African wild dog, Lycaon pictus, is less precarious than claimed. African Journal of Ecology 52 466-470.
4 Paul AA, Southgate DAT. 1978. McCance and Widdowson’s The Composition of Foods. Fourth revised and extended edition. London: HMSO.
5 Speakman JR, Gorman ML, Mills MGL, Raath JP. 2015. Wild dogs and kleptoparasitism: some misunderstandings. African Journal of Ecology. doi: 10.1111.aje.12258