by Dr. David Croft, School of Biological Science,
UNSW, Sydney NSW 2052.
Bias in Kangaroo Culling
Large males - prime targets Kirkpatrick and Nance (1985) clearly showed a strong bias towards males in the commercial kangaroo harvest in Queensland (Fig. 1). The result was strongest in the smallest of the three species, the common wallaroo, but equally present in red and grey kangaroo culls. Pople (1996) showed that this trend has continued into the 90's with a clear bias towards older males. If we compare Kirkpatrick and Nances' average values for red kangaroos against the proportional representation of male age classes in an unharvested population at Fowlers Gap station (Edwards et al. 1994) then clearly 4-9 year olds are strongly selected for in the cull (Fig. 2). The consequence may be the virtual elimination of males in this and older age classes as Pople (1996) found at Amaroo station in 1992. Figure 1. The proportion of males in the commercial harvest of three species of macropodids in Queensland: 1978 to 1983. (From data presented in Kirkpatrick & Nance 1985).
Click on graph for larger image Figure 2. (1) The average proportion of red kangaroos in three age classes harvested in Queensland from 1978 to 1983 (data from Kirkpatrick & Nance 1985) relative to (2) the proportion of males in the same three classes from an unharvested population at Fowlers Gap in north-west NSW (data from Edwards et al. 1994).
Click on graph for larger image The current objectives of Commonwealth approved kangaroo management programs are:
These objectives do not explicitly address the aims of long-term wildlife conservation (eg. MacNab 1991) which are:
Thus a clamour of concerned voices has been raised as to whether the imposition of a strong artificial selective force (the practices of the commercial kangaroo industry) is compatible with the long-term conservation of kangaroos across their range. Should we be concerned? The short answer is that we know insufficient about the genetics of the species in the commercial harvest to determine whether appropriate genetic diversity is being conserved. If a 'good gene' is being eliminated in the cull then we would not know until those individuals missing it are tested by some vagary in their environment and found wanting. Even so our knowledge of kangaroo mating systems and evolutionary theory (especially mate choice and sexual selection) prompt some alarm. Reproductive success is a complex interaction between size, age and condition of males and females; a very few individuals enjoy high success, most miss out. Amongst males, the largest and fittest fight their way to dominion over mating rights to most of the females in their local population. To achieve this status they have grown for around ten years and on average survived at least one drought. Where the opportunity arises, females will selectively seek out and mate with these alpha males, and attempt to thwart, often successfully, the mating attempts of lesser males. Females advertise their oestrus widely, often moving to the margins of their home ranges with the consequence of attracting a bevy of competing male suitors, amongst which a 'superior' mate will emerge. As figure 2 shows, relatively few males in the unharvested Fowlers Gap population live long enough to challenge for alpha status. If we then impose an artificial selective pressure through the current regime of male-biased 'top-down' culling fewer still will survive and the result is severe disruption to the natural social organisation, changes to population dynamics and the introduction of unnatural selective forces. Why? Breeding males will become smaller and younger, more common than would other wise be the case, and those that would otherwise not breed. Likewise, the taking of large females leads to breeding females that are smaller, younger and inexperienced. Culled populations obviously become more female biased, but also sinks for dispersing, under-sized individuals with high relative energy demands. There is strong selection for small body size. We lose genotypes that have been strongly tested by the environment because survival to an old age is a matter of good foraging skills, disease resistance, competitive abilities and, of course, chance. In choosing a mate, an male or female cannot to do much about chance but can assess the qualities of a potential partner's phenotype to look for correlates of 'good genes' that his/her offspring might inherit. We have sound behavioural evidence that female kangaroos do just this, although it is more difficult to prove genetic benefits in long-lived species because we cannot study sufficient generations. We also have some evidence that males may neglect poor breeders amongst the female population. Does it really matter if we cull out these older, larger more 'desirable' individuals? Any male reaching sexual maturity at around 3 years will produce more than enough sperm to fertilise all the females in his home range. True, but pastoralists do not maintain the wool quality of a merino flock or the meat quality of a cattle herd by letting any male breed. No, they choose stud rams and bulls with desirable characteristics that sustain a wool and beef industry. Natural selection operates, perhaps more slowly than selective breeding, in the same way. Could the commercial harvest of kangaroos eliminate 'good genes' in the kangaroo populations? Some would argue that a cull of 15-20% should leave plenty of copies of 'good genes' in the population. True, if the cull selected targets at random, but clearly this is not the case. The cull may effectively take close to 100% of large males. We do not know what is happening because this is not a priority of the kangaroo management programs but we should be nervous! Tuskless and skewed The strong selection pressure of commercial exploitation of Asian and African elephants, either for work or ivory, has increased the proportions of tuskless individuals in the wild populations. For example, Kurt et al. (1995) report a very high proportion (93%) of tuskless subadult and adult bull elephants in Sri Lanka yet this trait is generally rare in Asian elephants. They conclude that the loss of tuskers in the wild population is an anthropogenic (man-made) effect resulting from selective hunting and capturing of tuskers. Likewise, the proportion of tuskless African elephants in South Luangwa National Park in Zambia increased from 10.5% to 38.3% between 1969-1989 due to ivory poaching and, as better management has eased this pressure, the proportion declined again to 28.7% in 1993 (Jachmann et. al. 1995). Does this really matter? Perhaps it is better for elephant conservation if they are tuskless and do not fall victim to ivory poaching? Well not from the elephants point of view! Tusked elephants are preferred as mates over tuskless ones, tusked elephants have better foraging skills, and tusked elephants can better defend themselves and their offspring against predators . Furthermore, tourists prefer neighbouring Botswana's tusked elephants in the Chobe National Park rather than the perceived degraded populations of Zambia. Thus Zambia loses significant economic benefits from wildlife tourism. Evolutionary biologists have become very excited about the analysis 'fluctuating asymmetry' in recent years (eg. Swaddle et al. 1994). Fluctuating asymmetries result when individuals do not undergo identical development of a bilateral trait on both sides of the body; for example, one ear is bigger than the other. A negative correlation exists between fitness and asymmetry. Females of horned ungulates, such as deer and antelope, prefer males with symmetrical antlers. Damage to an antler can rapidly see a male fall from grace during the rut. Females presumably exercise this choice because males with skewed (asymmetric) antlers suffer developmental deficits, which could lower the female and her offspring's fitness (reproductive success). Unfortunately for deer and antelope, trophy hunters likewise prefer a symmetrical 'rack'. Thus in heavily hunted populations the 'developmentally challenged' males remain and the fitness of the population is threatened. What can learn from these two examples of wildlife under strong artificial selection from commercial exploitation? 'Good genes' may remain at low frequencies in these populations (eg. having tusks is the dominant trait in elephants) but the vigour of the population to resist the occasional 'catastrophe' is no doubt much diminished. Furthermore, these processes threaten other economic uses of wildlife, especially tourism. Does size matter? The kangaroo management programs seem to suffer from schizophrenia. One personality aims to mitigate damage to agricultural enterprises through a reduction in the biomass of kangaroos, and hence their grazing pressure. Thus if the artificial selection imposed by culling leads to a population of smaller kangaroos across the management zones then maybe this is good? However, the other personality wants utilisation of a valuable natural resource for its meat and skins to sustain a viable commercial kangaroo industry. Surely here big kangaroos with more muscle and hide are better than small ones? Selection against large body size is counterproductive! Pastoralists should be quite well informed about the allometry of metabolic rate and digestive efficiency (or gut size) in herbivores (Fig. 3) . They do not cast their weaners and yearlings of relatively small biomass into the worst pastures but rather the better ones. Small individuals have a larger surface area to body mass than larger ones. A small kangaroo demands much more energy in winter to keep itself warm and more water in summer to keep itself cool per kilogram than a large one. However, a small kangaroo has a proportionally smaller gut than a large one. As a consequence it has too small a fermentation chamber to efficiently digest fibrous plant material and no vat to draw water from. It thus eats 'clover' while a large kangaroo eats 'straw'. It takes moist green feed and grazes close to water while a large kangaroo gets by on dry feed and ranges widely. Figure 3. Relative scaling of metabolic rate (mass0.75) and gut size (mass1.0) against body mass.
Does it make any sense to create a population of small, numerous, thirsty 'clover' eaters? Yes, if you simply want a reduced biomass of kangaroos, but, no, if you want maximal production on 'green blade' and a balanced grazing pressure across the ecosystem. |
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