Costs of allogrooming in impala : distraction from vigilance

Reciprocal allogrooming is a unique but frequently performed behaviour of adult female and bachelor male impala, Aepyceros melampus (Hart & Hart 1992) and is a distinctive feature of newborn impala fawns (Mooring & Hart 1992). We have previously argued that the evolution of this behaviour appears to be related to its effectiveness in controlling ticks on the head and neck which are areas that an impala cannot reach with its mouth by self oral grooming. There appears to be no influence of kinship or dominance in choice of allogrooming partners or in the distribution of allogrooming between partners within an encounter (Hart & Hart 1992; Mooring & Hart 1993), and Hart & Hart (1992) have proposed that in impala, reciprocal allogrooming may represent one of the few bona fide examples of evolved cooperation in a non-primate species. The proposal of a behavioural system as a type of evolved cooperation necessitates the designation of benefits to the recipient and costs to the donor. The benefit of allogrooming to the recipient may be extrapolated from the value of self oral grooming in the removal of ectoparasites (reviewed in Hart 1990). The possible costs involved in delivering allogrooming include energy used, time taken from foraging, loss of water in saliva used in grooming, wear on the lateral incisors and distraction from vigilance for predators. The first three costs are likely to be small. Vertical attrition of the lateral incisors used in grooming has been shown, and these worn teeth subsequently become less efficient in removing ectoparasites (McKenzie 1990). The purpose of this study was to explore whether allogrooming appears to have a measurable effect on vigilance for predators. Within a group, a loss of vigilance would increase an animal’s susceptibility to predators. FitzGibbon (1989) found that cheetah were more likely to prey upon the less vigilant of a pair of Thomson’s gazelle. If stalked by predators, such as lions, wild dogs, hyenas or leopards, impala that take longer to alert to the predator are more likely to be killed. In the herd life characteristic of impala, overall individual vigilance might be expected to decrease as a function of increased group size (Underwood 1982). However, to take advantage of increased efficiency of vigilance in larger groups, the animals must be able to watch group mates for signs of reacting to a predator. Allogrooming activity would be expected to obstruct visual tracking of group mates, as does preening by dunlin, Calidris alpina (Redpath 1988). In our study of free-ranging impala, the plan was to examine the latency to alert to a predator for both the ‘allogroomer’ (delivering grooming) and the ‘allogroomee’ (receiving grooming), as well as the alert latency of non-allogrooming impala in the same group. To obtain this degree of control of the predator stimulus, and accurately record the alert latency, it was necessary to conduct the experiment with a person approaching a group of impala as a simulated stalking predator. We carried out observations on Fothergill Island, Matusadona National Park, Zimbabwe, during the tick-dense time of the year (March 1994) when allogrooming is common. Impala at Fothergill are preyed upon by lion, Panthera leo, cheetah, Acinonyx jubatus, leopard, Panthera pardus, and occasionally hyena, Crocuta crocuta, but are habituated to safari vehicles. We conducted observations from 0600 to 1100 hours and from 1630 to 1900 hours. The procedure involved driving a vehicle up to a herd of female impala to within 50–100 m of the nearest animals, waiting 1 min for the animals to adapt after the engine was turned off and focusing a video-camera on the group. As soon as a pair of impalas started to allogroom, we turned on the camera, and 3 s later passed a signal to the camera as a second investigator silently left the vehicle and began walking slowly towards the herd, ‘stalking’ until both *To whom all correspondence should be addressed.