Interspecific Competition and Anti-predator Behavior
Interspecific competition plays an important role in the regulation of natural populations and in the structuring of ecological communities (Connell 1983; Schoener 1983). Interspecific competition among mammalian carnivores often has particularly strong effects because it may involve direct aggressive interaction resulting in injury or death to competitors due to their morphological and behavioral adaptations for killing (Creel et al 2001). Interspecific competition among carnivores is usually asymmetric in that it does not have an equal impact on each competitor: the larger competitor usually wins (Palomares & Caro 1999; Creel et al 2001). Lions are much larger than hyenas (lions 130-260 kg; hyenas 45-86 kg), so in this case hyenas are subordinate. Interspecific competition between mammalian carnivores depends importantly, not only on relative densities of the two competitors, but also on whether or not each species can scavenge from the other. Thus for example, the dominant predator may compete with and kill the subordinate, yet also offer the subordinate an important source of scavenging opportunities, and thereby partially or totally offset the costs of interspecific competition (Creel et al 2001). Here the value of potential scavenging opportunities should vary among hyenas at consecutive stages of development as their energy needs, body size and abilities change. Furthermore, local prey density can affect the frequency and intensity of interspecific competition among carnivores. When scavenging is unimportant, high prey abundance tends to reduce the intensity of interspecific competition. However, if competition is for carcasses rather than live prey, as is often the case with hyenas and lions, then greater prey abundance can actually increase the intensity of competition by supporting a higher density of competitors (Creel 2001). Thus interactions among mammalian carnivores may be very complex, and their outcomes not necessarily predictable based on standard competition theory alone (Creel et al 2001). Adult behavior patterns known to be affected by interspecific competition in subordinate carnivores include circadian activity rhythms, habitat preferences, and prey selection (Gittleman et al 2001). Otherwise, we know little about strategies for coping with interspecific competition in adult carnivores, or about the relationship between competition and adult stress physiology, and we know nothing at all about these phenomena in juveniles.
Our broad goal here is to assess trade-offs between resource acquisition and risk of injury or death as these vary in spotted hyenas with life history stage and local ecological conditions. Specifically, we are testing an hypothesis suggesting that age (or developmental constraints associated with age) influences risk-sensitive behavior and adaptive decision-making in hyenas. We are also testing hypotheses suggesting that risk-sensitive behavior and adaptive decision-making are affected by circulating horones and by local ecological conditions, particularly the current regime of interspecific competition. Naturally-occurring risk-sensitive behaviors being monitored here include vigilance, use of space and time, subgroup size, communication and mobbing behavior exhibited during direct interactions with lions over food. Finally, we are determining whether stress physiology in hyenas is affected by age and/or by local competitive conditions.