Behaviorally Solitary and Spatially Grouped
Previously described as strictly solitary, in my earlier work in the Laikipia District of Kenya, we discovered that striped hyenas form small, stable spatial groups of up to three adult males and a single reproductively mature female. Individual home-ranges (HRs) for hyenas in the three main spatial groups on the primary Loisaba study site from 1999 thru 2003 are illustrated below. Note that the two females in the eastern group were successive, not contemporary, residents and the male with the small (purple) HR in the south was a short-term resident.
Although spatial group members shared ranges and occasionally rested together, striped hyenas always foraged alone. Superficially, the social system of the striped hyena appears similar to other carnivores that form male coalitions. However, it is only in the striped hyena that we find multi-male, single female use of common, exclusive ranges within which several males share a single female. Nevertheless, conventional explanations for group formation and social evolution have probably correctly identified the influential factors at work (e.g. abundance and dispersion of food and mates,as resources). However, the interaction between those factors has resulted in unexpected patterns of association in this species. Solitary female distributions likely reflect a lack of permissive and promoting conditions for group formation (e.g. uniformly distributed and small prey items comprising a substantial proportion of diet). However, by the same logic, we would expect males to be solitary in response to overdispersion of females (and food). Instead, the monopolization of single females by multiple males may reflect an additional trade-off between the cost(s) of sharing a female and the benefits of cooperative and effective defense of that female against a greater number of neighboring males.
Variation in Response to Resources
At the more recent field site outside of Lake Magadi in southern Kenya, food resources appear far more abundant. Comparisons of the patterns of striped hyena space use at Lake Magadi to those observed for Laikipia illustrate significant plasticity in spatial and social organizations across striped hyena populations. In Laikpia, striped hyenas lived in asocial spatial groups which contained up to four adult males but never more than a single adult female. In contrast, at Lake Magadi, female ranges are far smaller and multiple adult females will share ranges.
Point locations for one male (blue) and three female striped hyenas sharing a common range at the Lake Magadi study site.
In addition, some females sharing common ranges cooperate (to some extent) in raising young at Lake Magadi. A key project objective was to identify the major explanatory variables underlying these contrasting space use strategies. If we can determine more precisely how and why group structures and compositions differ between these populations, we will be able to predict what effects landscape (or other) variations might have. That insight will then greatly add to our understanding of the evolution of group formation behaviors in mammals, which is a necessary precursor to the evolution of sociality and social behaviors. These insights will also provide a valuable conservation benefit in that they will facilitate the tailoring of striped hyena conservation planning to local conditions.