The spotted hyaena is one the most highly gregarious of all carnivores; it lives in groups containing up to 90 individuals, and exhibits the most complex social behaviour. These animals live in social groups called clans that defend group territories. The society is characterised by a strict dominance hierarchy. The rank ordering among the adult females in one Kenyan study clan is shown below. Females are ordered on the left as winners in fights, arranged from top to bottom in descending order of the number of other females they can defeat in the group. The same females are listed as losers in fights across the top horizontal axis, and the numbers in the matrix represent the number of fights observed within female dyads in which there was a clear winner and a clear loser. Notice how few fight outcomes are inconsistent with the rank order assigned here.
Females are dominant over males, and even the lowest ranking female is dominant to the highest ranking male. Although males typically disperse from their natal clans when they are between two and six years of age (Smale et al 1997; Boydston et al. 2005), females usually remain in their natal clan, so large clans may contain several different matrilines. Although cubs of both sexes ‘inherit’ their mothers’ social ranks, males voluntarily forsake those to assume much lower ranks in the neighboring cans to which they disperse. (Smale et al 1997; Holekamp & Smale 1998). It appears that female mate choice drives male dispersal (Honer et al. 2007), although it remains controversial whether females merely prefer immigrants over natal males or whether they follow alternative rules of thumb.
Watch a number of spotted hyenas join forces to drive some lions off their kill at the following URL: http://www.youtube.com/watch?v=8zjPWpdyC74
Here you will hear the hyenas giggling (expressing nervousness) and whooping (calling in more recruits to help deal with the lions) and lowing. Lowing appears to be a vocalization that functions to bring all hyenas present to the same state of arousal in order for them to engage in a coordinated attack on the lions. Notice that the vocalizations stop as soon as the hyenas displace the lions from thir kill.
Spotted hyaenas live in a “fission-fusion” society, so clan members do not remain together continuously, but instead frequently forage alone or in small groups. Clan members co-operate in communal defense of the territory, of food resources, and the clan den. As with many monkeys and apes, female spotted hyaena cubs normally acquire dominance immediately below that of their mother. “Social politics” among clan members are very important in hyaena society, with individuals regularly forging alliances and coalitions. Overall, spotted hyaena society is characterized by its flexible nature, as demonstrated by impressive variation in group size, territorial behavior, foraging tactics, and nursing behavior.
Clan Size: Unlike many other social species where all group members are usually seen together, spotted hyaena clan members frequently wander alone or in small groups and only sometimes meet in large numbers. This occurs at kills, at the communal den, or when clan members rally together to defend individual carcasses or group territories (Kruuk 1972a, Tilson and Hamilton 1984, Henschel 1986, Frank 1986a. Cooper 1989, Mills 1990, Hofer and East 1993a).
The average number of adults and subadults in a clan varies from three in desert and semi-desert areas of southern Africa, to nearly 90 in the prey-rich savannah areas of east Africa. Territory size and the density of prey inside a clan’s communal territory usually limit clan size, with a couple of interesting exceptions. First, in the Serengeti, an ecosystem dominated by migratory herbivores, hyaena density and clan size are not limited by resident herbivore density, as Serengeti spotted hyaenas regularly undertake foraging trips to feed on nearby migratory herds (Hofer and East 1993a,b, 1995a). Second, in Etosha national Park, Namibia, Trinkel et al. (2004) have found that spotted hyaenas engage in seasonal expansion or contraction of the size of their territories to accommodate distributional changes in their migratory prey.
Territories: Territory size in the spotted hyaena is highly variable, ranging from less than 40 km2 in the Ngorongoro Crater (Kruuk 1972a) to over 1,000 km2 in the Kalahari (Mills 1990). Clans defend communal territories through vocal displays (East and Hofer 1991b; Theis et al. 2007a), scent marking (Gorman and Mills 1984; Theis et al 2007b) and boundary patrols (Kruuk 1972a; Boydston et al 2001). Clan members also cooperate in defending territories during boundary disputes with neighbouring clans (Boydston et al 2001). Long distance calls, particularly whoops, are used to quickly rally clan members to such sites of conflict (Kruuk 1972a, Henschel and Skinner 1991, East and Hofer 1991 b; Theis et al 2007a). Spotted hyaenas scent mark their territories by pasting a secretion from the anal gland onto grass stalks, and by depositing a secretion from interdigital glands when they scratch the ground (Kruuk 1972a. Mills 1990′; Theis et al 2007b). Spotted hyaenas also scent mark their territories by defecating in communal latrines (Kruuk 1972a. Mills 1990). Pasting sites and communal latrines are normally scattered throughout a clan’s territory and this “hinterland” scent marking strategy (Gorman and Mills 1984) may be a way of optimising the distribution of scent marks over a large area with a limited amount of scent and time (Mills 1990). Some spotted hyaena territories in the eastern portion of the Masai Mara National reserve, Kenya, are shown in the figure below.
In the Serengeti, clans defend territories against neighbouring clans but individual animals may move in transit through other clan territories when they commute to distant migratory herds (Hofer and East 1993b). When migratory herds are present inside a clan territory, many non-residents also enter the territory to feed. Non-residents typically signal submission and retreat when detecting residents, and at kills non-residents usually wait at a distance and feed after residents have departed. Aggression between residents and non-residents is rare when commuters are in transit. Aggression is more common when residents encounter intruders searching for food, and most intense at kills where agonistic encounters may escalate into fights causing serious damage (Hofer and East 1993b). The commuting system of Serengeti hyaenas and the flexible response of territory owners to intruders illustrate the flexible nature of the social behaviour of spotted hyaenas (see also Knight et al. 1992).
Female dominance: Spotted hyaena society is female-dominated (Kruuk 1972a), with a clear, linear dominance hierarchy amongst first the female and then the male clan members (Frank 1986b). Top-ranking females have priority of access to large carcasses and this provides increased reproductive success in comparison with low-ranking females (Frank et al. 1995a; Holeamp et al 1996; Hofer & East 2003). Apart from males dispersing from natal territories, clans may split (fission) if current clan size exceeds a threshold above which the food base of the territory is insufficient (Mills 1990), or if a territory in the neighbourhood has become vacant (Holekamp et al. 1993).
Vocalizations: The highly social nature of the spotted hyaena has led to the evolution of a wide variety of vocalisations (Kruuk 1972a, Henschel 1986, Mills 1990). The best known spotted hyaena vocalisation is the whoop, which can be heard over several kilometres. Spotted hyaenas can recognise each other individually by their whoops, at least within their clan (East and Hofer 1991a). Whoops can function as a rallying call to gather scattered clan members together to defend territory boundaries, food resources, and the communal den. Mothers whoop to locate their wandering cubs and some animals whoop to recruit hunting partners. Whoops are also used as a form of individual display, particularly by animals of high rank (East and Hofer 1991b). Adult males whoop more frequently than females, and top-ranking males put more effort into vocal displays than lower ranking males (East and Hofer 1991b). Another well-known vocalisation is the laugh or giggle, which is a signal of submission. A submissive individual giggles to signal to its partner that it accepts a lower status.
Greeting ceremonies: The spotted hyaena has a ritualised greeting or meeting ceremony during which two individuals stand parallel and face in opposite directions. Both individuals usually lift the hind leg nearest to the other and sniff or lick the anogenital region of the other. The unique aspect of greetings between individuals is the prominent role of the erect “penis” in animals of both sexes. This is used to signal submission. Greetings occur between all ages and both sexes, although greetings between adult females and males are uncommon and are typically restricted to males above median rank, principally the alpha male. Cubs can erect their penis or clitoris and engage in greeting ceremonies as early as four weeks after birth (East et al. 1993).
The social life of a clan is centered around the communal den. Some clans use particular den sites for years whereas others may use several different dens within a year or even several den sites simultaneously. These may be separated by up to 7 km (Hofer and East 1993a). The dens are not excavated by hyaenas. Instead, dens have usually been abandoned by other species, mostly warthog, aardvark and bat-eared fox (Kruuk 1972a). The structure of dens does not normally permit the access of adult animals, so cubs must emerge at the den entrance to have contact with their mother. This structure of small channels underground has been considered an effective anti-predator device which protects cubs during the absence of their mother (Kruuk 1972a). Circumstantial evidence suggests that predation on cubs by other hyaenas (infanticide) or other carnivores may occur but is considered rare (Mills 1990). Infanticide has been observed in the Masai Mara (Holekamp et al unpublished data) and in the Serengeti (Kruuk 1972; Hofer and East 1995a). In Serengeti, only high-ranking females have been observed killing the offspring of low ranking females in the same clan (H. Hofer and M.L. East, unpublished data), but in the Masai Mara, low-ranking females have also been observed killing infants of higher-ranking females (Holekamp et al unpublished data).