Hyracoidea
Biological synopsis by Hendrik Hoeck, Max-Planck-Institut für Verhaltensphysiologie, 82319 Seewiesen, Germany
Phylogeny
Rock Hyraxes, Procavia capensis
Fossil and morphological evidence shows that hyraxes share many features with elephants and seacows. Recent research using molecular sequence data provides additional support for the association of the paenungulates (elephants, hyraxes and sea cows), which together with sengis (elephant-shrews), the aardvark, tenrecs and golden moles, are called the Afrotheria (Springer et al. 1997). Fossil beds in the Fayum, Egypt, indicates that 36 million years ago hyraxes were the most important medium-sized grazing and browsing ungulates in Africa. During this period there were at least seven genera, ranging in size from that of contemporary hyraxes to that of a hippopotamus. During the Miocene (beginning about 25 million years ago), at the time of the first radiation of the bovids, hyrax diversity was greatly reduced, with species persisting only among rocks and in trees - habitats that were not invaded by bovids (Hahn 1935). Contemporary hyraxes retain several primitive features, notably their feeding mechanism, which involves cropping with the molars instead of the incisors as most modern hoofed mammals, imperfect endothermy, and short legs and feet.
Taxonomy
Hyraxes are members of the order Hyracoidea, family Procaviidae. Three living genera contain 5 species superficially similar in size and appearance. In southern Africa hyraxes are called dassies. The word hyrax derives from the Greek word hyrak, which means 'shrew.' Some authors consider the Rock Hyrax, Procavia capensis, to be monospecific with 17 subspecies (Olds and Shoshani 1982). However, studies on the geographic variation in mitochondrial DNA in South Africa indicate that at least 2 species exist in that region (Prinsloo and Robinson, 1992). The genus Heterohyrax include 1 species, the Bush Hyrax H. brucei, with 25 subspecies (Barry and Shoshani 2000). Tree Hyraxes in the genus Dendrohyrax include 3 living species: D. dorsalis, D. arboreus and D. validus. They have long, woolly gray or brown fur and a dorsally concave cranium (Bothma 1966; Jones 1978).
Distribution
Hyraxes are endemic to Africa with the exception of the Bush Hyrax that is also found in Sinai and the Rock Hyrax that extends into the Arabian Peninsula from Lebanon to Saudi Arabia (Barry and Shoshani 2000; Hahn 1935). Rock and Bush hyraxes are dependent on the presence of suitable refuges in rocky outcrops (kopjes) and mountain cliffs. They have the widest geographical and altitudinal distribution (Hahn 1035). Tree Hyraxes are found in arboreal habitats, but in the alpine areas of the Ruwenzori Mountains in Uganda and Congo, they are also rock dwellers. The Eastern Tree hyrax (D. validus) might be the earliest type of forest-living Tree Hyrax, being a member of the primitive fauna and flora of the islands of Zanzibar and Pemba in East Africa (Hahn 1935; Jones 1978).
Morphology
Hyraxes are all small to medium-sized herbivores (1.5-5 kg), with short legs, a rudimentary tail, and round ears. They have a rabbit-like appearance, hence the vernacular name, "rock rabbit". Males and females are approximately the same size. Average size of adult Rock Hyraxes varies greatly across Africa, and seems to be closely linked to average annual precipitation (Klein and Cruz-Uribe1996). The plantigrade feet have rubbery pads with numerous sweat glands, and are ill equipped for digging. While the animal is running, the feet sweat, which greatly enhances its climbing ability (Fischer 1992). Species living in arid and warm zones have short fur, but Tree Hyraxes and species in alpine areas have thick, soft fur. Hyraxes have long vibrissae (tactile hairs) widely distributed over their bodies, probably for orientation in dark fissures and holes. They have a dorsal gland, surrounded by a creamy to yellow-colored or dark brown to black circle of hairs that can be erected when the animal is excited. The pair of upper incisors are tusk-like, ridged or triangular in cross-section in males. The faces of these incisors are rounded in females (Hahn 1935). The digestive system is complex, with 3 separate areas of microbial digestion, the forestomach, the caecum, and the paired colonic appendages (Rübsamen et al. 1982). Testes are permanently abdominal and the uterus is duplex. Mean distance between anus and penis is 8 cm in the Bush Hyrax, 3.5 in the Rock Hyrax and 2.0 in Tree Hyraxes. Anatomy of the Bush Hyrax penis is complex, and the penis measures greater than 6 cm when fully erected (Coetzee 1966; Hoeck 1978, 1982B).
Physiology
Procavia johnstoni
Hyraxes have a poor ability to regulate their body temperature and a low metabolic rate for their body size. Body temperature is maintained mainly by gregarious huddling, long periods of inactivity, and basking. Although their physiology allows them to exist in very dry areas and use food of relatively poor quality, they are dependent on shelters (boulders and tree cavities) that provide relatively constant temperature and humidity (Bartholomew and Rainy 1971; Rübsamen et al. 1982). Hyraxes do not ruminate, but because of their complex gut their ability to digest fiber efficiently is similar to that of ruminants. Their efficient kidneys allow them to exist on minimal moisture intake. In addition, they have a high capacity for concentrating urea and electrolytes and excreting large amounts of undissolved calcium carbonate (Rübsamen et al 1982). As hyraxes have the habit of urinating in the same place, crystallized calcium carbonate forms deposits that whiten the cliff faces below latrines. These crystals were used as medicine (hyraceum) by several South African tribes and by Europeans (Hahn 1935).
Ecology
In several parts of Africa (e.g. the Serengeti in Tanzania, Matobos in Zimbabwe) Bush and Rock hyraxes occur together and live in close associations on rock outcrops (Hoeck 1975, 1982A, 1982B; Barry and Mundy 1998; Barry and Mundy in press ). On Serengeti kopjes the 2 species are the most important resident herbivores, their numbers depending on the size of the kopje. The population density for Bush Hyraxes varies from 20-81 animals per hectare, and the density of Rock Hyraxes is from 5-56 animals per hectare. Group size varies from 5-34 for the former and 2-26 for the latter. The adult sex ratio is skewed in favor of females, but the sex ratio of newborns is 1:1 (Hoeck 1982A). Rock Hyraxes feed mainly on grass, a relatively coarsefood, and therefore have hypsodont dentition (high crowns with relatively short roots), whereas the browsing Bush hyraxes and Tree hyraxes consume softer food and have a brachydont dentition (short crowns with relatively long roots). This difference in feeding is probably the main factor that allows both species to live together (Hoeck 1975, 1978, 1982C, 1989). Examination of C13:C12 ratios of carbonate and collagen fractions of bone (DeNiro and Epstein, 1978) and microwear patterns of the molariform teeth (Walker et al. 1978) confirmed that the Bush Hyrax is a browser and the Rock Hyrax changes between grazing and browsing (Hoeck 1975). Tree Hyraxes occur in riverine and tropical closed-canopy forests in Africa (Jones 1978) and feed on leaves, fruits, twigs, and bark in the upper canopy (Kingdon 1971).
Behavior
Rock and Bush Hyraxes are diurnal and live in groups (Hoeck 1982A; Hoeck et al 1982), but Tree Hyraxes are mainly nocturnal and are usually solitary, although groups of 2-3 animals can be found (Kingdon 1971). Where Rock and Bush Hyraxes occur together, they huddle in single mixed groups in the early mornings after spending the night in the same holes. They also use the same urination and defecation sites. Parturition tends to be synchronous (Barry 1994). Newborns are greeted and sniffed intensively by members of both species, and they form a nursery group and play together. Most of their vocalizations are also similar (Hoeck 1982B). However, Bush and Rock hyraxes do differ in key behavior patterns. They do not interbreed because their mating behavior is different and they have different reproductive anatomy. The male territorial call, which might function as a "keep out" sign, is also different. Rock and Bush Hyraxes live in cohesive and stable family groups consisting of 3-7 related adult females, 1 adult territorial male, dispersing males, and the juveniles of both sexes. Larger kopjes may support several family groups, each occupying a traditional range. The territorial male repels all intruding males from an area largely encompassing the females' core area (Hoeck et al 1982). The females' home ranges are not defended and may overlap. Rarely, an adult female from outside a group will be incorporated into the family group (Hoeck 1982A). In the Bush Hyrax these immigrants are responsible for bringing new alleles into local populations, preventing inbreeding and consequently reduce the risk of local extinction (Gerlach and Hoeck 2001). Long-term observations in the Serengeti and Matobos (Zimbabwe) show that hyrax populations fluctuate and small colonies are prone to extinction (Barry and Mundy 1998; Hoeck 1989).
Sexual Behavior
Females become receptive about once a year, and a peak in births seems to coincide with the rainy season. Gestation is 7.5 months. Within a family group, the pregnant females all give birth within a period of about 3 weeks. The number of young per female Bush and Tree Hyraxes ranges from 1-3 and in Rock Hyraxes from 1-4. The young are fully developed at birth, and suckling young of both species assume a strict teat order (Hoeck 1977). Weaning occurs at 1-5 months and both sexes reach sexual maturity at about 16-17 months of age. Upon sexual maturity, females usually join the adult female group, while males disperse before they reach 30 months. Adult females live significantly longer than adult males (Hoeck 1982A; Hoeck et al 1982). There are 4 classes of mature male: territorial, peripheral, and early and late dispersers. Territorial males are the most dominant. Their aggressive behavior towards other adult males escalates in the mating season when the weight of their testes increases 20-fold. These males monopolize receptive females. On small kopjes, peripheral males are those unable to settle, but which on large kopjes can occupy areas on the periphery of the territorial males' territories. They are solitary, and the highest ranking among them takes over a female group when a territorial male disappears. The majority of juvenile males -the early dispersers - leave their birth sites at 16-24 months old, soon after reaching sexual maturity. The late dispersers leave a year later, but before they are 30 months old.
Dispersal
Individuals of Rock and Bush Hyraxes were observed to disperse over a distance of at least 2 km. However, the further a dispersing animal has to travel across the open grass plains, where there is little cover and few hiding places, the greater are its chances of dying, either through predation or as a result of its inability to cope with temperature stress (Hoeck 1982A, 1982B, 1989).
Predation and Parasites
The Black or Verreaux's eagle (Aquila verreauxii) feeds almost exclusively on hyraxes (Gargett 1990). Other predators are Martial and Tawny Eagles, leopards, lions, jackals, Spotted hyena and several snake species. External parasites such as ticks, lice, mites and fleas, and internal parasites such as nematodes, cestodes and anthrax also probably play an important role in hyrax mortality. In Kenya and Ethiopia Rock and Tree Hyraxes might be an important reservoir for the parasitic disease Leishmaniasis.
Conservation
The Eastern tree hyrax is heavily hunted for its fur in the forest belt around Mt. Kilimanjaro. Because the forests are disappearing at an alarming rate in Africa, the tree hyraxes are probably the most endangered of all hyraxes.
Web Links
IUCN/SSC Afrotheria Specialist Group
Literature Cited
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