Courtesy of Ross Warner, http://rosswarner.com

 

SOCIAL SPACING

No groups of fully-grown males are observed. Adult males are generally found alone. Females, on the other hand, may roam by themselves, or they may form small groups with other females and/or subadults. Such associations are not very stable. The only animals that truly remain close to the female are her youngest offspring. Observed group sizes vary from 1 to 12, those consisting of 3-5 individuals being the most common (Leuthold 1977). Subadult males may also form small groups (especially with peers who are the same age as them) or wander alone until they acquire a territory, which can occur around the age of 3, i.e. adulthood (Leuthold 1978). The mean size of 607 groups of gerenuks observed in Tsavo National Park was 2.16 overall, with non-significant difference between the green season and the dry season (Leuthold and Leuthold 1975).

The size of the female’s home range varies between 2.4 and 4.5 km2. The home range of the male [all or most of it being defended territory, see below] is slightly larger – between 4.1 and 6.4 km2. These numbers are comparable to the range sizes of many other African ungulate species, with some remarkable exceptions, such as the zebra and the elephant (Leuthold 1977). The small size of the gerenuk, meaning the animal does not need exceptionally abundant food resources, coupled with its independence of water, limit its daily and seasonal movements. As Leuthold puts it, “hardly any species below the size of Thomson’s gazelle is known to make wide-ranging movements,” even though, he claims, the size of a home range does not always correlate with body size. The relatively small propensity of the gerenuk to aggregate or to form large social groups also precludes the need of an extensive range, since resources will generally be enough for the animals that live in a particular area (Leuthold 1977). Gerenuk feeding habits vary with the season, depending on what is available, so that the members of the species rarely have to move across great distances to follow a unique food source. Of course, the quality of the habitat itself is a major factor as well.

In contrast to other species in which the “internal anatomy of the home range” (Leuthold 1977) is well-defined and important, how the home range is structured seems to be of little significance in the gerenuk. Gerenuks don’t use fixed sites for sleep, comfort behavior, drinking, or observation. They do have, however, urination/defecation sites (in the male) and special places which the male marks with his antorbital glands secretion.

Even though the “internal anatomy” does not matter a great deal, gerenuks clearly inhabit a home range of known limits. Why is that? A home range contains the resources that the animal needs to survive and reproduce (food, hiding places). The animal familiarizes itself with the area, which grants it better knowledge of the availability and location of resources, as well as better capacity to flee predators (Leuthold 1977).

In adult male gerenuks, an individual’s home range usually coincides with a more or less classical territory. Adult males are spaced out and do not infringe on each other’s space. Female gerenuks, on the other hand, are nonterritorial. Their home ranges may coincide with a single male territory, or they may overlap with several of them. Females are completely independent of the males in their movements (as Leuthold puts it, “they couldn’t care less”) (1977).