Social Spacing
Note: photo taken by Michael Osgood and posted here with his permission
The wildebeest is one of the most gregarious of all ungulates (Estes, 1968). An individual wildebeest depends upon the herd and has a high tolerance for crowding with other wildebeests. As a result of the wildebeests' gregariousness and its adaptation to its environment, a herd of wildebeests may reach phenomenal numbers. For example, in 1967 the Serengeti gnu population reached 380,000 wildebeests (Estes, 1968). Although numbers have shrunk in recent years due to fencing, constructed water points, and abnormal levels of disease and predation, wildebeests are still able to reach extremely large population sizes (Berry, 1982).
Subclasses
often form among wildebeests, and these subclasses are especially structured
during the calving season
(Estes, 1968). Adult males and females form separate aggregations.
In the male heard, subclasses are divided by yearlings, two-year olds,
and adult wildebeests. In female
herds those that have the same reproductive status join together and form groups
of pregnant females, females with calves, and barren females.
The wildebeest social organization gets more complex because migratory
and sedentary subpopulations may coexist.
Estes reports finding the following groups: females in sedentary herds,
females in aggregations (subdivided into pregnant, nursery or maternal groups,
and unbred or barren females), males in bachelor herds, males subdivided into
age classes, males attached to female aggregations, transient and resident territorial
males (Estes, 1968).
During the rainy season, 10-30% of the wildebeest population (measured in the Crater) live in subpopulations that separate adult males and females (Estes, 1979). The females and their calves generally occupy a home range with a core area of less than 1 km2. The adult males occupy a separate territory. The remainder of the population during the rainy season is distributed in the migratory pattern. In this pattern females and the young tend to aggregate, and the males tend to be closely associated. In the dry season, the number of small herds declines and females abandon their home ranges in order to join aggregations. Many males also abandon their territories (Estes, 1979).
The Mating Only Territory:
Within these large, somewhat chaotic looking herds, social organization does exist (Estes, 1968). The social spacing of the wildebeest can be best described as a mating only territory. As an occupant of a mating only territory, the wildebeest actively defends a particular piece of land. A male wildebeest cannot mate without a territory, and thus the territory serves as a necessary status symbol as well as an actual area in which mating can occur (Estes, 1968).
Why a mating only territory?
The feeding pattern, the herd effect, predation, and the nature of a male wildebeest's sexuality help explain why a mating only territory governs the social organization of the wildebeest. The sections below detail the reasons why the wildebeest's territory is not used for feeding or nesting, and suggests why a territory is essential for mating purposes.
Feeding Patterns of the Wildebeest
The wildebeests are grazers, and compared to other antelopes, they are not picky eaters (Estes, 1979). Ninety-seven percent of the wildebeest diet consists of grass, and nutrition sources are generally not limiting (Berry, 1982). Because approximately 87% of female wildebeests reproduce each year, researchers believe that nutrition levels are more than adequate for this species (Berry, 1982). Furthermore, instead of selectively picking from the available grass in its area, the wildebeest selects a place to feed based on where the grass is at a growth stage that it prefers (Estes, 1979). Wildebeests tend to prefer grass that is short, growing, easily digested, and full of nutrients. As the dry season begins, wildebeests must become less picky and settle for older longer grasses. Because of this particular feeding style, wildebeests often clump together in areas of good grazing conditions. Thus, as grazers, food is not a limiting factor for the wildebeest. When the resources in one area become depleted, and when the grass is gone, the wildebeests move on to a better feeding area (Estes, 1979). Research by Ben-Shahar et al. (1992) examined the relationships between soil factors, nutrients in grasses, and foraging behavior of the wildebeest and zebra. They conducted their study in a semi-arid nature reserve located in South Africa. They hypothesized that (a) Soil nutrient levels determine both the abundance and distribution of grass species, (b) Nutrient levels within grass species are correlated with soil nutrient levels, and (c) The spatial distribution and diet composition of ungulates is influenced by the nutrient availability in grasses. The distribution of soil nutrient levels in upper ground levels significantly affected the distribution of grass species in the reserve. However, because grasses on fertile soils did not accumulate higher nutrient levels than grasses on poor soils, the grass nutrient levels did not correlate with soil nutrient levels. Most relevant to the social spacing of the wildebeest, Ben-Shahar et al. found that as the levels of nitrogen and phosphorus varied throughout the year, wildebeest and zebra moved to habitats containing a high proportion of nutritional species instead of selecting particularly nutritious species within a community (Ben-Shahar et al., 1992).
Because the wildebeest readily travels to areas abundant in nutrients, defense of a food resource by territorial males is impossible. First, a male is physically unable to defend the over-abundant grass found on the plains. Second, a male cannot attract a female by offering her grass to graze upon since the female can find high quality grass throughout the plains. Finally, if a male were to defend the grass on his territory as food for the female, his supply would soon run out. In this case, the female would leave the male during migration with a grass-depleted territory and no mate. Hence, as migratory grazers, defense of a territory for feeding purposes is unreasonable and does not occur in the wildebeest social organization (Estes, 1979).
The Herd Effect and Predation of Young Wildebeests
Nesting territories often provide areas of protection such as dens, burrows, etc. where the young may hide after birth. Because young wildebeests are protected from predation largely through the herd effect, males' territories do not serve nesting purposes. In the wildebeest social system, predation of the young by hyenas is a necessary concern, and thus must be addressed by the wildebeest population. For example, each year approximately three quarters of calves may be lost to predation, mostly the hyena (McFarland, 1982). Hyenas strive to locate the most vulnerable potential prey and thus seek the disease, injured, or incapacitated calves (Estes, 1979). Less frequently the wild dog and the cheetah may prey upon wildebeest calves. In the wildebeest system of protection for the young, several tactics are employed: (a) the young keep a safe distance from predators and stay within a large group, (b) they may attempt to outrun a predator if singled out, and (c) mothers may attempt to defend their young against an aggressive predator (Estes, 1979). Although (b) may seem impossible for a newborn calf, studies have shown that calves can run along side their mothers within a few minutes after being born (McFarland, 1982). Despite this ability, the best protection tactic is (a) the herd effect as a young wildebeests chance of being picked off are tremendously reduced when it is surrounded by a large aggregation of other wildebeests (Estes, 1979). As members of a large herd, the young are often shielded by their mothers, who place themselves between the predator and their offspring. Additionally, other wildebeests in the herd attempt to screen the mother and her offspring from the predator's eye (Estes, 1979). Thus, because the herd effect is the wildebeests' best tactic to avoid predation of offspring, a male need not defend his territory for purposes of defending newly born wildebeest calves. On a defended territory away from the masses of the herd, a young wildebeest would become easy prey for hyenas and wild dogs among others.
Predation of Adult Wildebeests
Research by Mills et al. (1992) examined the role of lion (panthera leo) predation on the dynamics of zebra (Equus burchelli) and blue wildebeest (Connochaetes taurinus) populations. Their work took place in the southeastern region of the Kruger National Park (KNP) during a time period with normal amounts of rainfall. They constructed simulation models based on intensive observations made over four years, and addressed the following two aspects of the predator-prey relationship: (i) the effects of lion predation on a sedentary (wildebeest) versus semi-migratory (zebra) population and (ii) the effects of predator selection of various sex and age classes on prey populations (Mills et al., 1992).
According to their model, the way in which lions select their prey and the sedentary or migratory behavior of that prey caused predation to affect wildebeest and zebra populations in the KNP differently. Mills et al. found that 7.7 lions were needed to stabilize the sedentary population of wildebeests. In the KNP the average number of lions in contact with the wildebeest population was about 7.7, and the results of Mills' study showed that the wildebeest population remained stable throughout the four-year period. Hence, they concluded that lion predation served as the main cause of death in the wildebeest population. This study did not investigate the mortality of wildebeest calves (Mills et al., 1992).
In the migratory zebra population, Mills et al. (1992) found that lions took foals more often than expected and adult zebras were killed less often. They found that zebras could support a similar number of lions (6.8) even though the fecundity rate of the zebra is lower than that for the wildebeest. Interestingly, even though zebra population numbers fluctuate due to migratory patterns, the zebra predation rate did not change. Since the number of zebras available to lions during a part of the year is diminished, zebra numbers might limit the number of lions (Mills et al., 1992).
Overall, Mills et al. found that lion predation affected wildebeest more severely than zebra in the KNP. They suggest that greater predation occurs on the wildebeest because of its sedentary nature in this region as opposed to the migratory behavior of the zebra. In uncovering the important differences between the sedentary wildebeest and the migratory zebra, the authors shed some light on the mechanism of predator-prey relationships among these animals. Future studies could use such a simulation model to compare lion predation on sedentary versus migratory wildebeests (Mills et al., 1992).
Sexuality and the mating only territory
The importance of the territory thus lies in its relation to mating. A male must hold a territory in order to mate. Because of this association, sexual and aggressive behaviors in the wildebeest are closely tied to a male's piece of property (Estes, 1968). Estes describes the wildebeest territory as a psychological state of mind, and explains the motive force behind territory seeking as the sexual territorial drive. While holding a piece of property, a male possesses the necessary self-assurance to engage in both sexual activity with females and aggressive encounters with other males (Estes, 1968). A male with a territory feels able to fight with other males over a female because he feels assured to win (Gnu, 1968). Thus, mating and fighting are inseparable. Interestingly, aggression generally dominates over sex in territorial behavior. For example, a bull will exert far more energy and time aggressively discouraging males from entering his territory than he will exert in mating with a female - even during the rut and with a female in estrus on his territory. Although inhibition of sex and aggression is typically associated with a dominance hierarchy, the wildebeests have no hierarchy. The only division that exists is the status of territory holder vs. non-territory holder (Estes, 1968). Therefore, only males who own territories feel confident enough to fight with other males over females, and only males with territories are able to mate with these females. Hence, while wildebeests need not hold territories for the purposes of nesting and feeding, holding a territory for mating is absolutely essential if the male is to pass his genes on to a future generation.
The Territory
From the text above, it becomes a apparent that the territory is absolutely essential for mating purposes. Nevertheless, a male wildebeest is unable to win and hold a territory until approximately three years of age. What is life like for the bachelor wildebeest?
Pre-territorial Behavior: The Life of the Batchelor
Between one and two years of age, a male leaves his mother or is driven away by territorial males (Estes, 1968). Because the wildebeest is naturally gregarious, he seeks the companionship of other males, and these young male wildebeests form bachelor herds of about 50 to over 500 wildebeests. A bachelor herd provides the male with social opportunities, safety, and security. Because males of a bachelor herd are not yet mature, they are generally docile when challenged by a territorial bull. A member of a bachelor herd will move his head out of the way and keep it low like a female if approached by a territorial male. The young bachelor will run if pursued. As a result of this submissive behavior, bachelors usually reside on the fringes of the wildebeest habitat. In this area the bachelor takes second-best nutrition and places himself in a location more easily spotted and attacked by lion predators. However, at times the bachelor herd is able to mix with the female nursery herds and can then gain access to the best pastures (Estes, 1968).
Compared to female nursery herds, bachelor herds are more loosely knit, and no hierarchy appears to exist (Estes, 1968). Despite their gregariousness, bachelors abstain from fighting and playing with one another. Thus, while a member of a bachelor herd, a male is simply marking time until he may hold his own territory. The sexual and aggressive tendencies necessary for holding a territory will not emerge until the male is reaches the proper place and time. Nevertheless, males in a bachelor herd will practice territorial behaviors. They will engage in pawing, rubbing the face and the preorbital glands on the ground, defecating, and rolling. They will perform the main steps of the Challenge Ritual, which includes low-intensity fighting and cavorting. If near a female in estrus, bachelors may exhibit sexual behaviors such as the Ears-down display, perineal smelling of females, and erection followed by spontaneous ejaculation. Finally, bachelors may also practice the advertising call to be later used in territory defense and female attraction, but a male will not achieve the perfect grunt until about two and a half years of age (Estes, 1968).
The Territorial Solitary Bull
At about three years of age, the wildebeest has matured
enough to be able to win and hold a territory (Estes, 1968). As described above, the
male uses this territory for purposes of mating and must actively defend his
ownership of that particular piece of land. Defense of the land against
other males happens on highly regular basis, and the ritualistic behavior
performed has been termed the Challenge Ritual.
The Challenge Ritual
Research by Estes has demonstrated that territorial bulls encounter their neighbors at least once per day. Sometimes several encounters may occur. Aggression between the neighbors is rather ritualized and has thus been named the Challenge Ritual. This ritual meeting procedure may last anywhere from 1 - 15 minutes (Estes, 1968), and has been described as playful (Fagen, 1981). An average challenge takes about 6.5 minutes. Although this meeting procedure is ritualized, no two protocols of over 100 that were recorded by Estes were identical. Each bull may choose from over 30 different actions to perform when interacting with his neighbors (Estes, 1968).
After reading the protocols above, the following actions emerge as characteristic of the Challenge Ritual:
However, despite the seeming ease with which the Challenge Ritual is described above, it should be noted that these behaviors might be performed in any combination and in any sequence and thus exhibit high levels of variability. Obviously, many of these actions are performed by the defending wildebeest as a threat to the invading wildebeest. The two major components of wildebeest threat behavior are pawing and horning (Estes, 1968).
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Click here to view how frequently Estes observed key activities in a sample of 100 Challenge Rituals. |
Thus, it has clearly been established that a male must hold a territory to mate. Somewhat paradoxically, the male must rule as master of his territory before he can hold dominance over other males. However, the male can only become a territory master by standing up to other bulls. What is a bull to do?
How are territories Established?
Unfortunately for the male wildebeest, the act of winning a territory is a formidable undertaking. The difficulty in this act depends on the density of the other territorial males in the area. A young male will encounter resistance and aggression from a resident territory holder whenever the territory-seeking male is visible by the resident male. Despite the aggression needed to gain such territories, center territories where the population density is the highest are the most desirable territories (Estes, 1968).
A male wildebeest will attempt to conquer the paradox of territory acquisition by seeking out and frequenting a weakly defended area. He will enter this area but yield when he meets aggression. He will return repeatedly with high levels of persistence. Eventually, the resident bull will fall to fatigue and habituation. The invader will be chased less persistently and/or less frequently, and as the invader is chased less, he gains in self-confidence. At some point the invader will take the territory as his own and begin herding females, bullying transient males, and moving about actively in the territory. He will not, however, act aggressively towards the advance of a neighbor. Territory ownership is granted to the invading wildebeest when the repelling force of his aggression is equal to the force exerted by other males in neighboring territories (Estes, 1968).
The time necessary for this acquisition process to occur depends upon terrain, season, the spacing of other males nearby, the temperament and motivation of the invading bull, and the temperament, age, and previous experience of the resident bull. If all conditions are good for the invading bull, he may be able to acquire a territory within a few days or hours of his first attempts. If the bull runs into more difficult conditions, he may take weeks to gain a territory, and the possibility remains that the invading bull will never gain a territory. Estes proposes that the most important factor in territory acquisition is the temperament of the resident bull, which will then determine the degree of resistance that he puts up. The territory-holding bull's temperament may depend on his physiological condition as well as the space available for newcomers (Estes, 1968).
Generally, territorial status quo is interrupted only twice per year: during the rutting season and the first month or so after the end of the dry season. During rutting season, so much activity is taking place and so many estrual females are on a bull's territory that an invader can break in and win a territory during the confusion with surprisingly little friction. Status quo may be challenged after the dry season because of poor nutrition. Because territory holders cannot leave their territories for long periods of time (for fear of invasion), they must graze only upon the grass in the nearby vicinity. Often after the dry season, their vegetation is of poor quality and the physical conditions of the territorial bulls declines rapidly. During this period of poor physical health, a bachelor with more strength and energy can invade with relative ease (Estes, 1968).
If a bull loses his territory, he will persistently attempt to regain that same territory (Estes, 1968). However, sometimes it is actually more difficult for a bull to regain his lost territory than it is to gain a new different one. This phenomenon occurs because a bull's emotional attachment to a piece of land may actually hinder his ability to acquire that piece of land. Secondly, the new resident bull may treat the previous owner with greater hostility than a stranger (Estes, 1968).
Vigilance and the Wildebeest
Research by Burger et al. (1994) explored the vigilance behavior of the wildebeest. Burger et al. (1994) found that animals on the physical edges of a wildebeest herd devote more time to vigilance than central animals. This type of behavior would be expected since outside animals are more directly exposed to attack. Burger et al (1994) also found that wildebeest males on the edge of the herd devote more than twice as much time to vigilance than males located elsewhere within the herd. Both male and female wildebeests were significantly less vigilant in herds without young than in herds with young. This characteristic remained true even if the wildebeests themselves had no young. Finally, Burger et al. (1994) also determined that females with young were more vigilant than females without young and that males were more vigilant that females without young. As this vigilance occurred in order to protect young from predators or to prevent them from straying, such behavior is evidence of the wildebeestsâ important parental role when raising their young.
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