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eastern grey kangaroo (macropus giganteus)
grouping
Eastern grey kangaroo interaction is characterized by the small groups that are formed. On average, there are about 3.6 individuals for every small group (Dawson, 1995). Kangaroos are by nature gregarious and form open-membership groups (Colagross and Cockburn, 1993). These are often referred to as mobs and are seen most frequently in the early morning and in the evening. These are the times of day when they are most vulnerable to predation and as such take advantage of having other individuals in the mob (Colagross and Cockburn, 1993). A mob usually consists of 2-3 females and their offspring and roughly the same number of males; one of whom is dominant (BBC - Science and Nature Wild facts). Dominance hierarchies occur within both sexes and provide different benefits for the different individuals (Dawson, 1995). The dominant individuals gain access to better sources of food and areas of shade (Dawson, 1995). Dominance is established mainly as a function of the size of an individual. Ultimately, Dawson posits that the goal of this dominance is to be able to gain access to females when they become fertile. He also points out that once a male achieves this level of dominance he rarely maintains that position for more than one year. Among the grey kangaroo there is also a bias towards male mortality. Dawson notes that it is not a rarity to see a dominant males die shortly after a change in favorable environmental conditions. This is because the maintenance of his position and continual reproductive activities take a huge toll on its energy. Energy conservation is one of the most important activity shaping behaviors for kangaroos in general. According to a study by Hohn, Kronschnabl and Ganslober, large and medium grey kangaroos are the most active individuals, while smaller males and females have significantly lower levels of activity throughout the day (2000).
Following a study conducted by Peter Jarman concluded that, "young females with living female relatives reproduce more successfully than those without, suggesting that the kin-discriminatory behavior may have an adaptive value." Females are also more likely to have closer relatives around them than are males (Jarman, 1993). because the grey kangaroo is known socially for its loose aggregations with other individuals, it is more likely that there is an element of cooperation that takes precedence over these mobs. Kin altruism may be a possibility in the females, but given the low degree of relatedness between the males, it is likely that they aggregate for cooperative reasons. The primary reason for this cooperation would probably be a response to predation pressure. It is easier to see predators when there are more eyes and to reduce the probability of being preyed on by a predator.
Picture courtesy of Damon Ramsey, Educational Tours Australia
vigilance
Vigilance behavior is defined as, "Any behavior indicative of alertness to potential danger" (Hurnik et al., 1995) A study by Colagross and Cockburn investigated what factors affected grouping in the grey kangaroo and specifically how vigilance related to grouping. They found that the most telling factor in influencing the alertness of an individual was predation pressure. It was found that the individuals adjust their behavior more to reduce the risk of predation than to the size of the group. As the size of the group increases, one would expect the vigilance to reduce. However, this was not the case with the grey kangaroo that the studied. Individual vigilance did not show any significant decrease with an increase in the group size. The three groups of kangaroos that showed the greatest amount of adjustment were reproductive females, individuals on the periphery, and individuals furthest from cover (Colagross and Cockburn, 1993). The only instance where the kangaroos showed a response to group size was a result of the edge effect: “the tendency of the proportion of individuals on the periphery of the group to decline as group size increases” (Colagross and Cockburn, 1993).
Sentinel behavior was also noted in these kangaroos. In this instance, they rely on a large number of individuals to reduce the probability that a particular individual will be picked off by a predator. Given that the degree of relatedness among the members of an aggregation (except the mother-offspring relationship) is relatively low this points to the selfish behavior of the kangaroos. The open-membership groups allow them to judge for create for themselves a situation in which the group can be used conveniently as a buffer against predation pressure. Blumstein and Daniel conducted a study where they compared eastern grey kangaroos on a predator free island with those on the mainland still exposed to predation pressure. The researchers found that the eastern grey kangaroos that were placed on the island showed less vigilance than those that were still exposed to predation pressure (Blumstein & Daniel, 2003). This serves to reinforce the idea that predation pressure is most important affecting vigilance.
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