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eastern grey kangaroo (macropus giganteus)
mating system
Hierarchical promiscuity best describes the reproductive system. This is explained by the lack of association across sexes and by the dominance hierarchies that allow the males a higher chance for female insemination (Lee & Cockburn, 1985). Promiscuity means there is no prolonged association between sexes and that there are multiple matings by members of at least one sex. It is possible that this promiscuous mating strategy evolved as a result of the low fecundity of the female which only allows males to realistically mate only once a year. This is also supported by the fact that grey kangaroos express a great deal of sexual dimorphism (Fisher, 2002). This is because as the level of dimorphism increases there is also usually a trend toward species being promiscuous. This suggests a high level of competition between the males for mates. The dimorphic characteristics of the grey male kangaroo can also help it establish dominance and can allow females to have some form of selection in who they mate with.
Another important factor to consider in the reproductive schema of the kangaroo is that the dependence on the mother by the offspring extends for a long period of time. This is made even more salient by the fact that the joeys (kangaroo offspring) stay in their mothers pouches for up to 300 days (Firth & Calaby, 1969). This accounts for the reason why females tend to form the more stable core in a mob that is facilitated by the bond to their offspring. At times females have been noted to let out their young as early as 200 days after birth. These excursions, however, are very limited and are conducted under the very watchful eye of the mother (Dawson, 1995). The eastern grey female has one of the longer gestation periods of all the other kangaroos at 37 days. Sexual maturation in females is achieved between the ages of 17 and 28 months, while males achieve it at roughly 25 months (Firth and Calaby, 1969). The estrus cycle of the eastern grey kangaroo is roughly four days longer than its gestation period (Lee & Cockburn 1985). The age of weaning in the grey kangaroo is after about 550 days of age in the joey. Even after leaving the mother’s pouch the joey continues to suckle from the outside (Ride, 1970).
Picture courtesy of Damon Ramsey, Educational Tours Australia
Females are the primary caretakers of young, with the male providing no assistance. At the same time, there is a higher mortality rate for males. This can be attributed to the larger size and thus higher demand for already sparse resources. Females have even been shown to have a preference for their females offspring during times when environmental conditions are harsh (Dawson, 1995). The likely reasons are that since males grow to almost twice the size of the females they require more resources. The grey kangaroo is a usually a seasonal breeder but has the ability to breed at anytime of the year (Dawson, 1995). According to Kirkpatrick a majority of the of the grey kangaroo births occur during the summer months, at a rate of almost three times the births in the colder months (1965). Given the scarcity of resources in the winter months of the year, a mother benefits by giving birth during the summer. She can have more resources for herself in order that she can produce more milk and be able to sustain her offspring.
An eastern grey joey
courtesy of Rose Darroch
According to Lee and Cockburn, eastern grey kangaroo falls into a category of known as the terrestrial herbivore. This group of species breeds seasonally and have a fecundity of one per year. This is due to the contrasting quality and quantity of food resources in the spring and early summer as opposed to those in the autumn and winter. This is also affected by the aforementioned extended reliance that joeys have on their mothers. The macropus giganteus is also identified as an obligate breeder (Lee & Cockburn, 1985). The Nature Conservancy website defines an obligate breeder as, "a species that is able to reproduce in only one kind of habitat" (2002). This is also explained buy the fact that grey kangaroos are sedentary creatures. They do not display a great deal of migratory behavior and have been know to die even though a water source was not more than 10 miles away from them. When food resources are scarce few females are known to ovulate during lactation, but when there is an abundance of resources, up to 85% of female ovulate during this period (Lee & Cockburn, 1985).
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