Friday, September 17, 2010
As described by a team of scientists led by the Wildlife Conservation Society's John Goodrich in the latest issue of the Journal of Mammalogy, a 14-year study of Amur tigers in eastern Russia's Sikhote-Alin Biosphere Zapovednik has shown that male and female tigers establish home ranges of different sizes for different reasons. After capturing and radio-collaring 32 individual tigers (adults and cubs), the team of Russian and American scientists was able to determine that male tigers maintained very large territories (about 1,385 square kilometers) which encompassed the home ranges of several females (about 390 square kilometers).
A map of Amur tiger home ranges between 1992-1997. Solid areas marked "F" designate females and dashed lines marked "M" designate males. From Goodrich et al, 2010.
The disparity in territory size was not much of a surprise. Among solitary big cats, males often have larger home ranges than females, and the reason for this difference between the sexes has to do with the different life strategies of male and female Amur tigers. Whereas young male tigers typically leave the home territory of their mother in an attempt to find a vacancy and gain access to as many females as possible, females stake out their territories based upon the resources they can provide for them and their cubs (thus their home ranges can be much smaller).
What intrigued the scientists, however, was that the home ranges of female Amur tigers contracted when they had female cubs, with their daughters taking up residence in the vacant areas. This favored the future reproductive success of the young tigresses as they did not have to face the risks usually encountered by individuals which try to establish themselves elsewhere (and often become victims of poachers). As the authors of the paper state, it appears that the adult female tigers in their study defended larger territories than they actually needed to survive, and by passing down a portion of this land to their daughters they enhanced the potential success of their offspring while mitigating competition for the same resources.
Yet, as the scientists saw firsthand, poaching can disrupt the matrilineal inheritance of territory among Amur tigers. During the early years of their study - from 1997 to 2000 - all but two of the radiocollared tigers living within the Sikhote-Alin Biosphere Zapovednik were killed by hunters. The vacancies were filled by a mix of "immigrant" animals from other areas and individuals which were related to those which had been poached, but, even after a new population became established, it took five years before a mother tiger passed down territory to her female offspring.
The spatial patterning of tigers over time detected by Goodrich and colleagues may very well complicate tiger recovery plans. If there is a large area of land in which tigers were nearly eliminated (as was the case in this study), the new tiger population will not quickly rebound to its maximum capacity. Instead female tigers which move into vacancies will defend larger territories than they require until they pass down some of that area to their daughters, and during this time the tiger population might be more susceptible to poaching as a smaller number of animals will be occupying an area which could actually support many more. But this news isn't all bad. If tigers can be successfully protected long enough for adult animals to become established, the population size and density can potentially double when the next generation of female tigers mark out their own territories within those initially carved out by their mothers. Given enough time, the tattered remnants of a tiger population can begin to recover.