When a territorial animal is young with little to lose, it tends to wander widely in search of food. But as it ages, its home range shrinks as it stays close to the more valuable territory it has acquired over its lifetime.

A new model developed at the University of California, Davis, and described in the journal The American Naturalist, identified this pattern. The model is the first to focus on how and why home ranges expand or contract in territorial animals, a trend noted in multiple animals ranging from deer and pandas to owls and flying lizards.

The study’s co-authors include lead author Yun Tao, a PhD candidate with the UC Davis Department of Environmental Science and Policy when the model was developed and currently a postdoctoral candidate at Penn State University . Tao’s former faculty adviser, Alan Hastings, from the UC Davis Department of Environmental Science and Policy, and Luca Börger from Swansea University in the United Kingdom.

The model identified three general patterns:

1) Wandering youth, homebody elders. Younger individuals are more exploratory and adventurous than older individuals, so they have a larger home range. “They’re usually too small to establish and secure their own territory, so they have nothing to lose by abandoning the idea of a territory,” said lead author Yun Tao, a PhD candidate with the UC Davis Department of Environmental Science and Policy when the model was developed and currently a postdoctoral candidate at Penn State. “For them, it’s more profitable to stick to distant feeding expeditions and forage as best they can. As they age, however, their ability to secure their own territory improves, so home ranges shrink in size as they get older.”

2) Range anxiety. The less an animal knows about its environment, or experiences “sensory noise,” the more home range dynamics vary, regardless of age. For instance, when forage outcome or threats of intrusion are unpredictable, an individual tends to either wander widely or become more territorial, defending what it has in preparation for the unknown. When pressure from competitors is particularly high, almost all individuals adopt a defensive strategy except for juveniles.

3) Opportunity also a risk . A rise in opportunity, such as a seasonal abundance of food, can also pose a risk for older individuals when it attracts outside competitors. In this scenario, the young would continue to wander afar as resource levels increase, whereas the old will contract their home ranges in order to strength territorial protection against the potential invaders.

Management implications

Home range refers to a spatially bound area animals routinely use over time to fulfill its needs. Previous studies considered home ranges as fairly static, while this study notes that they fluctuate throughout the year, a flexibility that helps animals to adapt to their environment. “Knowing this allows for us to have a better idea of how to manage animal distributions in terms of conservation or disease control for cases such as Nipah virus and rabies,” Tao said.

The study was funded by the Army Research office, National Science Foundation Graduate Research Fellowship Program and its Graduate Research Opportunities Worldwide program. Read the Article

Picture: Red deer (Cervus elaphus), a species whose territory varies over its lifespan. Animal World & Snake Farm, New Braunfels, TX, USA.
(© 2007 Jeff Whitlock, The Online Zoo, used with permission)