INTRODUCTION

In order to improve their survival and reproductive success, some animals will establish a territory and restrict access to its resources, whether those resources be food or access to receptive females. The animal will advertise that it owns the area and, if necessary, chase others away. But advertising and chasing takes time and energy that could have been used for other beneficial purposes, such as foraging for food and watching out for predators. Ecologists often use a cost-benefit approach to establish a framework for understanding the evolution of this type of behavior.

ANIMATION SCRIPT

One common way for an animal to improve its survival and reproductive success is to establish and defend a territory. This involves advertising ownership as well as chasing away other individuals from the territory.

Advertising and chasing take energy, however, and this energy could have also been put to use in finding food and watching out for predators. Thus, defending a territory comes with significant costs, and to understand the evolution of this type of behavior, ecologists often use a cost–benefit approach.

The total cost of any particular behavior has three components. First, there is the energetic cost. This is the difference between the energy the animal would have expended had it rested and the energy expended in performing the behavior.

Next, there is the risk cost—the increased chance of being injured or killed as a result of performing the behavior, compared with resting.

Finally, there is the opportunity cost. This is the sum of the benefits the animal forfeits by not being able to perform other behaviors during the same time interval. An animal that devotes all of its time to foraging for food, for example, will not achieve high reproductive success.

Let's review the cost-benefit approach using Yarrow's spiny lizard. In nature, Yarrow's spiny lizards become highly territorial only in September and October, at a time when females of the species are most receptive to mating. During this period, males will vigorously defend territories to exclude conspecific males.

One obvious benefit of defending the territory is in the form of improved reproductive success—potential mates living in the territory are a resource for the males. However, there are also costs. We can examine some of these costs by conducting a simple experiment.

Territoriality in Yarrow's spiny lizards is mediated by testosterone. With this information in hand, Catherine Marler and Michael Moore designed a series of experiments that would measure the costs associated with defending a territory. LetŐs explore those experiments.

Territoriality can be artificially increased by treating males with testosterone. To examine this effect, let's assign lizards to one of two experimental groups. The first group will receive an implant containing testosterone. A second group will receive an identical implant that contains an inactive saline solution, which would not be expected to have a behavioral effect, but will serve as a control for the effect of the implantation procedure itself.

We now return the lizards back into a rock pile and monitor their behavior. As a measure of activity, we'll determine the percentage of active lizards throughout the day in each group.

Plotting the data reveals the graph shown.

The testosterone-treated lizards are represented in the top graph, which indicates a higher activity level in this group throughout the day. The experimental group spent considerably more time patrolling their territories and performing aggressive advertising displays. In addition, Marler found that the testosterone-implanted lizards expended about one-third more energy than control males. Spending so much time patrolling obviously requires more energy, but what are some other costs associated with this behavior?

Time spent defending a territory leaves less time for foraging for insects and also increases the risk of being injured or captured. This becomes clear when we compare the survival rates of the two groups. The results suggest an explanation as to why, in nature, these lizards vigorously defend their territory only when receptive females are present—that is, at a time when the reproductive benefit outweighs the energetic and risk costs.

CONCLUSION

To understand the evolution of a behavior like defending a territory, ecologists often use a cost-benefit approach. This approach assumes that an animal has only a limited amount of time and energy to devote to its activities. Animals seldom perform behaviors whose total costs are greater than the sum of their benefits—the improvements in survival and reproductive success that the animal achieves by performing the behavior. A cost-benefit approach provides a framework that behavioral ecologists can use to design experiments and make observations that enable them to understand why behavior patterns evolve as they do.

The results of Marler and Moore's experiment on Yarrow's spiny lizards suggests that spending time patrolling a territory requires significantly more energy (energetic cost), leaves less time for capturing insects (opportunity cost), and also increases the risk of being injured or captured (risk cost). It's therefore not surprising that, in nature, these lizards vigorously defend their territories only during the months when females are the most receptive to mating—a time when the benefits outweigh the costs.