Animation 51.1: The Costs of Defending a Territory

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.

Experiments by Catherine Marler and Michael Moore on Yarrow's spiny lizards provide insight into the energetic costs of defending a territory.

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.

Textbook Reference: Key Concept 51.4 Selective Pressures Shape Behavior