Costs and benefits of non-predator eavesdropping in mammal-bird

Author: Emily Prehn

Introduction

Nearly all animals communicate in some way or another, be it through visual cues, noises or sounds, smells, touch, or even taste or chemical interactions. Communication includes at least two individuals: the sender and the receiver. The sender is the animal that sent the signal in the first place; the receiver is the intended audience of the signal. But in many cases, a third party is introduced to the communication: an eavesdropper. An eavesdropper is an animal that receives a signal meant for someone else. Within interceptive eavesdropping, the eavesdropper gains information from signals; within social eavesdropping, the eavesdropper gains information from signal interactions ( [link] ) (Peake 2005). In this chapter, we will focus primarily on interceptive eavesdropping found in mammal-bird interactions.

The specific type of animal communication we will be considering here is alarm calls. Many social animals, from titmice to mongooses, make use of alarm calls. When an individual, either an assigned sentry or otherwise, notices a predator, it communicates the presence of that predator to the rest of the community by using one of the earlier mentioned types of signal. For instance, many types of birds employ loud, shrill sounds to alert others to the danger; this behavior is assumed to have evolved to aid close relatives. Many animals other than the birds’ conspecifics, however, listen (eavesdrop) to the alarm calls.

Picture an eastern chipmunk foraging under a tree. Suddenly a hawk swoops overhead. The chipmunk, concentrating on its eating, does not notice it; a tufted titmouse sitting above it in the tree does ( [link] ). The titmouse calls out, warning its own family, and at the same time inadvertently warning the chipmunk. The chipmunk looks around, realizes its danger, and scampers to safety. We will call this kind of interaction “increased vigilance following another’s call,” and will examine other examples of it and the costs/benefits of this type of behavior later in the chapter.