Chapter 5

The Physiological Basis of Behavior

1. Because an animal’s environment provides various stimuli that can trigger contradictory responses, and because its physical and social environment often change over time, animals have evolved proximate mechanisms that help individuals set behavioral priorities, which they can adjust to changing conditions.

2. Biological clock mechanisms, acting in conjunction with neural and hormonal systems, enable individuals to shift their behavior in accordance with predictable changes in the environment that occur over periods ranging from a day to a year.

3. Circadian and circannual clocks have environment-independent components, but they can also adjust their performance by acquiring information from the environment about local conditions, such as the time of sunrise or sunset or of high or low tide. This information can entrain (reset) the clock so that changing behavioral priorities do not get out of synchrony with the physical environment.

4. The endocrine system also contributes to the operation of neurophysiological mechanisms that respond to predictable changes in the physical environment (such as seasonal changes in photoperiod) as well as to unpredictable changes in the social environment (such as the presence of potential mates) or ecological environment (severe weather events). Hormones produced by endocrine organs often set in motion a cascading series of physiological changes that make reproductive activity the top priority at times when the production of surviving offspring is most likely to occur.

5. The fact that so many species employ essentially the same hormonal tool kit is a reflection of the conservative consequences of evolution by descent with modification (from ancestral species). Like any phenotypic trait, hormones themselves can be targets of natural selection.

6. Hormonal control, like all proximate mechanisms, comes with costs as well as potential benefits. The many damaging effects of testosterone or glucocorticoids—things such as a compromised immune system that can cause sickness and lead to death—provide a case in point. The cost–benefit approach helps explain why different animal species use the same hormone, such as testosterone, for very different functions.