At the heart of perception lies an animal's ability to recognize change and to make predictions based on the way change has played itself out in the past. Critical to this behavior is the ability to measure time. Without suggesting that it is the basis of predictive science, the perception of time is a primary ingredient in causal understandings.

Animal event timing refers to the process that an animal undergoes in order to recognize an interval of time. In a very anthropomorphic way, one can ask the question this way: How does an animal know the difference between 2 min and 2 h? Or does it know at all? Absolute time may be an artificial consequence of man-made clocks, but animals do behave on temporally defined schedules, and many of them are observed to solve problems in the wild that require a specific estimate of time. Central place foragers must be able to find their way home, and many of them then communicate information about distance to forage sites after they do so (e.g., Kacelnik, 1984; Seeley, 1995). Prey species adjust vigilance schedules to match predator density (e.g., Arenz and Leger, 1997). Prospective mates choose future partners based on the temporal specificity of mating displays (Kyriacou et al., 1992; Michelsen et al., 1985). Foraging animals move on when the rate of resource intake drops too low (Stephens and Krebs, 1986; Charnov, 1976). In all of these cases, animals must perceive the temporal duration of events.

A unified theory of animal event timing requires that we know two things. First, how do nervous systems perceive and learn key aspects of temporal information? What are the molecular and cellular mechanisms required to build perceptual clocks? And second, why have these clocks come to exist? What evolutionary forces have driven their evolution? The answers to these two questions are generally referred to as the proximate and ultimate definitions of a behavior. However, it is a major assumption of this review that, because molecular mechanisms and evolutionary forces feed back on one another, one cannot fully understand one without the other.

To our advantage, psychophysical studies have already established that some animals do measure short intervals of time. This will provide us with a basis for conceptualizing event timing in a definitive way. The purpose of this review will be to formalize what we know about animal event timing by addressing contributions from psychophysics, molecular genetics, neuroscience, and evolutionary ecology, while hopefully combining that information in a way that furthers our understanding of animal event timing.

I begin by reviewing what is known about animal event timing from psycho-physics, establishing that animals can measure time and that time measurement appears to obey specific conserved properties over a wide range of species. I then discuss the molecular mechanisms of circadian clocks, which to date are our most well understood molecular clocks. Circadian mechanisms will help us to understand what the event timer is not and also how an event timer might operate at the cellular level. I then suggest possible mechanisms for event timers that agree with the psychophysical and physiological evidence for where these clocks are and how they operate. Finally, I take a tour of ecologically relevant behaviors where we may hope to find event timers at work in the world. This will further our understanding of the evolution of event timers and also allow us to make predictions about what kind of event timers specific animals are likely to have.

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