Time is easily one of the more slippery subjects in four dimensions. Our linear and subjective experience of it makes it rather difficult to define. Understanding how other organisms experience it is even more problematic. Although when one stops to take it all in, a considerable amount is actually known about how animals perceive time. Molecular geneticists and neurobiologists are in fact making numerous inroads into the mechanisms controlling event timing. Ecologists as well are refining their ideas to incorporate evidence from psychophysical studies of timing and time perception (see Bateson, this volume; Bateson and Kacelnik, 1998; Hills and Adler, 2002).

A hopeful contribution of this review is the distinction between circadian time and event timing. Circadian timers are temperature compensated, while event timers are most likely not. Circadian timers do not appear useful for recording event intervals that deviate from the 24-h LD cycle, whereas event timer accuracy appears to be a function of the linear increment in duration (Weber's law). Vertebrate circadian and event timers also seem to be isolated to different areas of the nervous system, but this remains to be corroborated in invertebrates. Circadian time also affects the variance in event timer responses, but not the duration. These distinctions provide us with a basis for understanding the relationship between the two mechanisms and for understanding why, in an adaptive sense, an animal is biased toward certain environmental stimuli and patterns of behavior that follow geophysical rhythms.

I have also presented possible mechanisms for event timers that are in agreement with the available evidence from psychophysics and neurophysiology. Time fields are an analogous structure to space fields and may operate by the same mechanism. Molecular decay timers are presented as an explanation for essentially all timed events (as the hippocampus contains decay timers, potentially in the form of ion channel conformation changes). The difference between decay timers and time fields may simply be in the number of cells used and the allocation of receptors to specific sensory cues. This difference may also explain failures to show evidence for event timers in invertebrates, as a consequence of sensory bias.

The problem with event timers is one that is shared by much of the literature on the adaptive value of cognitive mechanisms. We do not yet understand the costs of nervous tissue (Aiello, 1997). Costs associated with changes or new development of nervous tissue are still far from quantified. Genetic perturbations may be bringing us closer to this, but the distributed nature of nervous tissue will presumably confound our efforts for some time. In the SCN example, a portion of the brain was removed with no observable effect over a 2-year span. This suggests that a trait of nervous systems in general may be their plasticity. The costs and adaptive value of phenotypic plasticity, even though animal event timing is one of them, are still very much in the dark. Why not perceive everything, record perfect memories of it all, and be able to tell me to the minute (without looking at a clock) how long you have been reading this?

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