Introduction

The natural world is full of temporal regularities, and it makes sense that animals should have evolved clocks that permit them to maximize their fitness by exploiting these regularities. However, proving that animals use internal clocks to schedule their behavior is rarely possible in natural environments. To provide proof that an animal is timing, it is necessary to eliminate any external cues to the passage of time that the animal might be using in place of an internal clock (Killeen et al.,

1997). Because this is usually difficult in natural environments, data showing that animals can time intervals come from the controlled conditions of the laboratory, where the cues available to the subjects and the temporal properties of their experience can easily be manipulated. Such laboratory experiments have traditionally been the domain of operant psychologists who typically restrict their studies to the behavior of laboratory-reared rats and pigeons tested in Skinner boxes on various schedules of reinforcement. Due to the lack of a clear connection between the behavior of rats and pigeons in the artificial environment of the Skinner box and the problems faced by wild animals in their natural environments, research on interval timing has so far failed to attract widespread interest from ethologists and behavioral ecologists who usually focus on understanding behavior patterns initially identified in wild animals. Evidence for interval timing in animals has mostly been published in the psychological literature, and it is psychologists that have been responsible for setting the agenda in research on interval timing.

As a consequence of the domination of the interval timing literature by psychologists, the focus in most timing research has been on describing the psychophysics of interval timing, with the ultimate goal of elucidating the cognitive and neural mechanisms underlying the interval timing clock (e.g., Hinton and Meck, 1997; Matell and Meck, 2000; Paule et al., 1999). For example, psychologists have paid particular attention to inaccuracy and imprecision in interval timing on the grounds that imperfections in the system can be particularly important for revealing the underlying mechanisms (e.g., Gibbon and Church, 1984). It is taken for granted that the ability to time intervals is useful to animals, and the interval timing clock has consequently been conceived of as a general-purpose stopwatch-like timepiece. A result of this approach has been that questions regarding the evolution and current function of interval timing have remained largely unasked.

In contrast to the psychological approach, ethologists believe that no account of behavior is complete until both its proximate mechanisms and its ultimate evolutionary functions are understood (Tinbergen, 1963). Skinner (1989) stated that "by looking at how a clock is built, we can explain [how] it keeps good time, but not why keeping time is important." Although Skinner was correct in recognizing that questions about mechanism and function are logically distinct, the strength of Tin-bergen's ethological approach lies in the belief that finding the answer to one type of question will often provide valuable insights into the answer to the other. Thus, although logically it is not necessary to know why the clock has evolved in order to understand how it works, there is reason to believe that considering both questions simultaneously could bring considerable benefits of understanding.

There are a number of reasons why it might be beneficial to study function and mechanism simultaneously. First, it is generally much easier to understand how a mechanism works if you know exactly what it is designed to do. By analogy, deciphering someone else's computer code is always facilitated if you know precisely what the function of the code is. Although we may feel we understand what clocks are for, it remains the case that a clock that functions to measure the intervals between successive prey captures in a foraging starling may have different design requirements than a clock that functions to measure the rate of the mating display in a male sage grouse. Second, interval timing clocks are likely to have evolved in response to specific types of temporal regularities that confront animals in their natural environments. As a result, the clock may behave very differently when it is probed with the natural stimuli that it has evolved to respond to than when it is probed with unnatural stimuli that are outside the range of variation it encountered during its evolutionary history. In other words, modeling the biological clock as a flexible stopwatch may be misleading; the biological clock may behave differently depending on what it is asked to do. Third, psychologists have described several ways in which the performance of the interval timing clock departs from perfect accuracy and precision. In order to understand the significance of these imperfections for the adaptive behavior of animals, we need to know whether the imperfections of the clock are true constraints of its mechanism or whether perhaps they are artifacts of studying the clock under conditions for which it was not designed. By analogy, if you rev a car on a surface such as ice or sand, the pair of wheels that drive the car will spin. This gives you some clear information about how the car works: you now know whether it has front- or rear-wheel drive. However, the behavior of the car on the sand is very different from what you would observe if you were to rev the same car on a road. Thus, studying timing in the lab may provide very useful insights into the mechanisms underlying the clock; however, it may be misleading if your aim is to understand the role of timing in the generation of adaptive behavior. The evolutionary process has resulted in a complex relationship between function and mechanism in biological systems, and it is only by considering both questions simultaneously that we can ever hope to understand animal behavior fully.

My aim for this chapter is to take an ethological approach to interval timing and attempt to integrate mechanistic and evolutionary approaches. I begin by asking the functional question of why animals need to be able to time intervals, and I present evidence suggesting that interval timing is likely to have a central role in foraging behavior. Next I introduce optimal foraging theory and show exactly how temporal information is important in making optimal foraging decisions. An important component of all optimal foraging models is the constraints that are assumed, and I go on to consider timing as a potential constraint. The most sophisticated attempts to integrate models of timing and foraging have come from studies that focus on scalar timing theory. I give a brief description of the basic scalar timing model and review how it has been adapted to model a range of foraging problems. Throughout, my aim is to highlight the benefits of understanding that have resulted from the combination of scalar timing with optimal foraging.

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