Interval Timing And Foraging

So why do animals need the ability to time intervals? Although timing seems a generally useful ability, are there specific behavioral problems faced by animals that are particularly likely to have selected for interval timing abilities? In trying to answer these questions, I am going to argue that several lines of evidence point to the likelihood that interval timing is of major importance in foraging behavior (see Hills, this volume).

Interval timing has been identified in the majority of vertebrate species in which the ability has been investigated (Lejeune and Wearden, 1991), and it is probably safe to assume that the ability is universal in the vertebrates (Bateson, 2001). This implies that interval timing is an evolutionarily ancient ability, and in searching for behavioral problems that might have specifically selected for timing, we need to identify very general problems that are faced by all vertebrate animals and are likely to have also been faced by their common ancestors. All animals need to find food, making foraging behavior a potential candidate for a general problem that might have selected for the ability to time.

Warm-blooded vertebrates such as small mammals and birds have relatively high metabolic rates, the maintenance of which requires large quantities of food on a regular basis. This need is assumed to have imposed a strong selective pressure to produce efficient foraging behavior. Birds in particular have been at the forefront of studies of optimal foraging because the high energetic demands of flight have meant that birds need to ingest particularly large quantities of food of high nutritive value. A small bird may need to eat its body weight in food per day. A blue tit (Parus caeruleus) weighing 11 g needs 11 kcal per day in winter, which is equivalent to around 300 small insects, and a rufous hummingbird (Selasphorus rufus) weighing 3.5 g will visit a nectar feeder approximately every 10 to 15 min from dawn until dusk. These high rates of foraging coupled with the fact that most birds forage only during the hours of daylight make birds attractive subjects for studies of foraging behavior.

Arthropods are the most common food consumed by small mammals and birds, and they bring with them various interesting foraging problems because the distribution of arthropod prey in the environment is usually both spatially and temporally patchy. For example, for much of the year European starlings forage on leatherjack-ets, the larvae of tipulid flies. Leatherjackets are hidden beneath the surface of the soil and occur in patches. Starlings search for leatherjackets by probing the soil with their bills. Given that it is impossible to visually assess the density of leatherjackets in a particular location, starlings rely on their cognitive abilities to form estimates of the rates of intake they have experienced in different locations. These estimates can then be used as a basis for making future foraging decisions.

It is possible to analyze all foraging behavior in terms of its costs and benefits to the forager. Finding, consuming, and digesting food all have both energetic and time costs associated with them, because time and energy spent foraging are time and energy taken away from other fitness-promoting activities, such as looking out for predators and reproducing. We therefore expect natural selection to have honed foraging decisions so as to optimize the trade-off between costs and benefits, and thus maximize the lifetime survival and reproductive success of the forager (e.g., Stephens and Krebs, 1986). Because the costs associated with foraging involve the length of time taken, it is likely that selection on foraging decisions has involved selection on the ability to measure these costs accurately.

The final piece of evidence linking timing with foraging comes from the observation that the majority of the comparative evidence for interval timing comes from animals performing on fixed-interval (FI) schedules of reinforcement in which food is used as the reinforcer. In a typical free-operant FI schedule, reinforcement, usually the delivery of a small amount of food, is contingent on a response made by the subject after some fixed period of time has elapsed. The interfood interval serves as the only discriminative stimulus, and the interval requirement is reset after each food reinforcement is delivered. The optimal strategy in a subject trying to maximize the frequency with which it receives food while minimizing the number of responses it has to make when faced with such a schedule is to time the fixed interval and make a single response as soon as the interval has elapsed. Although well-trained subjects never achieve this optimal strategy, they do show a postreinforcement pause that averages about two thirds of the FI value, after which they start responding at a high rate until food is delivered (e.g., Schneider, 1969). Thus, we have good evidence that animals are able to time interfood intervals and also that the delivery of food can serve to reset the animal's interval timer (Matell and Meck, 1999).

In summary, therefore, we have established that (1) all vertebrates need to forage, (2) foraging behavior is likely to have been under strong selective pressure to increase efficiency, (3) efficient foraging involves making decisions that involve timing, and (4) animals can time intervals between food deliveries and that food can reset the clock. Taken together, I suggest that the above evidence points strongly toward the possibility that interval timers may play a major role in natural foraging behavior. Of course, none of the above evidence proves that interval timers initially evolved for the purposes of foraging, or that interval timers are used solely for the purposes of foraging. However, the likely fitness consequences of inefficient foraging do suggest that the selection pressures to improve the efficiency of foraging are very likely to have been an important force in the evolution of the interval timing clock.

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