Any organism has a limited amount of time, matter, and energy available to devote to foraging, growth, maintenance, and reproduction.
The way in which an organism allocates these resources among various conflicting demands is of fundamental interest. This apportionment determines the ways in which the organism can conform too many aspects of its environment and thus indicates a great deal about its ecological niche.
Time and energy budgets vary widely among organisms; for example relatively r-selected organisms, allot more time and energy to reproduction at any instant than do more K-selected ones. Varying time and energy budgeting is a potent means of coping with a changing environment while retaining some degree of adaptation to it.
Thus a male marsh wren expends a great deal of energy on territorial defense during the breeding season but little at other times of the year. Similarly, in animals with parental care, an increasing amount of energy is spent on growing offspring until some point when progeny begin to become independent of their parents, whereupon the amount of time and energy devoted to them decreases.
Indeed, adult female red squirrels. Tamiasciurus, at the height of lactation consume an average of 323 kcal of food per day, whereas the average daily energy consumption of an adult male is only about 117 kcal (C. Smith, 1968); time budgets of these squirrels also vary makedly with the seasons. In a bad dry year, many annual plants “go to seed” while still very small, whereas in a good wet year, these plants grow to a much larger size before becoming reproductive; presumably more seeds are produced in good years, but perhaps more (or very few) would be produced in a bad year if individuals grew to the sizes they reach in good years.
Organisms can be viewed as simple input-output systems with foraging or photosynthesis providing an input materials and energy which are in turn “mapped” into an output consisting of progeny. Fairly extensive bodies of theory now exist both on reproductive tactics and on optimal foaging.
In optimal foraging theory, the “goal” usually assumed to be maximized is energy uptake per unit time (successful offspring produced during an organism’s lifetime would be a more realistic measure of its foraging ability, but fitness is exceedingly difficult to measure).
Similarly, among organisms without parental care, reproductive effort has sometimes been estimated by the ratio of calories devoted to eggs or offspring over total female calories at any instant (rates of uptake versus expenditure of calories have unfortunately not yet infiltered empirical studies of reproductive tactics).
To date, empirical studies of resources partitioning and niche structure have been concerned largely with “input” phenomena such as overlap in and efficiency of resource utilization and have neglected to relate these to “output” aspects.
In contrast, empirical studies of reproductive tactics have done the reverse and almost entirely omitted any consideration of foraging. Interactions and constraints between foraging and reproduction have barely begun to be considered. A promising area for future work will be to merge aspects of optimal foraging with optimal reproductive tactics to specify rules by which input is translated into output; optimal reproductive tactics (“output phenomena) surely must often impose substantial constraints upon “input” possibilities.
An animal’s time and energy budget provides a convenient starting point for clarifying some ways in which foraging influences reproduction and vice versa. Any animal has only a certain finite period of time available in which to perform all its activities, including foraging and reproduction.
This total time budget, which can be considered either on a daily basis or over the animal’s lifetime, will be determined both by the diurnal rhythm of activity and by the animal’s ability to “make time” by performing more than one activity at the same time (such as a male lizard sitting on a perch, simultaneously watching for potential prey and predators while monitoring mates and competing males).
Provided that a time period is profitable for foraging (expected gains in matter and energy exceed inevitable losses from energetic costs of foraging), any increase in time devoted to foraging clearly will increase an animal’s supply of matter and energy. Necesarily accompanying this increase in matter and energy, however, is a concomitant decrease in time available for nonforaging activities such as mating and reproduction.
Thus profits of time spent foraging are measured in matter and energy while costs take on units of time lost. Conversely, increased time spent on nonforaging activities confers profits in time while costs take the form of decreased energy availability. Hence gains in energy correspond to losses in time, while dividends in time require reductions in energy availability. (Of course, risks of foraging and reproduction also need to be considered).
The arguments above suggest that optimal allocation of time and energy ultimately depends on how costs in each currency vary with profits in the opposite. However, because units of costs and profits in time and energy differ, one would like to be able to convert them into a common currency.
Costs and profits in time might be measured empirically in energetic units by estimation of the net gain in energy per unit of foraging time. If all potential foraging time is equivalent, profits would vary linearly with costs; under such circumstances, the loss in energy associated with non-foraging activities would be directly proportional to the amount of time devoted to such activity.
Optimal budgeting of time and energy into foraging versus nonforaging activities is usually profoundly influenced by various circadian and seasonal rhythms of physical conditions, as well as those of predators and potential prey. Clearly certain time periods favourable for foraging return greater gains in energy gathered per unit time than other periods.
Risks of exposure to both harsh physical conditions and predators must often figure into the optimal amount of time to devote to various activities. Ideally one would ultimately like to measure both an animal’s foraging efficiency and its success in budgeting time and energy by its lifetime reproductive success, which would reflect all such environmental “risks”.
Foraging and reproductive activities interact in another important way: many organisms gather and store materials and energy during time periods that are unfavourable for successful reproduction but then expend these same resources on reproduction at a later, more suitable, time. Lipid storage and utilization systems obviously facilitate such temporal integration of uptake and expenditure of matter and energy. This temporal component greatly complicates the empirical measurement of reproductive effort.
Prey density can strongly affect an animal’s time and energy budget. Gibb (1956) watched rock pipits, Anthus spinoletta, feeding in the intertidal along the English seacoast during two consecutive winters. The first winter was relatively mild; the birds spent 6V4 hours feeding, l3/4 hours resting, and 45 minutes fighting in defense of their territories (total daylight slightly exceeded 9 hours).
The next winter was much harsher and food was considerably scarcer; the birds spent 8V4 hours feeding, 39 minutes resting, and only 7 minutes on territorial defense! Apparently the combination of low food density and extreme cold (endotherms require more energy in colder weather) demanded that over 90 per cent of the bird’s walking hours be spent feeding and no time remained for frivolities.
This example also illustrates that food is less def endable at lower densities as indicated by reduced time spent on territorial defense. Obviously food density in the second year was near the lower limit that would allow survival of rock pipits. When prey items are too sparse, encounters may be so infrequent that an individual cannot survive. Gibb (1960) calculated that, to balance their energy budget, during the winter in some places, English tits must find and insect on the average once every 2V2 seconds during daylight hours.
Time and energy budgets are influenced by a multitude of other ecological factors, including body size, mode of foraging, vagility, trophic level, prey size, resource density, environmental heterogeneity, rarefaction, competition, predation, and reproductive tactics.