The development of an ant colony’s foraging range

This study examines how an ant colony’s foraging range changes as the colony grows older, and whether colonies preserve their foraging ranges from year to year. Foraging ranges were measured in 88 colonies of known age over the course of 5 years. A seed-eating ant, Pogonomyrmex barbatus, colony lives for 15-20 years, reaching a stable size of about 12 000 workers when it is about 5 years old. On average, foraging ranges of colonies 2 years and older were similar in size, so a colony with about 2000 workers uses as much area to forage as one with 12 000. A colony’s foraging range increased most from ages 1 to 2 years, when the colony is still very small; increase of foraging area appears to be related to colony growth rather than overall colony size. From year to year, colonies did not retain or expand from a core foraging range. Instead, only about half the foraging range a colony used one summer was used by it the previous summer. However, the overlap of foraging ranges from year to year was significantly greater than expected if foraging ranges were distributed each year at random. The extent of year-to-year turnover in foraging range did not depend on colony age. Most colonies in which reproductives were observed were 5 years or older. These results are considered in the light of previous work showing that 3-4-year-old colonies, at about 75% of mature size, are more persistent in conflict with neighbours over foraging space than older, larger colonies. Effects of colony size on the rate of encounter with neighbours, and the costs of reproduction, may determine developmental changes in the territorial behaviour of an ant colony. Territory size can change during the lifetime of an individual. Such changes may result from fluctuations in resource abundance (Lott 1991). Models that predict optimal territory size usually compare the benefits available from resources on the territory with the costs of maintaining and defending a territory (Schoener 1987). These models, which link territory size and costs of defence, suggest that at a given level of resource abundance, ability to win territorial conflict can determine territory size (e.g. Davies & Houston 1981). Territory size and territorial behaviour may also change during the ontogeny of territory owners. This has been demonstrated in vertebrates (Mackintosh 1970; Nolan 1978; Tooker & Miller 1980; Michener 1981; Doncaster & Macdonald 1991) and, to a lesser extent, in invertebrates (Koenig 1990). Life-history constraints may influence the costs and benefits associated with a territory; for example, costs of reproduction appear to influence the timing of territory acquisition in the ontogeny of some cooperatively breeding birds (Stacey & Ligon 1987). A territory may have prospective value for a juvenile animal 0003-3472/95/030649+ 11 $08.0010 Cd 1995 The Association for the Study of Animal Behaviour if it can obtain a territory for use later in life, as in anole lizards (Stamps & Tollestrup 1984; Stamps 1987; Stamps & Eason 1989). Prospective value thus may contribute to the benefits of territory in young animals, and this may help to explain ontogenetic changes in territorial behaviour. Resource availability, costs of defence, and life history may all interact to influence territory size. The relationship between individual size and territory size is an example. Everything else being equal, it seems reasonable to assume that larger individuals, if they are more likely to win in territorial conflicts, should have larger territories. But this does not always hold when individuals grow larger as they grow older. Individuals may establish territories when they are young and small, while older, larger individuals devote more energy to reproduction than territorial defence. When young, small individuals value territory more than older, larger ones, the largest territories may not be occupied by the largest individuals (e.g. Stamps 1984). Ant colonies tend to grow larger as they grow older, and colony size is often related to colony