Temporal Activity Patterns in Two Competing Ant Species (Hymenoptera: Formicidae)

Most ant species are known to exhibit some degree ot patterning in oraging rate. Interspecific dif/erences in ]oraging rate can be noted both temporally and, in temperate latitudes, seasonally. Such dif/erences can contribute to an effective partitioning o resources among coexisting species. The ollowing seto observations documents a difference in oraging pattern that may be the significant component of coexistence ot the two closely competing species observed. The system will be described, and possible implicati.ons ot the observations will be discussed. The species observed wereDorymyrmex antarcticus and Tapinoma antarcticum , both members o the subfamily Dolichoderinae. Observations were made at Fundo Santa Laura, near Til Til, Santiago Province, Chile, during October and November o 97 and 972. The site was at ,ooo rn elevation on the east-acing slope o the low coastal cordillera. Vegetation was mixed shrubs and annuals orming the community known as matorral, which is characteristic o the Mediterranean climate zone o central Chile. On visits to the site in 97 I noted that the two species compete strongly or baits. A bait o honey on a small wad o cotton would attract Dorymyrmex antarcticus workers when it was placed on the ground in the early morning. These workers recruited a. small number o nestmates, and activity would continue until midmorning when the first ew workers o Tapinoma antarcticum appeared. These workers, once they located the bait, would quickly recruit many ot their nestmates, and the many small T. antarcticum workers aggressively repelled the ewer, la.rger, less aggressive workers o

Province, Chile, during October and November o 97 and 972. The site was at ,ooo rn elevation on the east-acing slope o the low coastal cordillera. Vegetation was mixed shrubs and annuals orming the community known as matorral, which is characteristic o the Mediterranean climate zone o central Chile.
On visits to the site in 97 I noted that the two species compete strongly or baits. A bait o honey on a small wad o cotton would attract Dorymyrmex antarcticus workers when it was placed on the ground in the early morning. These workers recruited a. small number o nestmates, and activity would continue until midmorning when the first ew workers o Tapinoma antarcticum appeared.
These workers, once they located the bait, would quickly recruit many ot their nestmates, and the many small T. antarcticum workers aggressively repelled the ewer, la.rger, less aggressive workers o  D. antarcticus. The Tapinoma workers continued on the bait throughout the midday, whereas the Dorymyrmex workers retreated to their nests, apparently to avoid the high midday temperatures.
Late in the aternoon, as temperatures dropped, the Tapinoma workers would retreat to their nest leaving the bait to be reoccupied by Dorymyrmex workers. Activity at the bait seemed in general to parallel the apparent activity patterns ot? the species, which I illustrated as in Figure I. The apparent tolerance of cooler temperatures granted an exclusive period of toraging activity to D. antarcticus; aggressive dominance yielded foraging success for T. antarcticum during the period when both species were active simultaneously.
In 972 I documented these patterns quantitatively. Closely adjacent nests of each species were chosen, and counts of workers passing the nest entrance during a two-minute period each half-hour were tallied with hand counters. Soil surface temperatures were monitored with a bulb-type thermometer placed touching the soil near the nests.
Solar intensity values were taken from a pyroheliometer (Belfort Inst. Co.) stationed 5o m from the study nests. The pattern documented for one nest of each species on 4 October is illustrated in Figure 2a.
On the following day I returned to the same nests to document the early morning activity missed the preceding day. The patterns for 5 October are shown in Figure 2b. The day was cloudy and cool, and differences in foraging activities of the ants are of interest. On 19 October I tested this hypothesis by using a Thermos Brand Space Blanket (a highly reflective material) to shade a nest of D. antarcticus on a hot, sunny day. Figures 3a and 3b show activity patterns at two neighboring nests of similar size. Workers of the unshaded nest illustrate the expected pattern, but workers from the shaded nest continued foraging throughout the day. Figures 4a   and 4b show a similar test with T. antarcticum. Figure 4a illustrates activity of a single colony on November; Figure 4b is the same nest, shaded, on 2 November. The shading yielded a marked reduction in foraging rate. The late afternoon peak is of workers carrying larvae and pupae and emigrating to a new nest site under unshaded stones about m from the shaded site.

Discussion
The two species studied have similar food and foraging site preferences. The observed differences in temporal oraging pattern are hypothesized to contribute significantly to the species' coexistence. Soil surface temperature seems to be the proximate factor by which the foragers regulate their activity. Endogenous activity rhythms, often keyed to environmental factors, are known for many species. This study illustrates how variation in environmental factors can yield alteration of the activity rhythms.
During the course of a year at this temperate latitude (33 S) it seems probable that Dorymyrmex antarcticus would have more foraging hours availabIe to it, especially during spring, fall, and winter days, than would Tapinoma antarcticum. Perhaps the aggressive dominance o,f Tapinoma is a necessary requisite for survival in competition with other species. The rapid recruitment of a large number of rather small workers in Tainoma may illustrate components of a foraging strategy that must yield successful foraging returns during activity periods that are more limited than those of a coexisting competitor.