J. H. Hunt.
Temporal Activity Patterns in Two Competing Ant Species (Hymenoptera: Formicidae).
Psyche 81(2):237-242, 1974.

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TEMPORAL ACTIVITY PATTERNS IN
TWO COMPETING ANT SPECIES
(HYMENOPTERA: FORMICIDAE)
BY JAMES H. HUNT
Dept. of Zoology, Univ. of California
Berkeley, CA 94720~
Most ant species are known to exhibit some degree of patterning in foraging rate. Interspecific differences in foraging rate can be noted both temporally and, in temperate latitudes, seasonally. Such differences can contribute to an effective partitioning of resources among coexisting species. The following set of observations documents a difference in foraging pattern that may be the significant component of coexistence of the two closely competing species observed. The system will be described, and possible implications of the observations will be discussed.
The species observed were Dorymyrmex antarcticus and Tapinoma antarcticum3, both members of the subfamily Dolichoderinae. Obser- vations were made at Fundo Santa Laura, near Ti1 Til, Santiago Province, Chile, during October and November of 1971 and 1972. The site was at 1,000 m elevation on the east-facing slope of the low coastal cordillera. Vegetation was mixed shrubs and annuals forming the community known as matorral, which is characteristic of the Mediterranean climate zone of central Chile. On visits to the site in 1971 I noted that the two species, compete strongly for baits. A bait of honey on a small wad of cotton would attract Dorymyrmex antarcticus workers when it was placed on the ground in the early morning. These workers recruited a small number of nestmates, and activity would continue until midmorning when the first few workers of Tapinoma antarcticum appeared. These workers, once they located the bait, would quickly recruit many of their nestmates, and the many small T. antarcticum workers aggressively repelled the fewer, larger, less aggressive workers of 'A Contribution of the Structure of Ecosystems Subprogram, International Biological Program.
"Present address: Dept. of Biology, University of Missouri, St. Louis, Missouri 63121.
Generic placement of this species in Tapinoma is almost certainly in- correct. Ecologically, the species is similar to Iridomyrmex pruinosum of the southwestern U. S.
Manuscript received by the editor May 2, 1974.



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Psyche
Time of Day
Figure 1. Activity patterns of the two ant species as observed in 1971. Solid line == solar intensity; dotted line == Tapinoma antarcticum foraging activity; dashed line == Dorymyrmex antarcticus foraging activity. D. antarcticus. Th'e 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 afternoon, as temper,atures 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 of the species, which I illus- trated as in Figure I. The apparent tolerance of cooler temperatures granted an exclusive period of foraging activity to D. antarcticus; aggressive dominance yielded foraging success for T. antarcticum during the period when both species were active simultaneously. In 1972 I documented these patterns quantitatively. Closely adja- cent 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 150 m from the study nests. The pattern docu- mented 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. The D. antarcticus continued active all day. The T. antarcticum workers emerged later and were out in fewer numbers than on the



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19741 'Hunt - Competing Ant Species 239
Hour of day
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Figure 2.
Activity patterns of the two ant species as documented in 1972; A ==: 4 October, B ==: 5 October. Triangles = Dorymyrmex antarcticus; circles ==: Tapinoma antarcticum (both in ants per minute passing nest opening) ; dotted line = soil surface temperature ( O C ) ; dashed line = solar intensity (cal/cmYmin).




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Figure 3. Activity patterns of Dorymyrmex antarcticus on 19 October with and without experimental shading of nests; A = an unshaded nest, B = a shaded nest.
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242 Psyche [June
preceding day, with activity peaks corresponding to peaks of soil warmth. It seemed that foraging rates of the species were mediated by soil surface temperatures.
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 illus- trates activity of a single colony on I 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 I m from the shaded site. DISCUSSION
The two species studied have similar food and foraging site prefer- ences. The observed differences in temporal foraging 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 (33O S) it seems probable that Dorymyrmex antarcticus would have more foraging hours available to it, especially during spring, fall, and winter days, than would Tapinoma antarcticum. Perhaps the aggres- sive dominance of Tapinoma is a necessary requisite for survival in competition with other species. The rapid recruitment of, a large number of rather small workers in Tapinoma may illustrate com- ponents of a foraging strategy that must yield successful foraging returns during activity periods that are more limited than those of a coexisting competitor.
ACKNOWLEDGMENTS
This research has been supported by NSF grant GB 31195 to R. K. Colwell. This paper is a contribution from the Structure of Ecosystems Subprogram, International Biological Program. I thank R. R. Snelling, who taxonomically determined the species studied, and E. Reid, who prepared the illustrations. For review of the manuscript I am grateful to R. K. Colwell, B. Holldobler and E. 0. Wilson.




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