Ecology of the Ant Pogonomyrmex mayri: Foraging and Competition

Abstract

Aspects of recruitment and behavior of Pogonomyrmex mayri (Myrmicinae: Formicidae) are described. Foragers leave and return to the nest in all directions, but each worker tends to forage only in one sector of the territory. When food is discovered, nestmates are recruited by a trail pheromone, but even with a persistent food source, a single file of foragers never develops. Foragers rarely cooperate to transport prey. Foraging is strictly diurnal with the amount of activity controlled by temperature, which in turn is affected by season, time of day, and nest location. Strong territorial defense by neighboring colonies and competition from other ant species often severely restrict direction and distance. VIRTUALLY NOTHING IS KNOWN OF THE SOUTH AMERICAN species of the genus Pogonomyrmex. The North American Pogonomyrmex of the subgenus Pogonomyrmex are well known for living in open habitats and collecting and storing large quantities of seeds in underground granaries, and for stinging fiercely when disturbed (Wheeler 1910). It is not known to what degree the subgenera Ephebomyrmex and Forelomyrmex, which dwell primarily in South America, share those characteristics. Pogonomyrmex (Forelomyrmex) mayri s a large, velvety black ant whose entire known distribution lies at the foot of the Sierra Nevada de Santa Marta in northeastern Colombia. Kugler (1978) and Kugler and Hincapie (1983) have shown that this species differs morphologically and ecologically from its better-known relatives. Instead of forming large colonies in open desert or grassland, it forms rather small colonies usually at the bases of trees and shrubs in dry forest. It scavenges dry arthropod remains and miscellaneous plant parts by on the soil or leaf litter surface. It does not sting or store seeds. This report makes further comparisons of diurnal activity, territoriality, and recruitment. MATERIALS AND METHODS Most observations come from a study area 33 x 40 m in the forest behind Gairaca Bay of Tayrona National Park, 8 km east of Santa Marta, Colombia. A description of the study site and the means used to locate, mark, and map nests are found in Kugler and Hincapie (1983). To record the temporal patterns of activity in wet and dry seasons and in the presence and absence of competing ants, I used what will be referred to henceforth as foraging plots. These plots were 1 x 2 m rectangles outlined by string held several centimeters off the ground by small wire stakes at the corners. They were placed within a meter of a P. mayri nest entrance, and each hour the numbers of P. mayri and its most likely competitors, Odontomachus bauri and Ectatomma ruidum, were recorded using the following procedure. First, I placed two mercury thermometers on the ground near the plot, one in shade and one in full sun, and stood immobile at the edge of the plot for five minutes, during which time I estimated the percentage of the plot in direct sun. Then, each species was counted in a separate inspection of the plot, and the thermometer temperatures were recorded. Those temperatures are of course only approximations of the temperatures of the ants. A record of activity throughout the day was obtained by repeating this procedure ach hour beginning and ending in darkness. I accumulated 21 such records of diurnal activity using 10 nests in a variety of microhabitats and spanning both wet and dry seasons (November and March, respectively). Intraspecific competition in was inferred from three types of evidence: (1) mapping the limits of the territories by offering peanut chips to foragers and following them to their nests, (2) luring marked foragers from one colony to the territory ofanother colony and observing the resulting interactions, and (3) measuring territorial expansion after excavation of a neighboring nest. Mapping was accurate to within 50 cm (Kugler and Hincapie 1983). Interspecific competition was also studied by removal experiments. Twice I tested whether P. mayri limits the of E. ruidum. In each experiment, I recorded hourly the numbers of both ants in a plot located 1 m from a P. mayri entrance. Then the foragers of that colony were removed by entrapment in the nest or by excavation, and the hourly activity of E. ruidum alone was recorded. In a third experiment, I tested whether E. ruidum restricts the of P. mayri. The territory ofa P. mayri nest was mapped and the positions of all E. ruidum entrances in and around that territory were marked with small flags. The normal levels of forI Received 26 July 1983, revised 6 December 1983, accepted 13 December 1983. BIOTROPICA 16(3): 227-234 1984 227 This content downloaded by the authorized user from 192.168.52.67 on Tue, 20 Nov 2012 12:43:37 PM All use subject to JSTOR Terms and Conditions Ants 35c o 2 sq. m. I 20~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 0 0 0 ~~~~~251