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Ant communities in Vermont and New York woods were sampled in four time periods to determine species composition, relative abundances, and nest locations in space. The Vermont community was richer, containing more species and higher nest densities than New York. Both communities followed the geometric distribution of species abundances, suggesting that a single resource was mediating competition. The resource most clearly implicated was suitable nest sites, principally pre-formed plant cavities. Nonrandom species associations, underdispersion in every season, and the occurrence of incipient nests overwintering aboveground all implicated shortage of such cavities. Furthermore, microhabitat differences which produce suitable nest sites occur over a very small scale in these communities.

Slavemaking ants are of great interest to biologists because of their highly specialized lifestyle. Slavemakers rely on the presence of heterospecific ''slaves'' to perform routine chores such as foraging and brood care. While we have considerable information on the behavior of these ants, mostly gleaned from laboratory studies, we know very little of their basic ecology. Here we report data collected over 20 yr in three geographic sites on the occurrence, spatial pattern, nest site use, and demography of two slavemaker species and three of the host species they enslave; all are small ants in the tribe Formicoxenini. We found geographic and temporal variation with respect to the interaction between social parasites and their hosts. The slavemaker Protomognathus americanus occurred in each geographic site, and in each its most common host was Leptothorax longispinosus; the other known hosts of this slavemaker, L. curvispinosus and L. ambiguus were less abundant. In a West Virginia site, slavemaker nests were smaller than in New York and Vermont sites. Additional data from West Virginia showed that the slavemaker is strongly constrained by its hosts there, but we found little impact of the parasite on its hosts. By contrast, in the New York site, we found strong evidence that the majority host was affected by the slavemaker: host nests showed important life history shifts between neighborhoods containing slavemakers vs. those that were parasite-free. Furthermore, the slavemaker was able to subjugate large host nests there and had large slave pools. In a third site in Vermont, parasite and host demography reflected their interaction weakly. We discovered a second slavemaking species there, and both parasites were preferentially found in areas of high host density. However, we uncovered no demographic or spatial evidence that slavemakers affected their hosts in Vermont. Likewise, we found no evidence in any site of demographic responses by the minority hosts to the presence of slavemakers. Thus our results imply that host-parasite interactions varied among host species and among geographic locales. The slavemaker system therefore is amenable to analysis in terms of parasite-host coevolution.

Colony structure and reproductive investment were studied in a population of Myrmica punctiventris. This species undergoes a seasonal cycle of polydomy. A colony overwinters in entirety but fractionates into two or more nest sites during the active season and then coalesces in the fall. Colony boundaries were determined by integrating data on spatial pattern, behavioral compatability, and genetic relatedness as revealed by protein electrophoresis. Colonies contained at most one queen. Consequently, a colony consisted of one queenright nest and one or more queenless nests. Furthermore, estimates of relatedness were fully consistent, with queens being single mated. M. punctiventris therefore has a colony genetic structure that conforms to the classical explanation of the maintenance of worker sterility by kin selection. Kin selection theory predicts that workers would favor a female-biased allocation ratio while selection on queens would favor equal investment in males and females. We predicted that in polydomous populations, queenless nests would rear more female reproductives from diploid larvae than queenright nests. There was a significant difference between queenright and queenless nests in sexual allocation; queenless nests allocated energy to reproductive females whereas queenright nests did not. At neither the nest nor colony levels did worker number limit sexual production. We also found that nests tended to rear either males or females but when colony reproduction was summed over nests, the sexes were more equally represented. The difference in allocation ratios between queenless and queenright nests was attributed solely to queen presence/absence. Our work shows that polydomy provides an opportunity for workers to evade queen control and thereby to sexualize brood.

Investigations of time budgets reveal that for many animals a surprising proportion of their active time is spent in inactivity. The question of why these beasts are often idle is investigated by examining their foraging behavior in a model which does not utilize optimization criteria. If an organism's goal is to stay alive, one satisfactory strategy is a thermostat feeding process whereby the animal initiates foraging when it perceives hunger and ceases when it becomes satiated. The simple model is formulated as a Markov chain and analyzed for three cases. Results from each case predict that for many combinations of activity levels and resource spectra, the time spent looking for food is smaller than the time spent not foraging, and laziness may result. Simple decision rules as well as optimization schemes are therefore useful for studying some types of foraging behavior.

For social insect species, intraspecific variation in colony social structure provides an opportunity to relate the evolution of social behavior to ecological factors. The species Myrmica punctiventris is a cavity-dwelling forest ant that exhibits very different colony structures in two populations in the northeastern United States. Combined data from seasonal censuses, allozyme electrophoresis, and worker hostility tests showed that a population of M. punctiventris in Vermont was strictly monogynous and seasonally polydomous. The same procedures showed that a population of M. punctiventris in New York was facultatively polygynous and predominantly monodomous. Genetic relatedness among colony-mates was not different from Hamilton's expected values in the Vermont population and was consistent with little exchange of ants between colonies and single-mating of queens. In contrast, relatedness was lower in New York, and examination of nest-mate genotypes revealed exchange of ants between colonies, high rates of colony loss and replacement of queens, or multiple-mating of queens. The genetic structure of the Vermont population was consistent with no inbreeding, but in New York, the population genetic structure reflected microgeographic subdivision and inbreeding. Previous study of the ant communities at these sites implicates nest-site limitation in New York as a primary constraint on social structure.

Abstract. The parasite pressure exerted by the slavemaker ant Protomognathus americanus on its host species Leptothorax longispinosus was analyzed demographically and genetically. The origin of slaves found in colonies of the obligate slavemaker was examined with nuclear and mitochondrial DNA markers to make inferences about the frequency and severity of slave raids. Relatedness of enslaved L. longispinosus workers in the same nest was very low, and our data suggest that, on average, each slavemaker nest raids six host colonies per season. Therefore, the influence of slavemaker species on their hosts is much stronger than simple numerical ratios suggest. We also found that slave relatedness was higher in small than in large slavemaker nests; thus, larger nests wield a much stronger influence on the host. We estimated that in the study population, on average, a host nest has a 50% chance of being attacked by a slavemaker colony per year. Free-living Leptothorax colonies in the vicinity of slavemaker nests did not represent the source of slaves working in P. americanus colonies, which suggests that raided nests either do not survive or migrate after being raided. Colony composition and intranest relatedness of free-living L. longispinosus colonies differed markedly between areas with slavemakers and those that are parasite-free. In the presence of slavemakers, host colonies were less likely to be polygynous and had fewer workers and a higher relatedness among worker brood. Host nests with slavemaker neighbors allocated more resources into sexuals, possibly caused by these shifts in nest demography. Finally, enslaved Leptothorax workers in P. americanus nests appeared to be less efficient than their counterparts in free-living colonies. Thus, slavemakers exert a much stronger impact on their hosts than had previously been suspected and represent an unique system to study parasite-host coevolution.Key words. Coevolution, microsatellites, parasite-host interaction, raiding behavior, social insects, social parasitism.Received December 21, 1999. Accepted September 18, 2000.Slavemaking ants have intrigued biologists because of their highly specialized behavior and life history, and the species Protomognathus americanus exemplifies that specialization. A P. americanus slavemaking colony is initiated when a newly mated queen invades a host colony, which in this case is one of three Leptothorax species; the slavemaker queen then proceeds either to drive out or to kill the adult workers and resident queen(s) of the invaded colony (Wesson 1939). Invasions occur in mid to late summer, when host colonies contain worker pupae. After invading, the slavemaker queen waits a few weeks until the pupae emerge. These ants as adults start to forage, care for brood, and engage in other tasks necessary for colony maintenance. Shortly after a takeover, then, Leptothorax workers are available to help the P. americanus queen raise her own offspring. Typically, the slavemaker queen produces one or two daughter workers in her first year. Thes...

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