Colby J. Tanner

1. Resource characteristics and competitive pressure can affect an ant colony’s foraging strategy. This study examined the ability of the wood ant Formica integroides to respond, at both the colony and individual levels, to changes in competitive pressure for access to terrestrial and arboreal resources. 2. Because foraging behaviours depend on resource characteristics, foraging for different resource types (e.g. terrestrial and arboreal habitats) produces different spatial or territorial arrangements. In this study, terrestrial contests for resources followed an interference‐exploitation tradeoff, while arboreal foragers defended entire trees as absolute territories. 3. Competitive pressure for access to arboreal resources was shown to increase with distance from F. integroides nests. 4. In this study, the ability of F. integroides to defend a resource varied with body size. Large foragers were better defenders than small foragers. For groups of foragers, the ability to defend a resource increased with the ratio of large to small foragers. 5. In response to competitive pressure, F. integroides colonies altered the size distribution of arboreal, but not terrestrial, foragers. An increase in competitive pressure was matched by an increase in the number of large foragers allocated to trees. This response to competition affected the relationship between body size and distance from the nest for arboreal foragers. 6. Foraging behaviours for individual arboreal foragers also varied with competitive pressure. As competition increased, large arboreal foragers spent more time in direct contact with the resource rather than standing between resource patches.

Laboratory cyclomorphosis experiments with crustacean zooplankton often fail to produce morphological trait expression as extreme as that observed in field populations. A seven-generation laboratory experiment found that the maximum expression of relative head area in a round-helmet morph of Daphnia mendotae was delayed until the third generation of non-lethal, chemical exposure to Leptodora kindtii. By the third generation and after, relative head area remained high and comparable in degree to field specimens. The induction pattern from parents to offspring was statistically positive into the fourth generation. Published studies on kairomone induction of morphological trait expression extending only one generation may fail to measure transgenerational effects, which could account for mismatch between laboratory and field results. In a second laboratory experiment with a pointed-helmet morph of D. mendotae exposed to L. kindtii and Chaoborus punctipennis, we show that induction levels are not the same for all predators or combinations of predators. The maximum relative head area for pointed morph individuals exposed to both predator cues was still not as extreme in the laboratory as in the field. This is the first demonstration that delayed generational effects of kairomone induction of helmet expression in the Cladocera can reconcile laboratory experiments with the field observations of cyclomorphosis.

The relationship between numerical advantage and competitive ability is a fundamental component in contests between groups of social animals. An individual's ability to correctly assess the numerical state of its group is of vital importance. In addition to numerical dominance, the group's fighting ability also plays an important role in competitive interactions. By staging experimental fights between two Formica ant species, I show that Formica xerophila are able to assess their own group's strength prior to any competitive encounter. Ants that perceive themselves as part of a large group act more aggressively toward a competitor than ants that perceive themselves as isolated individuals. This increase in aggression improves F. xerophila 's competitive ability. Furthermore, the number of individuals in a contest was found to affect competitive ability. In contests with equal number of competitors, groups of F. xerophila were more successful than individual F. xerophila . Contrary to previous predictions using Lanchester's laws of fighting, F. xerophila 's ability to kill competitors increased nonlinearly with group size. This nonlinearity was due to the collective fighting strategy of an F. xerophila group isolating and engaging a single Formica integroides competitors.