The dissolution of cooperative groups: mechanisms of queen mortality in incipient fire ant colonies

In several species of ants, queens often form temporary cooperative associations during colony foundation. These associations end soon after the eclosion of the first workers with the death or expulsion of all but one of the queens. This study examined competition between foundress queens of the fire ant Solenopsis invicta. Although attacks by the workers contributed to queen mortality, queens gained no advantage by producing more workers than their co-foundresses. Restriction fragment length polymorphism analysis of mitochondrial DNA showed that the queen producing more workers during colony founding was no more likely to survive than the less productive queen. In experimentally manipulated colonies in which all the workers were daughters of only one of the queens, the mother of the workers was no more likely to survive than the unrelated queen. Queens producing diploid males reared fewer offspring but were as likely to survive as queens producing only workers. These results suggest that workers do not discriminate between related and unrelated queens within colonies. Aggressive encounters between queens were common. Queens were more likely to die or be expelled if paired with heavier queens or if they lost more weight than their co-foundress during the claustral period. Finally, when queens were separated by screens through which workers could pass, the workers usually attacked and killed the queen farther from the brood. These results suggest that queen survival is promoted by a high fighting ability relative to co-foundresses, rather than by increased worker production, and that workers respond to queen differences that are independent of kinship. Received: 8 September 1995/Accepted after revision: 5 March 1996     

Ecology,feeding competition and social structure in baboons

Predictions of the model of van Schaik (1989) of female-bonding in primates are tested by systematically comparing the ecology, level of within-group contest competition for food (WGC), and patterns of social behaviour found in two contrasting baboon populations. Significant differences were found in food distribution (percentage of the diet from clumped sources), feeding supplant rates and grooming patterns. In accord with the model, the tendencies of females to affiliate and form coalitions with one another, and to be philopatric, were strongest where ecological conditions promoted WGC. Group fission in the population with strong WGC was “horizontal” with respect to female dominance rank, and associated with female-female aggression during a period of elevated feeding competition. In contrast, where WGC was low, females’ grooming was focused on adult males rather than other females. Recent evidence suggests that group fission here is initiated by males, tends to result in the formation of one-male groups, and is not related to feeding competition but to male-male competition for mates. An ecological model of baboon social structure is presented which incorporates the effects of female-female competition, male-male competition, and predation pressure. The model potentially accounts for wide variability in group size, group structure and social relationships within the genus Papio. Socio-ecological convergence between common baboons and hamadryas baboons, however, may be limited in some respects by phylogenetic inertia. Received: 22 April 1994/Accepted after revision: 9 December 1995     

Individual experience-based foraging can generate community territorial structure for competing ant species

To gain additional territory while defending existing territory, animals must acquire and use information regarding resource characteristics and competitive pressure. For social organisms like ants, individual workers have experiences to acquire information, but territory establishment is a colony level behavior. Colony behavior, in turn, affects community structure. Here, I investigate how an individual ant’s previous experience affects its future foraging behavior and how individual behaviors can scale up to community territorial structure for two coexisting Formica species. To do this, I combine a field survey, a multi-agent computer simulation, and a manipulation experiment. The field survey shows that workers of both species co-occur on many trees early in the season, but ants on trees become segregated by species as the season progresses. The simulation demonstrates how this segregated spatial distribution can result from ants using a foraging strategy in which individuals show a preference for foraging sites based on previous experience. The experiment suggests that these ants are indeed capable of experience-based foraging behavior; ants preferentially return to sites where they have had positive experiences and avoid sites where they have had negative experiences. Results from this study suggest that spatially explicit information can be collected and stored by individuals to facilitate colony territorial structure, and that future investigations of community territory formation should include effects of individual previous experience.     

What enables coexistence in plant communities? Weak versus strong species traits and the role of local processes

Explaining the coexistence of species that basically depend on the same resources has been a brainteaser for generations of ecologists. Different mechanisms have been proposed to facilitate coexistence in plant communities, where space is an important resource. Using a stochastic cellular automaton simulation model we analyze - separately and in combination - the influence of different species traits and processes which alter local competition on the coexistence of plant species over a fixed time horizon. We show that different species traits operate on different time scales in competition. We therefore suggest the concept of weak versus strong traits according to short- or long-term exclusion of species differing in these traits. As a consequence, highly non-linear trade-offs between weak and strong traits can result in communities. Furthermore, we found that trade-offs based on physiological species traits such as plant lifetime, dispersal range and plant growth, did not support broad and long-term coexistence—further processes such as density-dependent mortality and light-dependent colonization were necessary. This suggests that coexistence in plant communities requires (stabilizing) local processes to support the (equalizing) trade-offs in species traits.     

Evidence of behavioral co-option from context-dependent variation in mandible use in trap-jaw ants (Odontomachus spp.)

Evolutionary co-option of existing structures for new functions is a powerful yet understudied mechanism for generating novelty. Trap-jaw ants of the predatory genus Odontomachus are capable of some of the fastest self-propelled appendage movements ever recorded; their devastating strikes are not only used to disable and capture prey, but produce enough force to launch the ants into the air. We tested four Odontomachus species in a variety of behavioral contexts to examine if their mandibles have been co-opted for an escape mechanism through ballistic propulsion. We found that nest proximity makes no difference in interactions with prey, but that prey size has a strong influence on the suite of behaviors employed by the ants. In trials involving a potential threat (another trap-jaw ant species), vertical jumps were significantly more common in ants acting as intruders than in residents (i.e. a dangerous context), while horizontal jumps occurred at the same rate in both contexts. Additionally, horizontal jump trajectories were heavily influenced by the angle at which the substrate was struck and appear to be under little control by the ant. We conclude that while horizontal jumps may be accidental side-effects of strikes against hard surfaces, vertical escape jumps are likely intentional defensive behaviors that have been co-opted from the original prey-gathering and food-processing functions of Odontomachus jaws.