Abstract
Ants use a great variety of recruitment methods to forage for food or find new nests, including tandem running, group recruitment and scent trails. It has been known for some time that there is a loose correlation across many taxa between species-specific mature colony size and recruitment method. Very small colonies tend to use solitary foraging; small to medium sized colonies use tandem running or group recruitment whereas larger colonies use pheromone recruitment trails. Until now, explanations for this correlation have focused on the ants' ecology, such as food resource distribution. However, many species have colonies with a single queen and workforces that grow over several orders of magnitude, and little is known about how a colony's organization, including recruitment methods, may change during its growth. After all, recruitment involves interactions between ants, and hence the size of the colony itself may influence which recruitment method is used—even if the ants' behavioural repertoire remains unchanged. Here we show using mathematical models that the observed correlation can also be explained by recognizing that failure rates in recruitment depend differently on colony size in various recruitment strategies. Our models focus on the build up of recruiter numbers inside colonies and are not based on optimality arguments, such as maximizing food yield. We predict that ant colonies of a certain size should use only one recruitment method (and always the same one) rather than a mix of two or more. These results highlight the importance of the organization of recruitment and how it is affected by colony size. Hence these results should also expand our understanding of ant ecology.
Highlights
Many organisms rely on strength in numbers [1]
If we consider ants that are capable of more than one recruitment method, can we predict which recruitment method is likely to be used? Does the recruitment method that ants use result from a relationship between colony size and reliability of recruitment, defined as the per capita probability for a recruitment act to fail? (Note that we are not considering foraging efficiency here, but merely ‘recruitment competitiveness’, i.e., how one recruitment strategy compares to another when both are competing for recruits.) Are different recruitment methods likely to be mutually exclusive at any given colony size? We explore these issues with suitable mathematical models
N As colony size increases, ants should change from solitary foraging to pheromone recruitment, possibly with tandem running and/or group recruitment as an intermediate stage
Summary
Many organisms rely on strength in numbers [1]. Seabirds often nest synchronously and in huge colonies to reduce the chances that their offspring will be killed by predators [2]; wildebeest mass together, to reap similar selfish benefits, during migration across the African plains [3]. Animals cooperating with one another to increase their collective chances of survival may need to regulate how to share information such as the location of food. The reliability of such protocols often depends on the number of individuals involved [4]. Colonies of bacteria consist of such large numbers that they can rely on anonymous chemical signals to aggregate and form spawning bodies to reproduce [5] Behavioural displays such as those used by wolves, dolphins or monkeys are only effective when they are performed in front of conspecifics, and in relatively small groups (see [6,7], and many references therein). One taxon displaying tremendous variation both in colony sizes (over six orders of magnitude) and internal organization are the ants
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