Morphological volumes, trade-offs, and biodiversity maintenance in ants

Javier Ibarra-Isassi, Biology, Concordia University, Montréal, QC, Canada, I. Tanya Handa, Département des sciences biologiques, Université du Québec à Montréal, Montréal, QC, Canada and Jean-Philippe Lessard, Department of Biology, Concordia University, Montreal, QC, Canada

Background/Question/Methods
The maintenance of diversity is still the topic of much debate and a central focus of research in ecology. Ubiquitous, and highly diverse taxa such as ants have particularly puzzled many over the past decades. Evidence suggests that differences and trade-offs in morphological, physiological, phenological and behavioural traits are at the root of biodiversity maintenance. Differences in traits arise, and are maintained through time, as a result of selection leading to niche partitioning. However, the spectrum of trait variation observed in nature is constrained by developmental and ecological trade-offs. Thus, widely beneficial, but strongly constrained trait combinations will persist across the evolutionary tree. In ants, behavioural and physiological trade-offs have been explored in the context of local species co-existence. However, whether generalized morphological trade-offs exist across the ant phylogeny, and whether such trade-off play a role in the maintenance of global ant diversity, remains unexplored. In this study, we use data on 10 morphological traits of 808 ant species distributed in the New World to (1) explore the morphological trait spectrum of ants and (2) identify major morphological trade-offs, which may relate to the life strategies of ants.
Results/Conclusions
Our results show that workers occupy a larger volume of ant morphospace than queens. However, ant queens occupy morphological trait space more uniformly distributed than workers. Differences in the morphological volume occupied by the different castes indicate that even though they share the same genome, differential ecological, evolutionary and developmental forces impose different sets of constraints on queen and worker morphological diversification. Our results further show that ant morphological space is well defined along three axes of trait variation: eye size, appendage (leg and scape) length and head size. These three axes represent a three-way morphological trade-off for both castes, which may relate to the spectrum of life strategies observed in ants. Finally, these results provide the backdrop to investigate whether these trade-offs are consistent across phylogenetic scales, which would reinforce the hypothesis that trade-offs underline biodiversity maintenance.