Elucidating the drivers of variation in phenotypic complexity across the range of a broadly distributed ant species

Background/Question/Methods

While the geography of functional traits reflects adaptations to environmental gradients, the geography of phenotypic complexity might indicate the conditions promoting phenotypic innovations. In eusocial organisms, phenotypic complexity is easily quantifiable because it relates to morphology diversity within the super-organism (i.e. colony). In ant colonies with high phenotypic complexity, the workers exhibit polymorphism levels wherein workers vary in size and head-to-body scaling. Worker polymorphism is also associated with social complexity since morphology relates to task partitioning within the colony. The bigger workers, known as majors, often possess a role of colony defense or food storage, whereas the smaller workers frequently specialize in brood caring or foraging. Yet, the environmental factors modulating the development of polymorphic workers remains poorly explored. Here, we examine the environmental factors driving geographic variation of worker polymorphism among colonies of the carpenter ant Camponotus herculeanus. Specifically, we asked which biotic and abiotic factors best explained variation in the proportion of major workers among colonies. To answer this question, we collected 27 colonies along a latitudinal gradient spanning eight degrees of latitude and estimated canopy cover, nesting tree diameter, the presence of ant competitors and regional climate, which are factors expected to affect development and social regulation.

Results/Conclusions

We found that the proportion of major workers within a colony decreased with increasing latitudes and was positively correlated to regional climate. One possible explanation for this pattern is major workers are more costly to produce given that they consume more energy than minor workers during the long winter months. Therefore, investment in the production of major workers might be worthwhile only in warmer regions where the growing season is longer and winter shorter. These results lend further support to the view that geographic variation in the environment can regulate the level of phenotypic complexity in ant colonies, which has implication for the evolution of social complexity.