Cold Comfort: metabolic rate and tolerance to low temperatures predict latitudinal distribution in ants
Metabolic compensation has been proposed as a mean for ectotherms to cope with colder climates. For example, under the metabolic cold adaptation/metabolic homeostasis hypotheses (MCA/MHH), it has been formulated that cold-adapted ectotherms should display higher/more thermally sensitive metabolic rates (MRs) at lower temperatures. However, whether such compensation can truly be associated with distribution, and whether it interplays with cold-tolerance to support species climatic niches, remains largely unclear despite broad ecological implications thereof. Here, we teased apart the relationship between MRs, cold-tolerance, and distribution, to confront the MCA/MHH among 13 ant species. We report clear metabolic compensation effects, consistent with the MCA and MHH, where MR parameters strongly correlated with latitude and climatic factors across species distributions. The combination of both cold-tolerance and MR further upheld the best predictions of species climatic niches. To our knowledge, this is the first study showing that the association of metabolic data with cold-tolerance supports increased predictive value for biogeographical models in social insects. These results also highlight that adaptation to higher latitudes in ants involved adjustments of both cold-tolerance and MRs, potentially at the expense of metabolic performance at warmer temperatures, to allow this extremely successful group of insects to thrive under colder climates.