by Sean O’Donnell
Based on research for the paper “O’Donnell, S., S. Bulova, V. Caponera, K. Oxman, & I. Giladi. In press. Species differ in worker body size effects on critical thermal limits in seed-harvesting desert ants (Messor ebeninus and M. arenarius). Insectes Sociaux.”
A key puzzle in understanding animal biodiversity is how species with similar ecological needs, sometimes even closely related species, can coexist in a habitat. Why doesn’t competition for shared resources lead to some of the competing species going extinct? If we can understand how species manage to partition their environments, we may gain insights into how high species diversity is maintained.
In the deserts of the Mediterranean and Middle East, seed-harvesting ants of the genus Messor provide a great study case of this conundrum. Several Messor species often co-occur and all harvest the seeds of desert plants for food. Within nests, Messor ant workers differ widely in body size, and species can further differ in average worker body sizes. Body size affects seed choice, but Messor species still overlap widely on the types of seeds they harvest. Do other factors help reduce interspecific competition?
Deserts are challenging to animal life in many ways, including their famously extreme temperatures. Many deserts, including the Negev, swing from being exceptionally hot to very cold. Small-bodied animals like ants, unable to thermoregulate when working outside their nests, are especially vulnerable to local temperature extremes.
My lab’s earlier work on army ants (Baudier et al. 2015, 2018) showed that worker body size differences within and among species were associated with the ants’ abilities to withstand temperature extremes. Smaller workers are generally more vulnerable to extremely high temperatures. We hypothesized that Messor species differences in thermal sensitivity, perhaps associated with body size variation, could affect their ability to function in extreme desert temperatures. Differences in thermal biology could influence the species’ relative abilities to harvest seeds under different temperature conditions. A first step in exploring this possibility was to test whether worker ant thermal physiology was related to body size with and between species. We chose two Messor species that co-occur in the northern Negev desert in Israel as research subjects: M. ebeninus and M. arenarius. Messor ebeninus workers range smaller, and M. arenarius workers range larger, but there is some species overlap in worker body sizes.
We collected workers from several nests of each species. Then we brought them to Itamar Giladi’s lab at the Desert Research Center of Ben Gurion University for physiological analysis. We placed single workers in vials in either a digitally-controlled heating or cooling device. By slowly ramping the temperatures up or down and watching for cessation of the ants’ behavioral responses, we determined the maximum or minimum critical temperature for each worker. After the thermal physiology trials, we measured each worker’s head width as an indicator of body size variation. We asked whether worker body size was associated with critical thermal maxima and minima both within and between the species.
We found that body size was related to maximum thermal tolerance in complex ways. In the larger species, M. arenarius, workers of all sizes were similarly tolerant of high-temperature extremes. In contrast, body size was strongly related to maximum tolerated temperature in M. ebeninus: smaller workers were more thermally sensitive and could not function at temperatures as high as their larger nestmates; larger M. ebeninus overlapped in size with smaller M. arenarius and were similarly thermally tolerant. Worker size showed no relationships with low-temperature tolerance in either species.
These results suggest extreme high desert temperatures could differentially affect these two Messor species and their abilities to forage for seeds. We expect smaller M. ebeninus workers to be restricted to foraging at cooler temperatures. Whether the smaller workers only, or the entire foraging force, drop out at higher temperatures remains to be tested (Baudier & O’Donnell 2017). But our results do suggest that temperature effects differ between ant species that share the same habitat and food resource. Species differences in thermal physiology may promote species coexistence.
Baudier KM, D’Amelio CL, Malhotra R, O’Connor MP, O’Donnell S. 2018. Extreme insolation: climatic variation shapes the evolution of thermal tolerance at different scales. American Naturalist 192: 347-359.
Baudier KM, Mudd AE, Erickson SC, O’Donnell S. 2015. Microhabitat and body size effects on heat tolerance: implications for responses to climate change (army ants: Formicidae, Ecitoninae). Journal of Animal Ecology 84: 1322-1330.
Baudier KM, O’Donnell S. 2017. Weak links: How colonies counter the social costs of individual variation in thermal physiology. Current Opinion in Insect Science 22: 85-91.
All images taken by the author.