Climbing the mountain: it’s not all about physiology

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In the recently published article “Coping with temperature extremes: thermal tolerance and behavioral plasticity in desert leaf-cutting ants (Hymenoptera: Formicidae) across an altitudinal gradient” by Yela et al. in Myrmecological News, the authors use two sympatric leaf-cutter ant species, Acromyrmex lobicornis and A. striatus, to investigate what drives their distribution along a 1000 metres elevation gradient. They hypothesise that the species distribution depends on their physiological critical thermal limits, temperature of foraging activity, and/or fungus garden depth. Although they find some differences between species, their distribution cannot be explained by these traits. Here, Tom Rhys Bishop highlights their main points.

A Review by Tom Rhys Bishop

What explains the distribution of species? This question has vexed ecologists for centuries. While many factors clearly contribute to the where and when of species occurrence, ecologists have focussed most of their energy on understanding how climatic factors constrain distributions. The idea is that different species are adapted to different sets of thermal, rainfall, soil, and other environmental conditions. This thinking underpins many species distribution models, theories explaining biodiversity patterns, and, critically, our predictions of how species and ecosystems may respond to climate change.

Of all the different climatic and environmental variables we can measure, however, temperature appears to be particularly important. Most animals on Earth are ectotherms, and central to their response to temperature is their underlying thermal physiology. What is the highest and lowest temperature that a species can tolerate? These tolerances correlate with observed distributions in a variety of cases: thermal physiology often reflects geographic distribution. If you can’t handle the heat, you probably don’t live in the Sahara. If your physiological architecture breaks down as the temperature drops, you won’t be found in the Arctic.

In a recent article published in Myrmecological News, Yela et al. challenge some of our prevailing views connecting physiology to distribution. They use a case study involving two species of Acromyrmex leaf-cutter ants across an elevational gradient in Argentina. The authors ask the deceptively simple question: what explains the distribution of Acromyrmex lobicornis and A. striatus across over 1000 metres of elevational change?

Acromyrmex striatus (© Philipp Hönle)

The authors survey the elevational gradient to understand the distribution of the two leaf-cutter species along it. They also collect three datasets to test the potential underlying mechanisms driving these distributions.

First, they sample the thermal physiology of the two species across the gradient by estimating critical thermal minima and maxima. These are the lowest and highest, temperatures that each species can tolerate before losing all motor control. If thermal physiology drives the distributions of the two leaf-cutter ant species, we should expect to find the cold-tolerant species at the highest elevations.

Second, the authors record the times of day, and the temperatures, at which the two species forage. The authors argue that the species, which forages under the coolest temperatures, will live at the highest elevations.

Finally, the authors excavate several nests across the gradient and record the depth at which the colonies’ fungal gardens are found. The fungal gardens of leaf-cutter colonies, just like the ants themselves, also require certain thermal conditions in order to thrive. The authors propose the idea that species living at the cold high elevations will have fungal gardens located closer to the soil surface. These locations are hypothesised to be warmer than deep-soil gardens at these extreme elevations. 

The results are surprising.
The distribution patterns of the two species are almost identical, both occupy the full elevational gradient. They do differ in their abundance, however. Acromyrmex lobicornis is abundant in the lowlands but is much less common in the highlands. The reverse is seen for A. striatus. The thermal physiologist in you will be saying: “Ah! A. lobicornis must be have a high tolerance to heat, A. striatus must be comfortable in the cold!”. The authors show, however, that A. lobicornis has the cold adapted physiology, despite dominating in the warm lowlands. Clearly, our traditional view that physiology reflects distribution is not working here.

The authors then show that the two species do not differ in terms of the temperature at which they forage either. Both tend to forage at their optimum at 20-25˚C. So, temperature-dependent activity patterns cannot explain their different abundance distributions along the mountain.

Finally, the authors reveal that the depth of the fungal gardens varies as a function of temperature. Fungal gardens are located deeper in the soil in the warm lowlands, and shallower in the soil in the cold highlands. This suggests a plastic response of the ants to temperature variation. Both species, however, moved their fungal gardens in response to ambient temperature in almost exactly the same way across the gradient. In short, this fungal garden effect cannot explain the distribution of these two species either!

Acromyrmex striatus (© Philipp Hönle)

This largely leaves the contrasting distributions of the two species unexplained. Why is there such a difference in the abundances of these two species across the gradient, and why isn’t it explained by our traditional ideas? The authors suggest that the energetic costs of digging the deep nests of A. lobicornis at high elevations constrains its success to the lowlands. This is a promising idea but requires some formal testing. For now, though, the authors wisely point out that we need to go beyond thermal physiology if we are to fully answer our questions about species distribution. The fact that ants have two “bodies” – the individual worker and the nest and colony structure itself – clearly provide ants with a wide range of options for thermally regulating. This paper by Yela et al. point the way for exploring these alternatives when we try to understand the distribution of ants and other social insects.

Further Reading

Baudier, K.M., Mudd, A.E., Erickson, S.C. & 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.

Bujan, J., Roeder, K.A., de Beurs, K., Weiser, M.D. & Kaspari, M. 2020: Thermal diversity of North American ant communities: Cold tolerance but not heat tolerance tracks ecosystem temperature. – Global Ecology and Biogeography, Online Early.

Bishop, T.R., Robertson, M.P., Van Rensburg, B.J. & Parr, C.L. 2017: Coping with the cold: minimum temperatures and thermal tolerances dominate the ecology of mountain ants. – Ecological Entomology, 42, 105-114.

Jones, J.C. & Oldroyd, B.P. 2006: Nest Thermoregulation in Social Insects. – Advances in Insect Physiology (ed. S.J. Simpson), pp. 153-191. Academic Press.

Kadochová, Š. & Frouz, J. 2013: Thermoregulation strategies in ants in comparison to other social insects, with a focus on red wood ants (Formica rufa group). – F1000Research, 2.

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