Ants don’t change their behavior to avoid sublethal warming

This blog post is provided by Elsa Youngsteadt and tells the #StoryBehindthePaper for the paper “Can behavior and physiology mitigate effects of warming on ectotherms? A test in urban ants”, which was recently published in Journal of Animal Ecology. In their paper they explore how ants might react to climate change, and whether they can adapt their behaviour to new conditions.

Spring flowers are bursting earlier, growing zones are marching ever poleward, and bumble bees are shrinking away from their southern range limits. Climate change is well underway and some of its biological effects have been established for decades. But other effects are more subtle. For example, physiologists predict that, as temperatures increase, some animals’ metabolisms will also ramp up. They’ll need more food to get by, they’ll live faster, and die younger; some will go extinct. But are animals really getting hotter as the climate warms up?

Ectotherms are animals that don’t maintain a constant body temperature, instead letting it fluctuate along with the environment. But they do exert some control, for example by moving between sun and shade or retreating underground at certain times of the day or year. Given this ability to thermoregulate, ectotherms might also buffer their own exposure to climate change by subtly shifting their activity to cooler microclimates within their habitat. Although some modeling studies have suggested that this ability would save many insects from heat-induced extinction, it’s not clear that they really have the behavioral flexibility to pull it off. This is one of the main questions my team and I had heading into the spring of 2020.

Of course, the spring of 2020 didn’t go as planned. We had meant to ask this question in the warm tropics, where ectotherms are closer to their maximum heat tolerance and may take more urgent measures to stave off heat stress. But instead of spending the spring in the tropics, we spent it in lockdown, trying to stay sane and figure out some justifiable use of our time. Once we began to creep back into public, we decided to pilot our tropical methods at home in Raleigh, NC. That pilot project never made it out of the temperate zone, but it did get bigger. Ultimately, we found that our own local ants—although not obviously heat stressed—are surely warming up.

Checking for ants during a nighttime transect survey. Photo credit: Sara Prado

To detect behavioral thermoregulation, it’s not enough to just measure an ectotherm’s body temperature. You need to know its preferred body temperature, and what range of temperatures are available in its habitat—in the thermal mosaic created by shade, sun, darker surfaces, lighter surfaces, and little breezes. With those pieces of information, you can see how challenging the thermal environment actually is. (If the animal moved carelessly through the habitat, would it ever encounter intolerable conditions, or only pleasant ones?) And even if no part of the environment is truly pleasant, you can see whether the animal is making the best of it, spending time in the microclimates that bring it closest to its preferred temperatures, even if it’s still way off.

We chose ants as our focal ectotherms because they’re common models for thermal biology research, and because they’re generally important participants in their ecosystems, acting as predators, herbivores, seed dispersers, and so on. To look at a climatic challenge to ant thermoregulation, we worked on an urban warming gradient. All our sites were forested, but the ones located along urban greenways averaged about one degree Celsius warmer in the summer than the non-urban forest sites.

Map and examples of study sites. All sites were forested, but some were hotter than others due to their location within the urban heat island of Raleigh, NC.

We worked with several ant species that were common across most of the study sites, and collected four kinds of data. First, we measured thermal preference—the range of temperatures where each species chose to settle on a gradient from a heat block to an ice bath. (Because the university was still closed, I did this assay in my home office using an apparatus my husband helped build during lockdown. The frozen bowl of our ice cream maker helped keep the cold end cold.) We also measured each species’ maximum heat tolerance. (Sarah Ferriter, an undergrad researcher, took ants to her apartment for this assay.) And finally, research associate Sara Prado measured available and occupied temperatures along transects at each site in the field. For these last two, Sara used operative temperature models of three focal ant species; these were dead, posed ants mounted on thermocouple probes. You can think of them a little bit like a species-specific heat index thermometer: they incorporate effects of air temperature, surface temperature, solar radiation and wind, as well as the size, shape, and color of the ant species. It doesn’t necessarily tell you the body temperature of live ants, but it tells you how comfortable the thermal environment is. If the operative temperature model is hotter than the live ant prefers, then the live ant shouldn’t just hang out in that spot until her own body reaches that unpleasant temperature; she should move along and find some shade.

Operative temperature thermometer for the chestnut carpenter ant (Camponotus castaneus), made from a posed ant specimen (not alive) mounted on a thermocouple temperature sensor. Photo credit: Elsa Youngsteadt
Thermal preference arena. The lanes of the arena span a hot plate and an ice bath; ants indicated their thermal preferences by choosing where to settle within that range. Photo credit: Elsa Youngsteadt

If ants are actually good thermoregulators, then we should find them disproportionately often at places and times where the operative temperatures most closely matched their preferred temperatures. This was the case for four of our five focal species. (The fifth had such a broad range of preferred temperatures that we could hardly ask the question.) But if ants were capable of saving themselves from climate warming by shifting their activity patterns, they should also occupy cooler parts of the transect at sites that were too hot, and hotter parts of the transect at sites that were too cool. This did not happen. Cool-loving ants, for example seemed to use fixed behaviors (like being more active at night) to avoid the hottest conditions across the board—even at sites where the temperature was more comfortable during the day.

Black field ants (Formica subsericea) at a bait station. Ants were only slightly more likely to use a bait station if it was placed in a preferred microclimate than if it was placed in an uncomfortable microclimate that was too hot or too cold. Photo credit: Sara Prado

None of the ants got dangerously hot or cold during our study. But they did get warmer than they preferred to be, and they didn’t shift their behavior to compensate. That means urban ants are hotter than non-urban ants, and as climate change progresses, future ants will probably be hotter than past ants. If this pattern holds, their metabolisms will increase, and they’ll live faster-paced lives. Because ants are social animals, the bottom line for the superorganism—as opposed to just its individual workers—is still unknown. That’s something we’d like to know next. Only this time, I hope it doesn’t take a pandemic to send us back out to our local field sites to find out. 

Author bio

Elsa Youngsteadt is an assistant professor in the Department of Applied Ecology at North Carolina State University.

Lab website:

Read the paper

Read the full paper here: Youngsteadt, E., Prado, S. G., Keleher, K. J., & Kirchner, M. (2022). Can behaviour and physiology mitigate effects of warming on ectotherms? A test in urban ants. Journal of Animal Ecology, 00, 1– 12.

Effects of sea temperature on wild fish behaviour

This blog post is provided by Carla Freitas, David Villegas‐Ríos, Even Moland and Esben Moland Olsen and tells the #StoryBehindThePaper for their article “Sea temperature effects on depth use and habitat selection in a marine fish community“, which was recently published in the Journal of Animal Ecology.

A cod rests between two rocks at the bottom of a southern Norwegian fjord. It is a sunny summer day; children jump and swim happily – enjoying surface water temperatures of 23°C. Just below them, eelgrass and algae beds are full of life, hiding invertebrates and small fish, a dream banquet for a hungry cod. But cod remain at depth for the time being, sheltering among boulders. It is too warm at the surface and it may need to wait several weeks before resuming diel vertical migrations to its favourite feeding habitats.

Cod (Gadus morhua). (Photo credit: Erling Svensen / Institute of Marine Research, Norway)
Cod habitat use and temperature

Using acoustic telemetry in Tvedestrand fjord, in southern Norway, Freitas et al. (2016) found that cod do not venture to shallow feeding areas, such as eelgrass and vegetated hard substrate, when surface temperatures rise above 16°C, remaining instead in deeper, colder waters, which seem to be less favourable in terms of food resources.

Depth use by cod (Gadus morhua) during the night (black dots) relative to sea temperature. Cod avoid night-time excursions to shallow waters, when water temperature rises above 16 °C. Adapted from Freitas et al. 2016.
Temperature effects on a fish community

But, what about other fish species utilizing the same seascape? Sea surface temperature in this region ranges seasonally from 0 to over 20 °C, representing challenges and opportunities to the fish community which includes both cold‐, cool‐ and warm‐water affinity species. In this new study, Freitas et al (2021) acoustically tracked more than 100 individuals of Atlantic cod (Gadus morhua), pollack (Pollachius pollachius) and ballan wrasse (Labrus bergylta) and examined how coexisting species within a fish community adjusted their behaviour (vertical distribution in the water column and habitat selection) to cope with the thermal variation.

They found that cod used colder waters, compared with pollack and ballan wrasse. In contrast to cod, pollack and ballan wrasse occupied shallow areas when surface temperature increased in summer. During winter, when surface temperature dropped and the thermal stratification reversed (deeper waters were then warmer than the surface), pollack and ballan wrasse moved to deeper, relatively warmer areas, while cod occupied shallower, colder habitats.

Pollack (Pollachius pollachius) in a boulder seabed, one of the preferred habitats for the species. (Photo credit: Erling Svensen / Institute of Marine Research, Norway)
Habitat selection

Though habitat selection was affected by temperature, species‐specific habitat selection was observed. When sea temperature was similar throughout depths and habitats, cod selected eelgrass and vegetated hard substrate, which probably provide suitable food resources, such as small fish and invertebrates. Pollack, in contrast, avoided eelgrass throughout the year and selected instead boulders and other hard substrates. These habitats may be more suitable for the hunting strategies of this piscivorous species. Ballan wrasse are known to feed on invertebrates and showed a preference for eelgrass, vegetated hard substrates and rocky walls with anemones. Interestingly, high sea temperature at the surface makes eelgrass suitable for ballan wrasse during summer but inaccessible for cod. The reverse is found in winter, when surface temperatures as low as 0 to 5°C make eelgrass accessible for cod but unsuitable for ballan wrasse.

Ballan wrasse (Labrus bergylta). (Photo credit: Erling Svensen / Institute of Marine Research, Norway)
Climate change implications

This study shows how cohabiting fish species respond to thermal heterogeneity, showing that temperature regulates the access to the different depths and habitats. Behavioural plasticity may be an important factor for coping with temperature variability and potentially for adaptation to climate change. However, the fitness cost of temporary deprivation from key feeding habitats are unknown. There are signs that cod in this region grow less during summer, contrary to what is observed in colder northern regions.

The autumn has arrived, and the hungry and slim cod now resumes diel vertical movements to shallow feeding habitats. Air temperature drop as winter approaches and eventually the fjord surface will freeze over. Pollack and ballan wrasse will move to deeper waters, while cod will thrive in shallow habitats and compensate for the reduced growth during summer – and hope for no further temperature extremes next summer. Projected sea surface temperature increases are expected to be detrimental for this cold-water species, while profitable for their warm-water affinity cohabitants.

Read the paper

Read the full paper here: Freitas, C, Villegas‐Ríos, D, Moland, E, Olsen, EM. Sea temperature effects on depth use and habitat selection in a marine fish community. J Anim Ecol. 2021; 00: 1– 14.‐2656.13497

The multiple facets of behavioural plasticity: A symposium at Behaviour 2019 and a celebration for Louis Lefebvre

Behavioural plasticity is an important concept in ecology and evolution. The term has been applied to a broad range of biological phenomena, from developmental changes occurring in a canalized fashion during ontogeny, to dynamic responses of animals to prevailing ecological and social conditions, and longer-term behavioural changes brought about by learning and memory. On July 24th at the Behaviour meeting in Chicago, the symposium Behavioural plasticity: integrating variation within and among individuals and species will serve a dual purpose.

First, this day-long symposium will highlight recent advances in the study of within-individual variation in behaviour, transgenerational effects, and the evolution of plasticity through cognitive processes. The symposium co-organizers (Ned Dochtermann, Jennifer Hellmann, Kate Laskowski and myself) and their invited speakers aim to provide a forum for discussion and exchange that will stimulate future research into the causes of variation in behavioural plasticity and its evolutionary significance.

6The morning session of the symposium will serve the additional purpose of celebrating the career and accomplishments of Louis Lefebvre, Professor in Biology at McGill University, Montréal, Canada. Speakers include Louis Lefebvre, Laure Cauchard (United Kingdom), Alexander Kotrschal (Sweden), Daniel Sol (Spain), and myself (Canada). Those who know the work of Dr Lefebvre will remember his contribution to the study of social learning, whose work with Luc-Alain Giraldeau demonstrated plasticity in the use of social information: pigeons who could scrounge from a knowledgeable individuals did not socially learn from this demonstrator, while those who were prevented from scrounging did (Giraldeau & Lefebvre 1987). This work highlights an important general principle: failure to learn may not necessarily reveal an inability to do so, but may indicate an adaptive decision by the animal. Thus, the expression and evolution of cognitive processes is tightly linked to a population’s ecology and social organization, a principle that is fundamental to cognitive ecology.

1Another very important facet of Louis’ research consists in his innovative work on behavioural innovations. Before the turn of the millennium, most comparative cognition research quantified species’ cognitive abilities in standardized laboratory tests. As a trained psychologist engaged in behavioural ecology research, Louis was well-positioned to bring in a complimentary approach, which focused on behaviours expressed spontaneously by animals in their natural environments. But how can we identify behavioural plasticity in the wild that is brought about by cognitive processes such as learning and generalization? Louis had the idea of compiling published observations of birds engaged in unusual behaviours that were clearly not species-typical, and represented invented or modified behaviours allowing the innovator access to an unexpected food source. The comparative study of animal innovation was thus born in the basement of the Redpath library at McGill, where Louis and his students surveyed ornithological journals from all around the world. This work blossomed into >2000 innovation records from hundreds of avian species, and later a similar database on primates collated by Simon Reader and Kevin Laland, as well as exciting findings by Louis and collaborators on the links between innovation rate and: evolutionary diversification rates, the probability of colonization and invasion, the frequency of tool-use, social learning and the size of the whole brain as well as specific brain regions (Lefebvre et al 2013). His work on innovation also includes empirical research on birds of Barbados, studying several aspects of their behavioural plasticity and recently, differential gene expression in the innovative Barbados bullfinch and the less plastic black-faced grassquit (Audet et al 2018).



Those who know Louis will also think of his extreme plasticity in terms of personal accomplishments. Indeed, Louis was shortlisted twice for the ‘’Prix littéraire du Gouverneur Général du Canada’’ for his first two novels, ‘’Le Collier d’Hurracan’’ and “Guanahani”, published in 1990 and 1992. While engaged in an active research program at McGill and a busy family life as a father of two sons, Louis published two other novels and even developed into a fully-fledged visual artist, producing photographs, ‘’collages’’, and mixed media art pieces. You can see a list of his literary publications and some of his art at here.

4Louis also taught thousands of undergrads and supervised many theses, among whom I figure as a PhD graduate from his lab at McGill. Today, and on July 24th, I would like to celebrate his accomplishments and the impact his work had on the development of several research avenues combining natural history, evolutionary biology, animal cognition and neurosciences, as well its profound influence on the development of my own research career, which I look forward to discuss in my presentation in Chicago.

5Join us on July 24th at the joint meeting of the International Ethological Congress & Animal Behavior Society: exciting presentations, stimulating discussions, and celebrations throughout the day on the menu. Why not also take the opportunity to read a good book while on the plane or train, and visit some of the world-famous art galleries of Chicago? I think Louis would say that if you enjoy these activities, you should always make time for them, even in the midst of a busy research life.

Julie Morand-Ferron