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Ants don’t change their behavior to avoid sublethal warming

/ Beth Preston / Leave a comment
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: http://youngsteadtlab.org/

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. https://doi.org/10.1111/1365-2656.13860

Field experiments, ecology and physiology: studying cultural propensities in wild species

/ Julie / Leave a comment
This blog post is provided by Kelly Ray Mannion and tells the #StoryBehindthePaper for the paper ‘A multicomponent approach to study cultural propensities during foraging in the wild‘, which was recently published in Journal of Animal Ecology. In their paper, they review previous work done with field experiments, how to assess diet and ecology, as well as physiology to offer a framework to study cultural propensities in wild species.

Uganda is quite the hotspot for primate research. Home to Bulindi, Kibale and Budongo chimpanzees as well as Bwindi mountain gorillas. And with each passing month, thanks to the Bugoma Primate Conservation Project which has been working to habituate some of the several chimpanzee communities in Bugoma Forest Reserve in western Uganda, researchers learn more and more about the chimps living in the forest there.

Semi-habituated: that means there are days where we spend hours with chimps and days where it seems like they’ve hitched a ride with a passing boda boda and haven’t left a note about when they’ll be back. It’s an exciting time to be working in Bugoma because we get to learn about these “new-to-us” communities and see how they fit in with the rest of the Ugandan chimps. And the big question on people’s mind is, do they use tools?!

“Hey Google, what are the drivers of tool-use?”. As a PhD student with this as my thesis topic, it was worth a shot. The literature surrounding culture and cultural behaviors like tool-use is immense and yet still growing. Although not without its criticisms, learning that culture exists in non-human species has not lost its shock value or appeal to researchers. Moreover, disentangling the factors which influence cultural propensities is still full of debate with many projects and hypotheses aimed at understanding what the root causes of these behaviors are. So, doing a PhD investigating drivers of tool-use behaviors leads us down a rabbit hole.

Semi-habituated, i.e., we’ve still got a lot to learn about these chimps. The presence or absence of tool-use seems like a great place to start. Of course, directly observing these behaviors in fully habituated chimpanzees would be ideal, but field experiments in communities that are not fully habituated, such as the Bugoma chimpanzees can really help bring this information to light. And for my PhD specifically, I am conducting the honey-trap experiment which has been done previously in other Ugandan chimpanzee communities.

Figure 1: PhD student Kelly Ray Mannion sets up the honey-trap experiment with the help of Bugoma Primate Conservation Project field assistants. (photo: Nisheet Patel)

The honey-trap experiment uses a log with a hole in it to simulate a natural beehive which chimpanzees can encounter in the forest. The honey is in the bottom of the hole and only accessible through a small opening, thus creating a situation where the chimps cannot reach it with their fingers and must figure out how to extract the honey. Chimpanzee behavior is recorded via motion sensing camera traps. So, the honey-trap experiment can help answer the question of tool-use behaviors among others like social learning, curiosity, information diffusion, etc. Next, is to understand what influences these behaviors (or lack thereof).

Figure 2: Screenshot from a camera trap video of Bugoma Forest chimpanzees interacting with the honey-trap experiment. (photo: Kelly Ray Mannion)

The field is not lacking in ideas for what to study, how to study it, nor hypotheses to consider. For example, it’s hard to analyze tool-use behavior and not address the necessity or opportunity hypotheses: is necessity the mother of all invention, or are ingenuous behaviors arising from opportune moments? Here’s how I will explain these hypotheses and the overall goal of my project to my grandmother: Imagine coming home from work hungry-bordering-on-hangry. What do you do? A) cook a pasta dinner on the stove (takes some time and knowledge to do so) or B) eat the open crackers in the pantry, followed by cupcake on the counter, and topped off with the random carrot in the fridge (most readily available and you only need to know how to chew)? Well, I guess it depends… but depends on what? And that’s what I want to find out. The list of potential factors could go on and on. Maybe it matters what I have eaten earlier that day or if I went to the gym. Perhaps there’s a social component like do I have guests coming over or are my roommates around to witness me scavenge the kitchen area? Because my exhibited behavior is centered around food let’s consider diet, items that are eaten and those that are available, as a general category which can impact my decision. Then, let’s consider how I am feeling, which is related to my diet because it corresponds to what my body needs in terms of energy and nutrients. Even though there are other potential factors, diet and physiology are prominent and so I will start with these.

Following this logic, I incorporate the same factors with the chimps in Bugoma. Diet information comes from observations and inspecting fresh dung samples. Collecting urine samples provides vital information about the stress, energetics, and overall health of the individuals. Combining all this information together gives a fuller picture into the influences of behavior and specifically to know more about the physiological underlying of behavior, which is a core goal of the ECCEpan group, headed by Dr. Thibaud Gruber.

This all seems very primate-centric, human or not, but the same premise can be applied for many other species – field experiment, ecology (diet) and physiology – to understand potential cultural behaviors. It won’t be without some creativeness, imagination and tenacity for the case of some species, but combining as many potential factors as possible gives us a sharper image on what influences cultural propensities in the wild.

About the Author

I am a PhD student at the University of Geneva, part of the ECCEpan group led by Prof. Thibaud Gruber. As a part of my PhD project, I am investigating drivers of tool-use behavior in wild chimpanzees and trying to understand influences of tool-use behavior from an ecological and physiological perspective. I have experience working with chimpanzees, gorillas, orangutans and bonobos and love all things primate and nature related.

Read the paper

Read the full paper here: Mannion, K.R., Ballare, E.F., Marks, M. & Gruber, T. (2022). A multicomponent approach to studying cultural propensities during foraging in the wild.  Journal of Animal Ecology https://doi.org/10.1111/1365-2656.13820

Coexistence is possible: spotted hyenas exposed to daytime pastoralism do just fine

/ Beth Preston / 3 Comments
This blog post is provided by Arjun Dheer and tells the #StoryBehindthePaper for the paper ‘Diurnal pastoralism does not reduce juvenile recruitment nor elevate allostatic load in spotted hyenas‘, which was recently published in Journal of Animal Ecology. In his study, he explores the impact of pastoralism on spotted hyena populations in Ngorongoro, discovering that they don’t seem stressed and numbers of recruited cubs didn’t differ between areas with human influence and areas without.

The blog can be found in Swahili here.

Humans affect wildlife, but it’s not always easy to assess the conservation-relevant impacts. In fact, we affect them in so many ways – they might be forced to avoid us and become more active at night, alter where they look for food, or even change their diet. It is often assumed that these effects are bad for the animals, and that wildlife struggles to cope with those behavioral changes. Surely, this is an issue of serious conservation concern. But is it really?

Not necessarily. Many animals can adjust their behavior to changes in their environment without this affecting their ability to survive or reproduce. Changes in behavior thus do not always mean that the persistence of a wildlife population is actually threatened. To figure out whether a population’s persistence is threatened is difficult. And it is especially difficult when studying larger, longer-lived species – such as spotted hyenas (Crocuta crocuta) – which often come into conflict with humans and thus need to be prioritized to promote coexistence. Doing such an investigation requires long-term data, focused particularly on two main elements: fitness and physiology.

Fitness refers to the ability of an organism to survive, reproduce, and contribute offspring to the next generation. Physiology is a broad concept referring to the functions in an organism’s body that allow it to survive and reproduce. Measuring changes in these variables is very conservation-relevant because of how directly they impact a wildlife population’s continued presence. And that is exactly what we investigated.

A spotted hyena in proximity to Maasai pastoralists and their cattle grazing in the Ngorongoro Crater. (Photo: Oliver Höner)

We used a natural experiment to study hyena clans exposed and unexposed to human activity. For our study, we collected 24 years of data (1996-2019) from the eight spotted hyena social groups (“clans”) resident on the floor of the Ngorongoro Crater, Tanzania. Two of the Crater clans – which we call the Airstrip and Forest clans – were exposed to pastoralism conducted by the local Maasai community from 1996-2016. Pastoralism is a globally widespread human activity and lifestyle whereby people accompany their livestock to grazing pastures, water bodies, and mineral licks. The other six clans were not exposed to pastoralism. This provided us with a natural experiment: we could compare how the exposed clans performed compared to the unexposed ones.

Measuring juvenile recruitment provides key information on the “health” of hyena clans. To measure fitness effects, we compared juvenile recruitment – that is, the number of surviving offspring – in exposed and unexposed clans. As for physiology, we compared the concentration of glucocorticoids in their poop, or “stress”. The level of “stress” indicates the cumulative burden an organism experiences due to life events, be it social interactions with groupmates or repeated human disturbance. To do so, we collected 975 poop samples from 475 different hyenas across our study period and measured the “stress” in each and every one.

Last but not least, in order to disentangle the effect of pastoralism from other environmental conditions, we also collected data on other factors during the course of our study period – the occurrence of disease outbreaks, the risk of encountering lions, and prey availability.

Contrary to what we expected, juvenile recruitment in the exposed clans was slightly higher than in the unexposed clans. Similarly, “stress” was very similar between hyenas from the exposed and unexposed categories. Altogether, it seems that pastoralism did not pose a threat to exposed clans in the Ngorongoro Crater spotted hyena population at all. The exposed clans did just as well as the unexposed clans, if not slightly better!

Two potentially big reasons why there was no negative effect on the exposed clans are that the pastoralism (i) was predictable, as it occurred strictly on dedicated paths into and out of the Crater and (ii) happened only during the day. Because it was predictable, the hyenas may have been able to adjust to the daily rhythm of the pastoralism without much issue. And because it occurred during the day, it meant the pastoralism was not very disruptive to key behaviors in the hyenas – namely hunting and suckling. Suckling of very young cubs does often occur during the daytime, but the mothers may have been able to adjust the suckling times to the night in order to avoid the pastoralism.

A spotted hyena cub takes a break from suckling during the daytime. Photo: Arjun Dheer

On top of all that, the hyena social system might have played a role. Hyena clans have a strict hierarchy or ranking system. Top-ranked hyenas tend to contribute more cubs to the population, and due to several factors, are less likely to be directly exposed to challenges such as pastoralism. It is possible that the lower-ranking hyenas might have absorbed any effects of the pastoralism; most of their cubs die even in the best of times. When exposed to pastoralism, their cubs may just die earlier, leaving the clan’s overall performance unaffected.

Finally, the Ngorongoro Crater’s rich prey abundance relative to the size of the hyena population (i.e., very high “prey per capita”) might have protected the exposed clans from experiencing serious effects of pastoralism. The Crater has one of the highest densities of large mammals on Earth, and that means the hyenas enjoy a banquet of delicious food year-round, at least during most of our study period. This may have allowed mother hyenas to produce plenty of milk to raise healthy cubs, and ultimately may have kept the clans productive.

Our results have strong implications… but should be interpreted with caution. Our findings suggest that human activity may be sustainable and conducive to coexistence with wildlife provided they are not too disruptive to key behaviors. Second, we have highlighted the importance of considering the effects of human activity in light of an animal’s behavioral patterns and social system. And third, by measuring juvenile recruitment at the level of clans, we have offered a new approach to the study of human-wildlife interactions focused on group-living species.

All that being said, our findings apply to a very specific situation. In areas where human activities are more intense and environmental conditions less ideal than in the Crater, pastoralism may have a stronger effect even on behaviourally flexible species such as the spotted hyena. We encourage other scientists to conduct studies that focus on a variety of human activities and animal species with various social systems and behavioral patterns so we can continue developing evidence-based strategies for coexistence.

Author bio

Arjun Dheer is a doctoral student working with the Ngorongoro Hyena Project, based at the Leibniz Institute for Zoo and Wildlife Research. He studies human-carnivore coexistence in Ngorongoro Conservation Area, Tanzania. Follow him on Twitter @ArjDheer and the Hyena Project at @HyenaProject and on YouTube at www.youtube.com/c/hyenaproject!

Read the paper

Read the full paper here: Dheer, A., Davidian, E., Courtiol, A., Bailey, L. D., Wauters, J., Naman, P., Shayo, V., & Höner, O. P. (2022). Diurnal pastoralism does not reduce juvenile recruitment nor elevate allostatic load in spotted hyenas. Journal of Animal Ecology, 00, 1– 12. https://doi.org/10.1111/1365-2656.13812