Scientist discovers pikas’ strategies for dealing with heat
By Alanna Elder
Just like the creature she studies, Embere Hall spends much of the winter beneath the snow. Her office is tucked in a network of hallways beneath the University of Wyoming’s older science buildings. The floorplan seems parallel to the “subnivean,” a tunnel system between the earth and the snowpack, where the American pika stays active even during the coldest months.
This past winter, while the animals at her field sites hid from the frigid alpine air, Hall was toiling behind a wall of computer monitors, finishing her dissertation. Because pikas are impossible to study in person until their hiding places melt out in the spring, much of their life history is still mysterious. The elusive details of pikas’ lives beneath the snowpack fascinated Hall, but the question driving her research is a broader one. Hall wants to know how species like the pika are trying to adjust to “Human Induced Rapid Environmental Change,” or HIREC. Scientists use the term to describe shifts in an ecosystem that have been brought on or accelerated by human activities, such as the spread of invasive species, deforestation, and climate change.
Hall defended her PhD last March, opening the presentation with a well-known image: a polar bear drifting on a tiny block of ice. Like polar bears, pikas have adapted to survive in a cold environment. With HIREC, “we are exposing species to conditions very different from those that shaped their evolution,” Hall told the audience. “Climate change is one of the most pervasive examples of this.” Scientists know that mountain ecosystems are particularly vulnerable, and that that the Northern Rockies have experienced temperature increases three times more drastic than global averages. What they still do not know, Hall said, is what species like the American pika are doing to survive this transformation.
Sometimes called “rock-rabbits,” pikas are quick moving and can be hard to spot. But they make their presence known with stacks of dry leaves poking out of the talus rock, and a raspy “bleat” that has long startled hikers. One special characteristic of pikas is that they are some of the only mammals that spend their entire lives at high altitudes without hibernating. To pull this off, pikas spend the warmer seasons building networks of hay piles from the flowers, grasses, and shrubs that cushion the alpine landscape. By the time big snows fall, these heaps can be as large as a bathtub. During the winter, pikas keep warm in snow tunnels, restoring their energy reserves with food from the piles. Their thick coats and compact shapes also help protect them from bitter cold, but with a catch—they do not cope well with heat. Scientists have been studying the species’ unique physiology for decades, but with climate change in the picture, it is all the more urgent that they learn how the animals respond to heat stress.
“We have a pretty good sense for what their upper lethal temperature is,” Hall said. “All animals have those temperatures above which our proteins denature.” For pikas, just a few hours in conditions consistently hotter than 77 degrees can be deadly.
The evolutionary story of pikas is a story of climate, although the changes that drove the American pika’s distribution across the Mountain West were more gradual than those we are seeing today. Researchers have found pika fossils at relatively low elevations, places that are now sagebrush and prairie domain. They suppose that the species followed retracting glaciers off the plains and into the mountains. This hypothesis explains why pika populations now freckle high elevation regions of the western United States and Canada, completely separated from one another. This isolation makes the species more vulnerable to extinction, because it prevents populations from sharing genes that would help them resist or adapt to threats. The populations at lower elevations are in the most trouble, with areas in the Great Basin and Southern Utah emptying of pikas as temperatures have gotten hotter. Scientists worry that the species’ only option to escape the heat is to move to higher elevations until they have nowhere left to climb.
Hall said this picture is right for many places, but it is a broad stroke to describe what distinct populations of pikas are experiencing. She and her advisor, Anna Chalfoun, have uncovered a more complex story of how elevation determines where pikas can live. The splotchy temperature maps that we often use to visualize climate change are “too general to accurately describe how species are experiencing climate, especially in heterogeneous environments,” Hall said. Mountain slopes are heterogeneous because plants, rocks, and other local features can provide relief from extreme hot or extreme cold.
For the first of several approaches to learn how pikas respond to rising temperatures, Hall set up field tests across five mountain ranges in the Bridger-Teton National Forest in Wyoming. Instead of using average temperatures at the 146 plots in her study, Hall placed thermometers above and below the surface of the rock.
“We knew exactly what temperatures they were experiencing,” Hall said.
She then surveyed each site to see which ones had pikas, and made a surprising discovery. The best predictor of whether or not the animals were present—better than elevation, slope, or food availability—was the difference in temperature above and below the talus. For every one degree difference, the animals were eighteen times more likely to occur. In other words, the best places for pikas had nooks and crannies where the animals could cool off, despite elevation or other factors. The results supported Hall’s hypothesis that pikas use “microrefuges” to survive when the surface environment is less than ideal. Microrefuges are the spaces in snow or rock crevices that help soften harsh conditions, be they hot or cold. Pikas appear to huddle beneath the rocks to stay cool, just as they keep warm by insulating themselves in pockets of snow. According to Chalfoun, this more detailed understanding of pika habitats raises other concerns that animals in some areas may actually freeze due to thinner snowpack.
“Snowpack is going to be important for a lot of things—not just pikas,” Chalfoun said. “But there seems to be a balance between heavy, insulating snowpack, and earlier melts, which give them more foraging time.”
Hall and Chalfoun agree: this does not mean that the impacts of climate change will balance out, but only that the shifts that pikas and other animals are experiencing are more complex than a rising thermometer. Although they help the animals avoid lethal temperatures, microrefuges are not enough to buffer threat of heat. “You can’t shelter forever,” Hall said. “At some point, you have to be active on the surface, and you have to do the things that allow you to persist as a species, such as securing food or finding a mate.”
Hall’s latest project has been to discover how pikas are altering their behavior, so that they can collect food even as their habitats heat up. At a portion of her field sites, she set up cameras and thermometers near hay piles, and recorded videos, matching the temperature readings to the footage. Hall then analyzed 1,600 recordings to see how pikas foraged when it was hot outside.
Hall found that temperatures were keeping the animals from venturing out for forage. As averages crept from 60 to 77 degrees Fahrenheit, the portion of the day when pikas collected food slid from nearly 100 percent to just 30 percent. Individuals at hotter sites were losing two-thirds of a day’s work to the heat. Come winter, that time could add up, and pikas pulling from a skimpy food stock could starve.
Hall said those results were not particularly surprising, but they confirmed that warmer conditions leave pikas with less time to forage. She wanted to know what the animals were doing about that, so she looked for patterns in what the animals were eating as well as when they were eating. What she found provided new insights into pikas and other species dealing with HIREC.
“Pikas were foraging at all hours of the day,” Hall said. While there was anecdotal evidence that the animals sometimes gather food at night, her team was the first to document that happening. Nocturnal foraging provides pikas with a way to secure food and stay cool.
More exciting was Hall’s discovery that pikas were generally choosier when they collected food during hotter periods. Hall described this concept as “high risk, high value.” The warmer the weather, the riskier the foraging trip. To make it worth the effort, pikas selected plants that were more nutritious, with higher nitrogen and lower fiber content, especially when temperatures were hot.
In the third phase of her study, Hall watched the footage to see if pikas were avoiding heat by altering the timing of their foraging trips. The graph from those results is a tangle of lines: some steep and others flat. Hall said the variation in the slopes of the lines shows the variation in individuals’ “plasticity,” or their ability to adjust their habits as conditions change.
“Individuals are endorsing really different strategies,” Hall said. “Some will continue to forage, but at a lower rate, whereas others will do really intense foraging when temperatures are mild and then shut off foraging when it’s really hot.”
Hall said the steeper lines on the graph represented the individuals that were more tactical in their food collecting habits: the more plastic pikas. In order to find out whether working smart, not hard, had any benefits for pikas, she took note of the content of their hay piles. She then took samples matching those plants from the surrounding meadows back to the lab to test them for nitrogen content. Sure enough, those individuals who saved their foraging for cooler hours secured more nutrition.
These results do not necessarily mean that more plastic individuals will survive while the others perish. Hall said it is actually better that not every individual had the same response, because it widens the possibilities for how the species as a whole might survive.
“I think a really interesting question going forward is to understand more about those individuals that were not expressing a high degree of plasticity,” Hall said, meaning the slow-and-steady foragers.
Hall’s next step may be to learn more about those pikas that are not changing their habits to match the heat. They may be slower to adjust, or they may be toughing it out. Higher tolerance, Hall says, may be another strategy.
“[Body] temperature is also under selection pressure, so I think it would be really interesting to go back and look at their body size,” Hall said. Smaller animals have more surface area compared to their volume, which makes it easier for them to shed heat. Hall thinks the less-flexible individuals from her behavior study may have a size adaptation that makes it possible for them to forage all day long. This is only speculation, because Hall says, trapping the pikas at her plots and measuring them would have been too time-consuming for this study.
Because they evolved to be active in the alpine year-round, the American pika lives on the front lines of climate change. Wildlife advocates have made calls to list them under the Endangered Species Act, but so far, the US Fish and Wildlife Service has declined to list the species, citing a lack of information. Despite their visibility in science and media, little has been done to protect pikas. Hall is watching them for signs of resilience, but she does not want her research to give the wrong idea.
“I don’t want you to think they are fine,” Hall said. She pointed out that the creative behaviors she observed are not even options unless pikas have access to nitrogen-rich food, adequate microrefuges, and a way to shed heat when temperatures are deadly. As climate impacts accumulate, these tools may become less available. Even the industrious rock rabbit will likely need some help.
Alanna Elder is a senior in agroecology and environment and natural resources at the University of Wyoming, and a part-time reporter at Wyoming Public Radio.