Sloths may be pushed to their metabolic limits by global crisis

SWANSEA, United Kingdom — In the lush rainforests of Central and South America, sloths have perfected the art of energy conservation. These slow-moving mammals have evolved to exist on an extremely low-calorie diet, operating within a finely tuned energy budget where minimal expenditure is linked to minimal energy intake. But as our planet warms, this balance may be disrupted, potentially pushing some sloth populations to their metabolic limits.

A study led by Rebecca N. Cliffe and her colleagues from the Sloth Conservation Foundation and Swansea University has revealed that climate change could have profound implications for the survival of sloth populations, particularly those living in high-altitude regions. The research, published in PeerJ, demonstrates that sloths from different altitudes respond differently to temperature changes, with highland sloths potentially facing an uncertain future in a warming world.

Sloths are known for their incredibly slow metabolism, which is about 39% lower than what would be expected for a mammal of their size, according to general mammalian predictions. This adaptation allows them to subsist on a diet of low-nutrient leaves, but it also means they have little energy to spare. Sloths are presumed to exist within a narrow and finely tuned energy budget, which, combined with their limited ability to move to new areas, means that even small changes in temperature could have significant impacts on their survival.

The research team focused on two-fingered sloths (Choloepus hoffmanni) from both lowland and highland areas in Costa Rica. They measured the sloths’ oxygen consumption and body temperature when exposed to different ambient temperatures, ranging from 18°C to 34°C (64°F to 93°F). They also tracked changes in body temperature and posture over several days in response to natural temperature fluctuations.

What they discovered was surprising. While all sloths showed similar responses to temperature changes at the lower end of the scale, their reactions diverged significantly as temperatures rose above 32°C (90°F). Lowland sloths demonstrated an ability to depress their metabolism in response to high temperatures, effectively reducing their energy expenditure and body temperature. Highland sloths, however, lacked this capability. Instead, their metabolic rates continued to increase as temperatures rose, potentially pushing them into an unsustainable energy deficit.

This difference in metabolic response is likely an adaptation to the different climates these populations experience. Lowland areas frequently experience temperatures above 30°C, so the ability to reduce metabolism in these conditions is crucial for survival. Highland sloths, living in cooler environments, haven’t needed to evolve this adaptation.

The implications of these findings become stark when considered in the context of climate change. Current estimates suggest that average daily air temperatures in Central and South American rainforests could increase by 2-6°C by the year 2100. Using their data, the researchers modeled how sloths might respond to such changes.

For lowland sloths, a 2°C increase in ambient temperature is predicted to raise their body temperature by about 2.13°C. While significant, their ability to depress their metabolism should help them cope with this change. Highland sloths, however, face a more challenging future. The same 2°C rise in ambient temperature is expected to increase their body temperature by about 1.53°C, but without the ability to reduce their metabolism, their energy expenditure would continue to rise.

The model suggests that high-altitude sloths are likely to experience a substantial increase in metabolic rate as temperatures rise. Given their intrinsic energy processing limitations and restricted ability to move to new areas, this could make their survival challenging in a warming climate.

This metabolic challenge is compounded by the sloth’s unique digestive system. Sloths have extremely slow digestion, with food taking anywhere from 150 to 1,200 hours to pass through their system. This slow processing is believed to be necessary for detoxifying the plants they eat. While increased temperatures might speed up digestion slightly, it’s unlikely that sloths could process food much faster to meet increased energy demands.

The study also revealed interesting differences in body temperature between highland and lowland sloths. Highland sloths consistently maintained higher body temperatures, likely an adaptation to their cooler environment. This higher body temperature, combined with their thicker fur, helps them survive in chillier mountain forests. However, this adaptation may become a liability as temperatures rise.

So what does this mean for the future of sloth populations? While lowland sloths may have some capacity to adapt to warming temperatures, highland groups appear to be particularly vulnerable. This vulnerability is exacerbated by their limited ability to migrate to more suitable habitats, as mountain-dwelling sloths are often restricted to isolated forest patches.

The research underscores the urgent need for conservation strategies tailored to different sloth populations. The study shows that highland sloths may need targeted interventions to help them cope with rising temperatures.

“Sloths are inherently limited by their slow metabolism and unique inability to regulate body temperature effectively, unlike most mammals,” Cliffe says in a statement. “Our research shows that sloths, particularly in high-altitude regions, may not be able to survive the significant increases in temperature forecast for 2100.”

As our planet continues to warm, the fate of these iconic creatures hangs in the balance. The sloth’s strategy of extreme energy conservation, which has served them well for millions of years, may prove to be a liability in a rapidly changing world. Yet, by understanding these vulnerabilities, conservationists hope to develop strategies to protect these unique animals.