A popular hypothesis among scientists is that animals that live in the tropics, and thus tend to experience very little thermal variation, cannot tolerate large changes in temperature. One issue with this hypothesis is that some species may be found in the lowest valleys and the highest mountains of tropical forests, and these very different altitudes experience different temperature and humidity ranges, despite being close latitudinally. So how do tropical species respond to living in such different areas of a tropical forest? Does inhabiting an environment with greater changes in temperature, such as at the top of a mountain, mean that an animal has a greater tolerance for big temperature changes? To answer these questions, Rafael Bovo from the University of São Paulo, Brazil, and colleagues from various institutions in the USA, Brazil and United Arab Emirates studied the physiological responses of five frog species found at various altitudes in Brazil's Atlantic Forest by exposing them to different temperatures and measuring their ability to stay alert and active.
The researchers studied adult frogs from five different species at two altitudes in the forest – sea level and 1600 m. Among these species, they collected individuals from 3–5 populations from different locations across the mountain ranges. They measured how much water each frog lost via evaporation through the skin and how much water it absorbed, the lowest and highest temperatures the frogs could tolerate before being unable to sit up straight, and the environmental temperature at which their body temperature approached an unsafe physiological limit.
They discovered that living in a habitat with a broader temperature range doesn't necessarily mean that an animal will have a greater tolerance to wide temperature ranges. This may be due to the fact that these species avoid large temperature changes by hiding away during the hottest part of the day and being active at night. However, frogs living at higher altitudes tolerate low temperatures better than frogs that live at lower altitudes, suggesting that these highland populations are less vulnerable to larger variation in temperatures caused by climate change. Interestingly, there was no pattern linking altitude to the ability to absorb or evaporate water from the skin, despite the fact that areas at higher elevation are often drier than lower areas. While the researchers predicted that highland frogs would be better adapted to take in water and avoid evaporation from their skin, they found that none of the frogs tested were at high risk of stress-related water balance issues, probably because similar levels of water were available at high and low altitudes.
While we often generalize the characteristics of animal habitats (for example, all tropical rainforests are hot and humid), this study shows that it is important to also consider other factors that may affect the conditions of a habitat, such as elevation, because this changes how an animal may be affected physiologically by their environment. We also need more information about how tropical animals tolerate temperature changes at a range of altitudes, as many previous studies have not taken into account the effect that altitude might have on different populations. This is especially important when we consider how animals could respond to climate change, as the temperature and humidity at higher altitudes are likely to be very different from those found in lower altitudes. Understanding the complex relationship between where an animal lives and the elevation of this habitat can help us to protect our most vulnerable tropical species against the challenges of a changing climate.
