How melanism affects the sensitivity of lizards to climate change (2022) Mader et al. , Functional Ecology, https://doi.org/10.1111/1365-2435.13993

Image credit: Tony Rebelo, CC BY-SA 4.0, via Wikimedia Commons

The Crux

Climate change is a fact of life. Every day we uncover more of the negative effects it will have on the various animals, plants, and fungi in the natural world. Species range contractions are one such effect, and they occur when the area that a given species normally occupies shrinks. They are directly linked to a species’ risk of extinction, with this risk growing as a species inability to adapt to new environments grows. Though the theory sounds logical, many of the exact mechanisms behind range contractions are still unknown.

Ectotherms are organisms that depend on the surrounding environment to regulate their own body temperature, making them particularly vulnerable to climate change. Many different biological mechanisms are involved in regulating temperature, but the ability to reflect solar radiation is a key player. Indeed, the ability of organisms to reflect solar radiation (aka energy from sunlight) is part of the thermal melanism hypothesis (see Did You Know?). Melanistic (darker) organisms may be favored under climate change, due to the protection against UV radiation provided by melanin. However, melanistic individuals are more prone to increased heating, which can be bad. Today’s authors sought to understand how climate change would affect melanistic organisms.

Did You Know: The Thermal Melanism Hypothesis

Being able to reflect solar radiation can be very advantageous, but the opposite can also be true. This is especially the case in cold environments, where darker animals will not only be able to heat up faster than their lighter conspecifics, but they will be able to maintain a higher temperature as well.

What They Did

The authors used the Karoo gilded lizard (Karusasaurus polyzonus) as their test organism, a lizard species that lives in a variety of different environments around South Africa. Importantly, this is a species that is not sexually dimorphic in appearance, meaning that the males and females have the same coloration. Due to the minimal differences between the males and females, the authors could specifically study how melanization and climate change interacted to affect these lizards. The authors first collected 56 Karoo gilded lizards from five different field sites and measured their body size and skin absorbency. Then, they collected species-level data on the thermal preferences and behavioral adaptations of this species from the scientific literature for use in their later models.

Of particular interest is the activity time of the lizards, or the amount of hours per day between the minimum and maximum temperatures that a lizard could be active and forage for food. Activity time is useful in this case as a proxy for the viability of the organisms, because the more active an organism is the more food it will be able to acquire. This last bit was a very rough generalization on my part, but it gets the idea across.

Next, the authors collected climate data for the “present” (1960-1991), as well as the predicted climate data for two future time periods (2040-2059 and 2080-2099). Finally, using all of this data on the individual lizards, the species as a whole, and the climate, the authors ran two models to predict what the activity time of the lizards would be for each of these three time periods.

What They Found

Not surprisingly, temperature was predicted to increase over time across all five of the study sites. Furthermore, the authors found that the activity times of the lizards was also predicted to increase at all five sites during almost every time period analyzed. Interestingly, the more melanistic lizards were predicted to not only be more active than the lighter lizards in general, but they were predicted to become even more active in the future.

Problems?

The complexity of the models used here are an impressive feat in and of themselves, yet the results rely on a number of generalizations about the data. For instance, the data on mean lizard activity per year only contains 12 data points, meaning that the authors averaged a month’s worth of climate data into a single point. While this is great for analyzing general trends, it fails to capture the variability inherent in climate data. This is not an issue in and of itself, but one of the biggest effects of climate change is how much more variable the climate will be getting, so future models will need to be more nuanced and complex to capture such variation.

So What?

Today’s paper was a double-whammy of a scientific study. Not only did the authors test for a mechanism in which climate change would negatively affect species, but their results were also the opposite of their predictions, with darker lizards appearing to actually benefit from climate change instead of suffering a greater cost. The methods used in today’s study could be useful for understanding how other organisms respond to climate change, which will be important for making predictions about what is likely to happen to many other species in light of climate change.

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Dr. Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions who thinks it’s hot enough already, thanks. You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.