Sampriti Roy, University of Calcutta
“It has been surrounded by confusion since forever”, said Kaspar Delhey, an Australian ornithologist, on the famous Gloger’s rule that he believes was “very dense and awfully written” in the 1833 book containing the same.
It was in 1833 that Constantin Wilhelm Gloger first remarked that animals living in warmer or more humid climates tend to have greater skin pigmentation than those living in comparatively cooler climates. For instance, birds in more humid climates were found to be darker than their lesser humid habitat residing relatives. In the case of mammals, we see a tendency of tropical and equatorial ones having darker skin while their poleward counterparts tend to have lighter skin. Relying on Gloger’s rule, Li Tian (China University of Geosciences) and Michael Benton (University of Bristol), predicted the effects of climate change on animals’ bodies.
In early December, Kaspar Delhey and his colleagues responded to Tian and Benton (Current Biology). They stated that, if controlled for humidity, Gloger’s rule gets flipped. This brought up the following question: do temperature and humidity have the same effect on animal pigmentation as Gloger predicted?
It is commonly seen that humidity results in lush plant growth, which acts as a shelter to animals and protect them from predators. Thus, in wet places, animals tend to be darker to camouflage themselves (Eg: Tasmania has one of the darkest bird species).
In contrast to this general observation, Delhey has presented the thermal melanism hypothesis, which states that some animals like reptiles and insects that reside in cooler places and rely significantly on external heat sources are aided by dark exteriors to soak up heat from sunlight. However, in warmer habitats, they end up lighter. Thus, the aforementioned flipping of Gloger’s rule was noticed.
Changes in colouration are said to be mainly dependent upon the temperature-regulation system of an animal. General trends show that cold-blooded animals are lighter, but we also see a wide range of outcomes in birds and mammals. It has been acknowledged by the parties in the discussion that when temperature and humidity both change, climate-driven colour predictions do tend to get tricky. For example, if we take the instance of Amazon, which is predicted to get hotter and drier in the coming years, lightening of animals’ skin colour is likely (lowering of heat absorption) and have all parties in agreement to the same. However, in case of Boreal forests of Siberia getting hotter and wetter, there are conflicts with regard to temperature and humidity affect predictions.
The discussion has led ecologists to realize with renewed importance the work that is still left in the field of evolutionary biology regarding animal skin pigmentation. At the end of the day, we must remember, as said by Michael Benton, “laws in biology aren’t really fixed” and look forward to more research to confirm the findings that relate climate change and animal appearance- both old and novel.
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Source: https://www.sciencemag.org/news/2020/12/will-climate-change-make-animals-darker-or-lighter
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