A study of arthropods, a group of cold-blooded animals that includes insects and crustaceans, has revealed an intriguing anomaly in the way they respond to temperature changes. It turns out that arthropods found in warmer waters are smaller than members of the same species found in colder waters, but that the reverse is often true for those living on land.
Bergmann's rule holds that “In warm-blooded animals, races from warm regions are smaller than races from cold regions.” The observation was first made in 1847 and while exceptions have been found, the rule has stood the test of time well enough that global warming has been observed reducing the size of Australian birds and Scottish sheep.
Creatures that depend on external heat sources don't necessarily follow the rule. A meta-study of insects found that some seem to behave as if Bergmann's rule applied to them, but others go in the opposite direction.
In an attempt to make sense of this confused mixture, Ph.D. student Curtis Horne of Queen Mary University of London took a look at studies of not just insects, but of a wide range of arthropod species. In Ecology Letters, Horne concludes, “Body size in aquatic species generally reduces with both warming and decreasing latitude, whereas terrestrial species have much reduced and even opposite gradients.”
The fact that terrestrial arthropods often show a pattern that works against Bergmann's rule indicates that the forces driving the process are different from those for creatures that produce their own heat. While mammals and birds need large bodies in colder climates to keep in the body heat they generate, for aquatic arthropods the issue is air, not energy. All else being equal, oxygen levels decline with water temperature.
The paper observes, “These patterns support the prediction that oxygen limitation is a major controlling factor in water, but not in air.” Dr. Andrew Hirst, a co-author on the paper and Horne's supervisor, previously found that even for types of animals where warm climates are associated with smaller sizes on land, the effect is ten times stronger in water.
What is driving the changes for arthropods on land is a more complex question and one that is yet to be fully resolved.
The research matters because it introduces a means of measuring past climatic changes where more standard proxies are not available and because it could tell us what to expect as temperatures rise. As Horne says, "Increasing our understanding of what influences how big animals grow will mean we can start to make better predictions about how different groups of species will cope with climate change.”
A world where mammals and birds get smaller, but insects grow bigger, sounds just a little too much like a horror film. But if it's coming, we better be prepared.