A Case Study in Scientific Inquiry: Investigating Coat Coloration in Mouse Populations

Different coloration in beach and inland populations of Peromyscus polionotus.
Source: Urry, Lisa A.. Campbell Biology (p. 20). Pearson Education. Kindle Edition.

A Case Study in Scientific Inquiry: Investigating Coat Coloration in Mouse Populations (Campbell Biology)

Our case study begins with a set of observations and inductive generalizations. Color patterns of animals vary widely in nature, sometimes even among members of the same species. What accounts for such variation? As you may recall, the two mice depicted at the beginning of this chapter are members of the same species (Peromyscus polionotus), but they have different color patterns and reside in different environments. The beach mouse lives along the Florida seashore, a habitat of brilliant white sand dunes with sparse clumps of beach grass. The inland mouse lives on darker, more fertile soil farther inland. Even a brief glance at the photographs above reveals a striking match of mouse coloration to its habitat. The natural predators of these mice, including hawks, owls, foxes, and coyotes, are all visual hunters (they use their eyes to look for prey). It was logical, therefore, for Francis Bertody Sumner, a naturalist studying populations of these mice in the 1920s, to form the hypothesis that their coloration patterns had evolved as adaptations that camouflage the mice in their native environments, protecting them from predation.

Who is Francis Bertody Sumner? Francis Bertody Sumner (1874-1945), was a professor of Biology, and ichthyologist, at Scripps Institution of Oceanography. Sumner’s name will probably remain most closely and most favorably associated with his longest research, the analysis of speciation in mice of the genus Peromyscus, but most of his separate studies were conducted on fishes. He made significant contributions on the mechanism of color change in fishes and his research on color adaptation in flounders opened a new field.

The researchers built hundreds of models of mice and spray-painted them to resemble either beach or inland mice, so that the models differed only in their color patterns. The researchers placed equal numbers of these model mice randomly in both habitats and left them overnight. The mouse models resembling the native mice in the habitat were the control group (for instance, light-colored mouse models in the beach habitat), while the mouse models with the non-native coloration were the experimental group (for example, darker models in the beach habitat). The following morning, the team counted and recorded signs of predation events, which ranged from bites and gouge marks on some models to the outright disappearance of others. Judging by the shape of the predators’ bites and the tracks surrounding the experimental sites, the predators appeared to be split fairly evenly between mammals (such as foxes and coyotes) and birds (such as owls, herons, and hawks).

For each environment, the researchers then calculated the percentage of predation events that targeted camouflaged models. The results were clear-cut: Camouflaged models showed much lower predation rates than those lacking camouflage in both the beach habitat (where light mice were less vulnerable) and the inland habitat (where dark mice were less vulnerable). The data thus fit the key prediction of the camouflage hypothesis.

What is cryptic coloration? Cryptic coloration allows an organism to match its background and hence become less vulnerable to predation or recognition by prey.

Related:

Charles Darwin and Natural Selection

Source:

Urry, Lisa A.. Campbell Biology. Pearson Education. Kindle Edition. https://www.pearson.com/us/higher-education/series/Campbell-Biology-Series/2244849.html

https://library.ucsd.edu/dc/object/bb1434632f

https://groups.molbiosci.northwestern.edu/holmgren/Glossary/Definitions/Def-C/cryptic_coloration.html

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