Why Are Aquatic Small Mammals So Large?

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Abstract
Small mammals of the families Muridae, Cricetidae, and Soricidae that are associated with aquatic habitats are significantly larger than their terrestrial counterparts. A theoretical argument is presented to explain this relationship. It is suggested that in aquatic environments, small mammals below a certain body size are selected against by fish predation, thus favoring larger-bodied animals. In terrestrial environments, large size may make animals more vulnerable to avian and mammalian predators, and small sized bodies are more adaptive. Aquatic environments may also provide higher quality food and less competition than terrestrial environments. Several alternative hypotheses to explain the relationship between body size and habitat are presented and discussed.