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Long-term monitoring calls into question evolutionary biology assumptions

The white-footed mouse, Peromyscus leucopus, is the subject of a new study.

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November 30, 2021

White-footed mice can survive and thrive, even with physical impairments. A new study in Proceedings of the Royal Society B assessed 26 years of monitoring data on wild populations of white-footed mice -- representing more than 27,244 animals -- and found that mice with missing or deformed limbs, tails or eyes persisted at a rate similar to their unimpaired counterparts. 

Co-author Richard Ostfeld, a disease ecologist at the Cary Institute of Ecosystem Studies, said that the "results of our long-term study on wild white-footed mice do not support the long-held assumption, sometimes applied to vertebrates more broadly, that physical impairments reduce measures of fitness such as survival, movement and mass."

The U.S. National Science Foundation-supported research team analyzed data from a long-term mark-recapture program that Ostfeld initiated on Cary Institute's Millbrook, New York, property. Small mammals were trapped at six plots in a deciduous forest dominated by oaks and maples. Catch-and-release trapping occurred every three to four weeks over two to three consecutive days between May and November using live traps baited with oats. 

Mice were given metal ear tags upon first capture, and data on sex, age, mass, ectoparasite load and location were recorded. In addition, the scientists took detailed notes about the physical features of each animal.

To assess the impact of physical impairments on the fitness of mice, notes from 1991 to 2016 were reviewed. Mice with the following impairments were included in the study: missing, partially missing, or broken tails; missing, partially missing, or broken/deformed limbs; and missing eyes or cataracts. Survival was estimated by persistence time on the plots.

Doug Levey, a program director in NSF's Division of Environmental Biology, added, "This study is both powerful -- more than 27,000 wild mice over 26 years -- and surprising. Until now, few ecologists would have said that impaired mice live as long as mice that are free of impairments."