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Wild mice don't just survive, but thrive with physical impairments

According to the Darwinian theory of natural selection, organisms that are better adapted will have better chances of surviving and passing on copies of their genes to future generations through their offspring. This implies that less fit individuals, including those with physical impairments, will not enjoy the same chances. This long-held assumption has been tested in a new study on white-footed mice in the state of New York.

The study involved capturing, marking and monitoring a total of 27,244 white-footed mice. The project was initiated by Richard Ostfeld in 1991, and ran for 26 years. Catch and release trapping was carried out every 3 to 4 weeks, using live traps baited with oats. 

Upon first capture, the mice were given metal ear tags for identification, and data on sex, age, mass, ectoparasite load (e.g., bot fly larvae, ticks), and location were recorded. In addition, trappers took detailed notes about the physical features of each animal. In total, 543 (2 percent) of the mice had some form of physical impairment, including broken, deformed or missing tails or limbs, missing eyes or eyes with cataracts. 

“White-footed mice are survivors. We know they thrive in degraded forests and can tolerate high parasite loads. We were interested in determining if this tolerance extended to physical impairments,” said study lead author Francesca Rubino of the UC Davis School of Veterinary Medicine.

Mouse survival was determined by estimating persistence time in the trapping area, and body mass, movement, and ectoparasite burdens (larval bot fly and ticks) were also analyzed.

“To our knowledge, no prior study has included as big a sample size of individuals or as long a time series. Having mark-recapture records on a large number of mice both before and after their impairment provided us with a unique opportunity to distinguish cause-effect relationships from mere correlation,” explained Ostfeld, a disease ecologist at Cary Institute of Ecosystem Studies.  

Although mice with tail and limb impairments carried more black-legged ticks and mice with cataracts had more bot fly larvae, the overall results showed that mice with impairments were just as likely as their unimpaired counterparts to survive on the trapping plots. In fact, mice with eye and tail impairments had greater average mass than unimpaired mice, and mice with tail impairments had larger home ranges.

“We found no evidence that physical impairments in white-footed mice were associated with decreased fitness,” explained Rubino. “On the contrary, looking across standard measures of fitness, mice with impairments performed at least as well, on average, as mice without impairments.”

“Mice appear able to compensate for broken or missing major appendages and for partial or total loss of vision in ways that generally avoid compromising longevity, body condition, movement, or protection from ectoparasites,” said Ostfeld.

The apparent resilience of mice in this study is consistent with data from prior studies that shows the mice can tolerate infection by some parasites. Whether this tolerance to injury and infection is shared with other species, and the potential mechanism underlying such high tolerance, are as yet unknown. 

“This study is both powerful – more than 27,000 wild mice over 26 years – and surprising,” said Doug Levey, a program director in NSF’s Division of Environmental Biology, which provided grant support for the work. “Until now, few ecologists would have said that impaired mice live as long as mice that are free of impairments.”

“Our findings challenge broader assumptions that physical impairments reduce the fitness of their bearers and are inconsistent with the devaluation of impaired individuals that pervaded early thinking in evolutionary biology.”

The study is published in the journal Proceedings of the Royal Society B.

By Alison Bosman, Staff Writer

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