Impaired wild mice survive and thrive as well as unimpaired counterparts
Long-term monitoring calls into question evolutionary biology assumptions


Date:
November 3, 2021
Source:
Cary Institute of Ecosystem Studies
Summary:
A new study 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.



FULL STORY ========================================================================== White-footed mice are able to survive and thrive with physical
impairments. A new study in Proceedings of the Royal Society Bassessed
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 Cary Institute of
Ecosystem Studies, comments, "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 research
team analyzed 26 years of data from a long-term mark-recapture program
that Ostfeld initiated on Cary Institute's Millbrook, NY property
in 1991. Small mammals were trapped at six plots in deciduous forest
dominated by oaks and maples. Catch-and-release trapping occurred every
3-4 weeks over 2- 3 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 (e.g., bot fly larvae, ticks), and location were recorded. In addition, trappers took detailed notes about the physical
features of each animal.

Lead author Francesca Rubino of the UC Davis School of Veterinary
Medicine was a former Project Assistant in the Ostfeld Lab. She explains, "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." To assess the impact of physical impairments on the fitness of mice, trapping 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 trapping plots. Body
mass, movement, and ectoparasite burden (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." Ostfeld explains.

Of the 27,244 mice, 2% (543) had visible physical impairments. Although
tail and limb impairments were associated with more black-legged ticks
and mice with cataracts had more bot fly larvae, mice with impairments
were just as likely as their unmpaired counterparts to survive on the
trapping plots. Mice with eye and tail impairments had greater mass than unimpaired mice, and mice with tail impairments had larger home ranges.

Rubino explains, "We found no evidence that physical impairments in white- footed mice were associated with decreased fitness. On the contrary,
looking across standard measures of fitness, mice with impairments
performed at least as well, on average, as mice without impairments."
With Ostfeld adding, "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." The authors note that this resilience is consistent with prior studies revealing the ability of mice
to tolerate infection by some parasites. Whether this tolerance to injury
and infection is shared with other species, and if so, what mechanisms
might underlie high tolerance, are unknown, but worthy of further study.

"This study is both powerful -- more than 27,000 wild mice over 26
years -- and surprising," says 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," Ostfeld concludes.

This research was supported in part by the US National Science
Foundation's Long Term Research in Environmental Biology (LTREB) program
(grant numbers: DEB 0075277, DEB 0444585, DEB 0949702, and DEB 1456527).

========================================================================== Story Source: Materials provided by
Cary_Institute_of_Ecosystem_Studies. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. Francesca I. Rubino, Kelly Oggenfuss, Richard S. Ostfeld. Effects of
physical impairments on fitness correlates of the white-footed
mouse, Peromyscus leucopus. Proceedings of the Royal Society B:
Biological Sciences, 2021; 288 (1962) DOI: 10.1098/rspb.2021.1942 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/11/211103200430.htm

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