American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“Infection status as the basis for habitat choices in a wild amphibian”

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Gabriel M. Barrile, Anna D. Chalfoun, and Annika W. Walters (Jan 2021)

Infection status shapes habitat choices in a wild amphibian; boreal toads clear chytrid infection by switching habitats

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A boreal toad (<i>Anaxyrus boreas boreas</i>) waits patiently as researchers attach a radio transmitter around its waist via a belt harness. Boreal toads tracked in western Wyoming during 2016 were able to clear infection with the amphibian chytrid fungus by seeking out warm, open habitats that helped to elevate their body temperature. <br />(Credit: Jessica Ulysses Grant, <a href="http://www.jessicaulyssesgrant.com">www.jessicaulyssesgrant.com</a>)
A boreal toad (Anaxyrus boreas boreas) waits patiently as researchers attach a radio transmitter around its waist via a belt harness. Boreal toads tracked in western Wyoming during 2016 were able to clear infection with the amphibian chytrid fungus by seeking out warm, open habitats that helped to elevate their body temperature.
(Credit: Jessica Ulysses Grant, www.jessicaulyssesgrant.com)

Infectious diseases pose a serious threat to the health of people, domestic animals, and wildlife alike. The chytrid fungus, for example, has already caused the extinction of over 100 amphibian species and continues to plague hundreds more across the globe. Boreal toads, once common in high-elevation habitats across the Rocky Mountain West, have suffered severe declines due to the chytrid fungus in recent decades. Boreal toads in western Wyoming, however, persist despite high infection rates with the fungus.

Gabe Barrile, a graduate researcher from the University of Wyoming, along with faculty advisors Anna Chalfoun and Annika Walters, study the lives of boreal toads in the mountains of western Wyoming. Toads usually take refuge under riparian willows or in small mammal burrows. Occasionally, however, the researchers observe toads in open environments, often basking in the exposed sunlight. They hypothesized that use of sheltered habitats, which are cool and moist, versus the use of open habitats, which are warm and dry, may be related to whether an individual toad is infected with the chytrid fungus. To test this idea, they obtained disease samples and recorded body temperatures of radio-tracked toads during the summer of 2016. They then matched disease and thermal data to the habitats that toads used through time.

The researchers found that wild boreal toads use moist, sheltered habitats when disease-free but move to warmer, more open habitats when infected. Switching habitats in response to infection appears to be advantageous, as increased warmth in open habitats is associated with the clearing of infection, likely via the elevation of body temperature. These findings suggest small-scale microhabitat manipulation to create warm patches may comprise an effective mitigation action against the chytrid fungus, and possibly other amphibian diseases. More broadly, a deeper understanding of disease dynamics better equips us to improve human and animal health in the face on infectious onslaughts.


Abstract

Animals challenged with disease may select specific habitat conditions that help prevent or reduce infection. Whereas pre-infection avoidance of habitats with a high risk of disease exposure has been documented in both captive and free-ranging animals, evidence of post-infection habitat switching to conditions that promote the clearing of infection is limited to laboratory experiments. The extent to which wild animals proximately modify habitat choices in response to infection status therefore remains unclear. We investigated pre-infection behavioral avoidance and post-infection habitat switching using wild, radio-tracked boreal toads (Anaxyrus boreas boreas) in a population challenged with Batrachochytrium dendrobatidis (Bd); a pathogenic fungus responsible for a catastrophic panzootic affecting hundreds of amphibian species worldwide. Boreal toads did not preemptively avoid microhabitats with conditions conducive to Bd growth. Infected individuals, however, selected warmer, more open habitats, which were associated with elevated body temperature and the subsequent clearing of infection. Our results suggest that disease can comprise an important selective pressure on animal habitat and space use. Habitat selection models therefore may be greatly improved by including variables that quantify infection risk and/or the infection status of individuals through time.