Experimental susceptibility of two boreal toad populations to native Batrachochytrium dendrobatidis

Chytridiomycosis caused by Batrachochytrium dendrobatidis has been implicated in amphibian declines worldwide.  Susceptibility varies across species, but differences within species have been less widely observed.  Boreal toad (Bufo boreas) populations have declined throughout Colorado, yet remain stable at sites within Grand Teton National Park (GTNP) in Wyoming.  B. dendrobatidis infection is prevalent in both regions.  We compared the disease susceptibility of animals from two sites:  wild-caught toadlets from GTNP and toadlets from the Colorado Division of Wildlife's Native Aquatic Species Restoration Facility.  B. dendrobatidis used for infection was isolated from GTNP toads.  We also varied access to dry basking sites.  The resulting experiment was a 2 x 2 x 2 design:  pathogen (exposed or control), toadlet source (WY or CO), and dry-site availability (low or high).  We monitored toadlet behavior, disease response (incl. skin shedding), and survival over 5 weeks until all the 'exposed' animals had died.  The B. dendrobatidis from GTNP was lethal to all those exposed, but Wyoming toadlets survived longer.  Exposed toadlets shed more and sought out dry basking sites more frequently than controls.  Longer survival time in Wyoming toadlets may be due to higher pathogen resistance, or because their slightly smaller size permitted more access to dry sites within cages.  Our findings suggest that behavioral responses to B. dendrobatidis may ameliorate infection and that populations of B. boreas may differ in their disease resistance.  Our future studies focus on teasing apart the effect of these behavioral and evolutionary factors on disease-induced declines.