Effects of variable larval tick burdens on survival of white-footed mice (Peromyscus leucopus)

Background/Question/Methods

White-footed mice (Peromyscus leucopus) in the Northeastern United States are frequently parasitized by blacklegged ticks (Ixodes scapularis). Larval burdens can range dramatically from zero to over 250 ticks per host during larval peaks. Although ticks at this life stage are not known to transmit the casual agents of Lyme disease or other common regional human diseases, high ectoparasite burdens can carry other potential costs to small mammals. Ectoparasites can impact small mammal survival, potentially through direct negative effects on body condition or indirect costs of immunological responses, altered behavior, or resource allocation. We employed a Bayesian state-space mark-recapture model to estimate survival and capture probabilities of white-footed mice, along with an effect of maximum larval tick burden on mouse survival. We tested this model using simulation, then fit it using seven years of live-trapping data collected on the property of the Cary Institute of Ecosystem Studies in Millbrook, NY. 

Results/Conclusions

Mouse survival and recapture probabilities varied annually. Survival probabilities between trapping sessions (ca. three weeks) ranged from 0.631 to 0.837 (mean = 0.741), while capture probabilities ranged from 0.658 to 0.887 (mean = 0.813). Contrary to expectations, larval ticks did not reduce mouse survival. In five of seven years, mouse survival was not correlated with larval tick burden, but in two years higher tick burdens were positively correlated with survival. In these two years the credible intervals for beta, the regression coefficient representing the effects of tick burden on survival, did not contain zero. Although the mechanism for this positive correlation of tick burdens with survival is not immediately apparent, it could be that more vigorous animals encounter greater numbers of ticks, or that any negative effects of ectoparasitism are apparent in reproductive output rather than survival. Nevertheless, increased mouse survival associated with heavy tick burdens could feed back to increase feeding opportunities for ticks and risk of tick-borne diseases.