Bighorn sheep associations: understanding tradeoffs of sociality and implications for disease transmission

Abstract

Sociality directly influences mating success, survival rates, and disease, but ultimately likely evolved for its fitness benefits in a challenging environment. The tradeoffs between the costs and benefits of sociality can operate at multiple scales, resulting in different interpretations of animal behavior. We investigated the influence of intrinsic (e.g., relatedness, age) and extrinsic factors (e.g., land cover type, season) on direct contact (simultaneous GPS locations ≤ 25 m) rates of bighorn sheep (Ovis canadensis) at multiple scales near the Waterton-Glacier International Peace Park. During 2002-2012, male and female bighorn were equipped with GPS collars. Indirect contact (GPS locations ≤ 25 m regardless of time) networks identified two major breaks whereas direct contact networks identified an additional barrier in the population, all of which corresponded with prior disease exposure metrics. More direct contacts occurred between same-sex dyads than female-male dyads and between bighorn groups with overlapping summer home ranges. Direct contacts occurred most often during the winter-spring season when bighorn traveled at low speeds and when an adequate number of bighorn were collared in the area. Direct contact probabilities for all dyad types were inversely related to habitat quality, and differences in contact probability were driven by variables related to survival such as terrain ruggedness, distance to escape terrain, and canopy cover. We provide evidence that probabilities of association are higher when there is greater predation risk and that contact analysis provides valuable information for understanding fitness tradeoffs of sociality and disease transmission potential.

Keywords: Contact; Dilution effect; Glacier National Park; Habitat selection; Landscape of fear; Landscape of peril; Many eyes hypothesis; Ovis canadensis; Respiratory disease.

Figure 1. Map of the study area in the Waterton-Glacier International Peace Park and the Blackfeet Indian Reservation.

Centroids of all GPS locations collared bighorn sheep (Ovis canadensis) during 2002 –2012 that were used to document contacts are shown (blue for males, pink for females). Red lines indicate possible breaks in the bighorn sheep population structure and line type indicates relative permeability of barriers (solid for no indirect contacts, dashed for few direct and indirect contacts, and dotted for no direct contacts but many indirect contacts). (1) West Waterton, (2) North Glacier (a: East Waterton, b: Many Glacier), (3) South Glacier.

Figure 2. Network structure of bighorn sheep (Ovis canadensis) in Waterton-Glacier International Peace Park during 2002 –2012.

Each panel represents a subpopulation based on the connections of indirect contacts. (A) West Waterton, (B) North Glacier, (C) South Glacier. Indirect contacts were defined as GPS locations of 2 bighorn sheep within 25 m regardless of timing. Each node represents the centroid of GPS locations of each collared bighorn sheep. Node color represents sex (pink for females, blue for males) and line width represents the number of indirect contacts. Lines extending to the right of panel A, from the lowest dot, are connected to lines extending to the left of panel B and represents a single individual from West Waterton subpopulation connected to multiple individuals in the North Glacier subpopulation. See Fig. 1 for overall configuration.

Figure 3. Annual number of direct contacts per dyad by month during 2002–2012.

Direct contacts were defined as simultaneous GPS locations of two bighorn sheep (Ovis canadensis) ≤ 25 m. Colors represent dyad type (pink for female–female dyads, blue for male-male dyads, and black for male–female dyads). Only dyads with at least 1 direct contact during the month were included. Box and whisker plot depicts median as middle line, inter-quartile range as box, 1.5 times the inter-quartile range as whiskers, and outliers as dots. Note: number of direct contacts should not be compared between dyad types given uneven collar deployments throughout the study.

Figure 4. Comparison of ranked probability of general use and ranked probability of direct contact given general use by bighorn sheep (Ovis canadensis) in the Waterton-Glacier International Peace Park.

Probability of general use and probability of direct contact given general use ranked by decile (1 = least probable, 10 = most probable). Colors represent ranked probability of direct contact given general use. (A) female-only, (B) male-only, and (C) male and female dyads of bighorn sheep.