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Habitat Selection of Female Desert Bighorn Sheep: Tradeoffs Associated with Reproduction
AuthorBlum, Marcus Earl
AdvisorStewart, Kelley M
Ecology, Evolution and Conservation Biology
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Animals select habitat types that enhance their ability to survive, reproduce, and therefore enhance reproductive fitness. Selection for specific habitat types often varies within populations based on season, habitat availability, sex, age, reproductive status, and other characteristics. Therefore, we expect habitat selection to be changing throughout the lifetime of an individual to meet the metabolic and nutritional constraints of specific periods. For instance, reproductive status, especially provisioning dependent young, is commonly linked to changes in behavior of female ungulates that results in variation in resource selection around parturition. Around parturition, female ungulates are commonly linked to tradeoffs between maternal nutritional condition and survival of offspring. These tradeoffs are hypothesized to take place because they allow females to increase their reproductive fitness by enhancing the likelihood that their offspring survives to recruitment. Understanding these shifts in habitat selection are essential to proper management and the conservation of ungulates.I was interested in documenting the variation in habitat selection of female bighorn sheep (Ovis canadensis nelsoni) around parturition, in addition to characterizing composition and quality of diets of females based on time of year and the provisioning status of individuals. This period is critical to female ungulates because their choice of habitat components also influences survival of their offspring and ultimately, the female’s reproductive fitness. I used 2 populations of desert bighorn sheep in west-central Nevada, to study how ungulates adjust habitat selection around parturition. I investigated habitat selection of female desert bighorn sheep from the beginning of their third trimester until weaning of offspring. Furthermore, I investigated how females selected parturition sites and neonates selected bed sites immediately following birth. To accomplish this goal, I captured adult and neonatal bighorn sheep and equipped individuals with very high frequency (VHF) and global positioning system (GPS) radio-collars from 2016 to 2018. Additionally, I collected fecal samples from female bighorn sheep throughout the year and within the parturition season, based on pregnancy and provisioning status of individuals. I also used unmanned aerial vehicles and publicly available remote sensing data to characterize birth sites of parturient females and bed sites of female-offspring pairs. I found that females adjusted habitat selection, based on provisioning status, by trading off maternal nutritional condition for survival of offspring. Prior to parturition, females selected areas with higher forage availability, however, following parturition, females adjusted selection to areas with habitat features that are commonly associated with increased survival of young (i.e., steep slopes, rugged terrain, and more open habitats). Furthermore, as neonates aged, females adjusted habitat selection back to pre-parturition selection levels, where females would be able to fulfill nutritional needs and offspring were less vulnerable to predation. These apparent tradeoffs were also supported by my analyses of diet composition and quality. Females that were not provisioning offspring tended to have higher percentages of nitrogen in fecal pellets indicating that they were consuming higher quality diets than those females that were provisioning offspring. Diet quality of females also varied throughout the year, where spring and summer months had higher fecal nitrogen content than winter months. Additionally, females adjusted diet composition based on provisioning status and season. I found that female bighorn sheep differed in selection strategies for birth and bed sites at each study area. Overall, females tended to select parturition sites at broad scales with steep slopes, high visibility, and close to ridgelines. Microhabitat selection for birth sites was similar to broad scale selection, other than females selected for low downslope visibility and for less steep slopes. Finally, parent-offspring pairs selected bed sites with higher amounts of concealment cover than birth sites.