Post-Partum Habitat Use for Peninsular Bighorn Sheep (Ovis canadensis nelsoni) in Southern California

Peninsular bighorn sheep (Ovis canadensis nelsoni) live in the Peninsular mountain range of Southern California. Peninsular bighorns were federally designated as an Endangered species after a severe population decline occurred, likely caused by a combination of factors including fragmentation, urban development and exposure to disease. A better understanding of bighorn sheep habitat needs can lead to more comprehensive and beneficial protection. The neonate stage of any mammal’s life is one of its most vulnerable times. Research has shown that many ungulate species exhibit strategies to reduce predation during the pre-partum period. The primary predator of Peninsular bighorn sheep is the mountain lion (Puma concolor), predation is responsible for up to approximately 60% of sheep deaths. Many ewe groups in the Peninsular bighorn population divide their time between two locations throughout the year referred to as the pre-partum and post-partum periods. Ewe groups are formed based on availability of other females rather than relatedness (Boyce et al. 1999, Festa-Bianchet 1991). While ewe groups spend the majority of their time in pre-partum regions, they move to a post-partum location for a few months. The objective of this study was to quantitatively describe and compare habitat characteristics to find which might be of key importance to Peninsular bighorn ewes during the post-partum period. The habitat characteristics examined in this study were aspect, elevation, percent slope, and distance to alluvial fans, distance to predator corridors, and distance to water sources. The habitat characteristics were compared between locations collected using Global Positioning System collar data during 30-day windows during the pre-partum period (October) and presumed (actual lambing locations not typically known) post-partum period (March). Twenty-eight ewes were studied from 2009-2017 from four ewe groups, In-Ko-Pah (IKP), Carrizo Canyon (Carr), Coyote canyon (CoC) and the South San Ysidro Mountains (SSYM). Differences in habitat use between pre-partum (October) and post-partum (March) period for each of the habitat characteristics was modeled using mixed effects logistic regression for each group, with the ID of each ewe used as a random effect to account for repeated measurements. Standardized logistic regression coefficients (SLRC) and differences in mean between periods were used for interpretive purposes (pre-partum period means are x̅2 and post-partum means are x̅1 in all comparisons). Ewes moved further from water (Carr mean = 11,671 m, SLRC = 3.5; CoC mean = 5,476 m, SLRC =-12.4; IKP-A = mean 1,049 m, SLRC= -0.8) and predator corridors (Carr mean = 10,359 m, SLRC = -3.4; CoC mean= 4,607 m, SLRC= -11.0; IKP-A mean = 409 m, SLRC=-0.7), while moving closer to alluvial fans (Carr mean = 391 m, SLRC= 1.0; CoC mean = 370 m, SLRC= 0.8; IKP-A mean = 561 m, SLRC= 1.0) when choosing postnatal sites in three of the four ewe groups. Ewes also moved to post-partum regions with lower elevation in two out of the four groups (Carr mean = 92 m, FE= 0.8; IKP-A mean = 92 m, FE = 0.8). One group in particular, from the In-Ko-Pah area, had additional data, known lambing dates and locations. The location data from this group was additionally analyzed by comparing the 30-day October window to a 30-day period directly after the known lambing date (unique to each ewe). Both methods for the IKP group (30-day locations from presumed pot-partum vs. actual post-partum period) shared the same trends for elevation, distance to water, distance to predator corridors, and distance to alluvial fans, supporting the validity of comparing data from October to March in other areas, and demonstrating the importance of these factors to site use by ewes. This study showed Peninsular bighorn ewes select post-natal sites based on habitat characteristics and further that traits chosen could be to deter predators and increase neonatal survivorship.