Available and shared burrows within pairs of overlapping home ranges of Sonoran Desert Tortoises in two Arizona populations

At Sugarloaf we monitored female tortoises (range 184–289 mm midline carapace length [CL]) weekly using radio telemetry in 1991–1993 and 1997–2005 as part of a reproductive ecology study (Averill-Murray 2002; Averill-Murray et al. 2018). During the 11 years of radio-tracking, all burrows used by both telemetered and opportunistically encountered tortoises were marked with numbered aluminum tags (n = 522). We use the term “burrow” specifically to refer to a subsurface cavity &gt;1/2 the tortoise’s length either formed by erosion or excavated by a tortoise or another animal (Burge 1978). We categorized burrow types as “rock” (cover provided by rocks or boulders, including large boulder piles in which the tortoise could not be visualized), “soil” (cover provided by soil or vegetation with no substantial rock component), and “packrat” (freestanding white-throated woodrat Neotomoa albigula nest independent of other shelter types; packrat nests inside rock shelters were categorized as “rock”). We did not individually mark “pallets” (shallow, scraped out areas &lt;1/2 tortoise length) or other temporary shelters unmodified by the tortoise (e.g., under trees, shrubs, or rock overhangs).At FMR, as part of a habitat-use study we attached radio transmitters (ATS, AVM, Telonics, or Wildlife Materials) to the anterior carapace of adult tortoises as above (Riedle et al. 2008). During the winter months (November through February) when tortoises were inactive, we located tortoises once a week. During the activity season (March through October) we located tortoises 2–3 times weekly, obtaining both morning and evening locations. From 2000 to 2003, we searched areas in which tortoises might occur within FMR, concentrating on sites suitable for burrow excavation, especially including incised washes with caliche caves and the volcanic hill. We also searched all washes within the study area, whether incised or not, and spent considerable time on the alluvial fans. We marked burrows in which we observed tortoises with individually numbered aluminum tags (n = 124), and we mapped locations of all caliche caves large enough to shelter a tortoise ≥180 mm CL (n = 463). Burrow types included caliche, rock, soil, and packrat.In order to directly compare patterns of space and burrow use between sites and to minimize potential variation due to inter-annual environmental variation, we estimated home ranges at both sites for the years of common study, 2000–2003. We only included adult tortoises (all &gt;220 mm CL) in the analysis, and we excluded observations between the first and last dates of hibernation each year, estimating the date that each tortoise terminated hibernation as the last day the tortoise was observed inside or &lt; 10 m from its hibernaculum. We calculated variograms, fit movement models, and estimated home ranges with area-corrected, optimally weighted, autocorrelated kernel density estimation (wAKDEC) using the ctmm package in R. We estimated total home ranges across the entire study period using perturbative Hybrid REML (pHREML) wAKDEC conditional on the selected underlying movement model for each tortoise. We estimated core areas as the area encompassed by the 50% AKDE isopleth, the proportion of the total (95%) home range area contained by the 50% core area (PA), and the intensity of core area use (ICU = core area isopleth/[50% core area/95% AKDE area]). We used the ‘overlap’ function in <i>ctmm</i> to calculate the bias-corrected, Bhattacharyya coefficient overlap of AKDE home ranges between individuals. We counted the number of burrows available within the common portion of each overlapping pair of 95% AKDE home ranges. <b>Location data are not included because </b><b><i>Gopherus morafkai</i></b><b> is a protected species. Inquiries for these data should be made to the Arizona Game and Fish Department.</b>Looking into how tortoises shared burrows (at any time, not only simultaneously) within overlapping home ranges, we used a generalized linear model with a Poisson distribution in R package <i>glmmTMB</i> to evaluate the hypothesis that burrows were shared equally among sex dyads (female-female, female-male, male-male) and among sites. The log10 of the total number of burrows available within the overlapping portion of each individual dyad’s home ranges served as an offset for the count of shared burrows, we included the log10 of the total number of observations between individuals of each dyad to control for differences in sample sizes, and we included the log10 of the estimated home range overlap to correct for the degree to which sharing burrows was possible.