Time spent within burrows by Sonoran Desert Tortoises at 2 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.We analyzed tortoise burrow use with the recurse package in R. The function ‘getRecursionsAtLocations’ determined the segments of a tortoise’s movement trajectory by connecting the closest dates and added a revisit to each mapped burrow the trajectory intersected. Because each burrow a tortoise used was individually marked, GPS coordinates were consistent across observations within a burrow, and we applied a 1-m radius to the recursion analysis. We identified the number of unique burrows used by each tortoise, the mean number of visits by each tortoise to each burrow, and the mean duration each tortoise occupied each burrow, including those used during hibernation. Absolute duration estimates are imprecise due to the relatively long tracking interval, but we used them to investigate general patterns of use.We tested whether the time tortoises spent within burrows differed by site, sex, or the estimated date that the tortoise entered the burrow. We square-root-transformed the response variable, the estimated number of days spent within a burrow, and we used R package <i>glmmTMB</i> to evaluate effects of the predictor variables with a generalized linear mixed-effects model (GLMM) and Gamma distribution with a log link; we used burrow ID and tortoise ID as random effects.