Burrows are critical refugia for desert tortoises (Gopherus agassizii), buffering individuals from extreme temperatures and desiccation. This buffering is crucial over winter when surface temperatures can fall below freezing and tortoises remain largely inactive, surviving on limited energy and water reserves. While burrow use is well documented, less is known about how burrow structure and microclimate vary across life stages during overwintering. We compared burrow structure and microclimate used by adult and juvenile tortoises over two winters in the Mojave Desert. Microclimate data were collected using temperature and humidity loggers affixed to tortoises, representing the conditions experienced during brumation, spent primarily in their chosen winter burrow. On average, juveniles used shorter, narrower burrows and experienced lower temperatures, higher relative humidity, and lower vapor pressure deficits than adults. This suggests the narrower burrow structure and associated microclimate experienced by juveniles partially compensate for the greater water-loss susceptibility of juveniles due to their higher surface area-to-volume ratios. These results reveal ontogenetic differences in burrow structure and highlight that juvenile tortoises can access hydric microclimates equal to or more favorable than those available to adults. Juveniles may possess greater resilience to dry conditions than would be expected based solely on their physiological vulnerability.

Katie Rock is a Research Associate for the San Diego Zoo Wildlife Alliance Desert Tortoise Program. Since starting this role in 2023, she has been working on the desert tortoise headstart program, thermal ecology research, and radiotracking at Edwards Air Force Base in the Mojave Desert. Previously, she worked for two years as a consulting biologist at HDR in Sacramento. She is a California Polytechnic State University, San Luis Obispo alumna with a B.S. in Biological Sciences, who led research on the authorship gender gap in the herpetology field and assisted with thermal ecology research on blunt-nosed leopard lizards.