The Agassiz’s or Mojave Desert Tortoise (Gopherus agassizii, herein Desert Tortoise or tortoise) is a federally threatened species with declining populations in most parts of the geographic range (US Department of the Interior 1990; US Fish and Wildlife Service 2015; Allison and McLuckie 2018). The literature on wild adult tortoise behaviors is better known than behaviors of wild, unmanipulated prereproductive tortoises. Intraspecific sharing of burrows, dens, and caves by wild adults, especially during late fall and winter, was described by Woodbury and Hardy (1948), Burge (1977), Harless et al. (2009), and others. Woodbury and Hardy (1948) recorded as many as 17 tortoises of both sexes in winter dens with tunnels of up to 9.14 m. Harless et al. (2009) distinguished between shared burrows (no simultaneous use) and cohabitation in a study of adults in the central Mojave Desert; both types of occupation occurred during the study. Late summer and fall cohabitation or simultaneous sharing of burrows or dens by male and female adults coincides when male testosterone levels rise to a peak of 243.60 ng/ml, males engage in courtship, and effective matings occur (Rostal et al. 1994; Lance and Rostal 2002). Testosterone levels decline to a fraction of that level before brumation and reach a low point in spring, when females nest. Other species of vertebrates, invertebrates, and arachnids are reported to occupy burrows and dens of adults (e.g., Woodbury and Hardy 1948; Agha et al. 2017).

We asked the question: Do prereproductive Desert Tortoises cohabit burrows in the wild? This question is important because juvenile and immature tortoises have low survival due to multiple sources of death until > 100 to 125 mm in straight-line carapace length at the midline (MCL; Nagy et al. 2015a, 2015b, 2020; Berry et al. 2023). A greater understanding of the challenges facing survival could improve opportunities for more successful outcomes in head-started and wild tortoises. We first reviewed a compilation of observations of 1154 wild, free-living individuals on 1403 occasions in capture, mark, and release projects at 18 sites in the Mojave and western Sonoran deserts of California between 1971 and 1982 (Berry and Turner 1984, 1986). The tortoises in the study were juvenile and small immature sizes, < 60 to 140 mm (MCL). The observations were limited to spring when tortoises were more likely to be observed aboveground and foraging than during summer and winter, when temperature extremes cause tortoises to retreat underground in burrows. Observations of 324 tortoises basking on a burrow mound, inside a burrow or pallet, entering or exiting a burrow, or digging a burrow were compiled. None of the observations were of tortoises sharing burrows.

We then reviewed the literature (Berry et al. 2023) to address the question of solitary burrow use for a 9-yr study of 83 juvenile and small immature tortoises that we translocated from crowded head-start pens where some individuals shared burrows (Mack et al. 2018) comparable to observations in head-start pens at the Fort Irwin National Training Center in the central Mojave Desert (e.g., Morafka et al. 1997). In both cases, burrow sharing was a result of insufficient sites to construct burrows, often necessitating a lineup of 3 or more tortoises facing inward in a single, narrow burrow, with the posterior tortoises exposed to the elements. In our study, the head-start pens were on Edwards Air Force Base in the southwestern Mojave Desert, California, and the translocation site was also on that base (Berry et al. 2023). The 83 tortoises were translocated in 2 groups, 1 each in fall of 2013 and 2014 to a 15-km² site where only an adult male was infrequently observed. The site was depleted of tortoises, like most of the western Mojave Desert (US Fish and Wildlife Service 2015; Allison and McLuckie 2018). The tortoises were in 3 life stages based on MCL at the time of translocation: juvenile 1, < 60 mm; juvenile 2, 60–99 mm; and immature 1 (small), 100–139 mm (Berry et al. 2023). Prior to release and throughout the study, tortoises were fitted with radio transmitters of an appropriate size and weight for the individuals (BD-2, 2.4 g; PD-2, 3.6 g; and as they grew to large immature and small adult sizes, R1-2B, 10 g; Holohil Systems Ltd, Carp, Ontario, Canada). We tracked them monthly and more frequently when transmitters were changed (≥ 2 times per year) through December 2022 using 2 types of receivers (R2000, Advanced Telemetry Systems Inc., Isanti, MN; and R-1000, Communications Specialists, Inc., Orange, CA). We recorded locations with a global positioning system (Garmin GPS Map 62s, Garmin Ltd., Olanthe, KS) (Berry et al. 2023).

We looked for potential tortoise cohabitation at above- and belowground locations of the tortoises. During the first few years, when most tortoises were juveniles, the tortoises enlarged abandoned rodent or other burrows; the larger juveniles and small immature tortoises excavated their own new burrows in soils (Berry et al. 2023). Many burrows were in coppice mounds beneath or in the dripline of shrubs. As tortoises grew larger, they dug their own burrows and continued to enlarge burrows, particularly the burrows repeatedly used during periods of temperature extremes in summer (June–August) and fall–winter months (October–February). We recorded 594 such burrows. For example, we identified 13 tortoises that repeatedly used the same individual burrows consecutively for 2 to 9 seasons (average = 4.38 seasons) and grew 10.7 to 57.2 mm (average = 25.8 mm), requiring enlargement of the tunnels. Tortoises created additional burrows and pallets in spring, late summer, and early fall.

We observed 2 events where tortoises co-occurred at a burrow. The first observation (5 September 2016), an apparent brief cohabitation of 1 to several days, involved a juvenile (72.8 mm MCL), no. 632, with a small immature tortoise (118.6 mm MCL), no. 32. Number 32 had occupied the burrow in August and September of 2015 and since 8 June 2016. Tortoise 632 joined the larger tortoise between August and 5 September 2016, when the tortoises were disturbed to change transmitters and conduct health evaluations. After change of radio transmitters, both tortoises left the burrow and settled in new and separate locations, where they were observed during the 24-hr check conducted after transmitters were changed. Tortoise 32 returned to its previously used burrow and remained there through fall and winter of 2016–2017. The smaller tortoise, no. 632, left and was in a different fall–winter burrow during 2016–2017 at the same location used in winter of 2015–2016. Tortoise 632 was killed by a common raven (Corvus corax) in spring of 2017.

The second event appeared to be a brief defense of burrow. A small, immature tortoise (122 mm MCL), no. 46, was resting inside the entrance of a burrow on 27 April 2019, when a large immature female tortoise (179 mm MCL), no. 20, arrived and attempted to enter. Tortoise 46 rammed and jostled the larger tortoise, attempting to drive it away. Ultimately the smaller tortoise, no. 46, departed; no records existed for no. 46 at this location, either later in the year, the previous or following years. The larger tortoise, no. 20, occupied the burrow in the following month but was not observed there in the previous year, later in the year, or during the following year. Tortoise 46 had excavated the burrow when no. 20 arrived and displaced it.

We conclude that prereproductive tortoises are unlikely to share burrows with another tortoise in the wild, having observed only 2 events of co-occurring locations (2/4622, 0.04%); or 1 at a burrow used during both summer and winter extremes (1/594, 0.17%). Because we did not observe tortoises more than once on most months and over a 24-hr basis when transmitters were changed twice a year, more cohabitations may have occurred than were recorded. On several occasions, we also observed Mohave rattlesnakes (Crotalus scutulatus) cohabiting and overwintering together with tortoises 110.2–170 mm MCL in their burrows. The snakes occasionally blocked the entrances, preventing extraction to change transmitters.

Juvenile tortoises have the capacity to occupy different desert vegetation and soil types when living naturally or when released from head-start pens (e.g., Berry and Turner 1984; Berry et al. 2023). Our observations indicate that those individuals likely to survive quickly establish and repeatedly used burrows for protection from temperature extremes and dig long tunnels with protective soil cover or rocks over the tunnel. We observed one such case when a 140-mm MCL tortoise died in its burrow: the tunnel was 1.7 m in length with 0.65 m of soil cover over the deepest end. For selecting sites to release head-started tortoises, the cover of shrubs, presence of coppice mounds, and soils suitable for digging deep burrows with long tunnels could be a consideration. To better observe use at such sites, trail cameras would benefit the collection of more detailed and potentially pertinent data for future selection of promising release sites.

Advantages exist for solitary living for young tortoises: they can emerge and retreat anytime, responding quickly to warm temperatures, emerging after brumation, and retreating quickly when environmental temperatures exceed body operating temperatures. They can rapidly clean out and expand the size and depth of burrows without having another tortoise obstruct these typical behaviors. Additionally, with 1 tortoise to a burrow, a predator can dig out and kill a single tortoise, but not multiples.