Ph.D. in Zoology 1994
Dissertation title: Patterns of burrow use by desert tortoises: influence of microclimate and chemical cues
xi + 219 pages
Burrow use was studied in free-ranging desert tortoises
(Gopherus agassizii) from June to October, 1992 in Clark County,
Nevada. Patterns of burrow use and co-occupancy differed between
the sexes and corresponded to the reproductive cycle. Most observations
were of individuals inhabiting shelters singly. Movement among shelters
was greater by females during the nesting season and by males during the
mating season, at which time male-female co-occupancy was commonly observed.
Immature tortoises are difficult to locate in the field; therefore, burrow use by captive adult and immature desert tortoises was observed in semi-natural enclosures. Adults used more burrows and switched more often than did immatures. A higher proportion of artificial burrows than natural burrows was found inhabited. Burrow sharing occurred within and among age groups, and tortoises did not share more frequently with individuals in their own age group.
Temperature and humidity varied among burrows measured between 1000-1200 h during three months of the active season and was influenced by burrow characteristics (e.g., length) and by surface conditions. Evaporative water loss predicted using biophysical models was consistently lower inside burrows than on the surface, except after rain and at night. Based on microclimate measurements made during one day in July 1993, a tortoise spending 24 hours in a burrow would lose less water than if on the surface and still less by spending the day in a burrow and the night on the surface.
Burrow use patterns may also be influenced by chemical cues left by previous inhabitants. To examine the role of socio-chemical cues in burrow choice, adult tortoises were presented a two-choice test between identical burrows, one an unscented control and the other treated with male chin gland secretion or another individualís feces. During both the nesting and mating seasons, male tortoises were more likely to use burrows treated with another maleís chin gland secretion rather than control burrows. Females were less likely to use burrows treated with feces from another female than control burrows during the nesting season.
The studies presented here have implications for tortoise conservation issues including relocation programs and disease epidemiology.