Abstract
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 individual’s feces. During
both the nesting and mating seasons, male tortoises were more likely to
use burrows treated with another male’s 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.