HELODERMA SUSPECTUM(Gila Monster). PREY

HELODERMA SUSPECTUM (Gila Monster). PREY. Gila Monsters are specialized nest predators; their diet includes eggs of
ground-nesting birds and reptiles, and juvenile mammals (e.g.,
Ammospermophilus leucurus, Neotoma albigula, and Sylvilagus
audubonii; Beck 2005. Biology of Gila Monsters and Beaded Lizards. University of California Press, Berkeley. 247 pp.). Here, we
report prey not previously known for H. suspectum, Desert Kangaroo Rats (Dipodomys deserti). We describe predation episodes
on juvenile kangaroo rats in the field, and we document adult kangaroo rat rescue of nestlings from H. suspectum predation.
We radio-tracked H. suspectum from March 2000 to August 2004
at a Mojave Desert site near Lake Mead, Nevada (36.5°N, 114.5°W;
elev. 600 m; Gienger 2003. Natural History of the Gila Monster in
Nevada. Unpubl. MSc Thesis. Univ. of Nevada, Reno. 55 pp.).
We located each lizard 2–4 times per day during the active season
(March–October). When we found H. suspectum surface active,
we followed each lizard from a distance of 5–10 m to record successful foraging bouts and specific prey.
At 0710 h on 30 May 2003, we observed an adult female H.
suspectum excavating an entrance to a rodent burrow complex at
the base of a sandy mound (Fig. 1a). After 2 min of excavation,
the H. suspectum disappeared into the burrow and an adult kangaroo rat ran out of a hole on the other side of the sand mound.
Immediately, two altricial (eyes still closed) kangaroo rat pups
were observed trying to crawl out of the burrow. The H. suspectum
then emerged from the burrow behind the pups (Fig. 1b) and seized
one pup by the mid-body. After consuming the first pup, along
with considerable sand, the Gila Monster then exited the burrow,
seized the second pup by the head (Fig. 1c), and consumed it as
well.
At 0758 h on 19 June 2003, we observed the same female H.
suspectum excavating a rodent burrow. After digging for ca. 1 min,
the burrow collapsed on itself and the lizard disappeared inside.
An adult Desert Kangaroo Rat then sprinted out of a second burrow entrance 70 cm from the collapsed entrance. Three juvenile
Desert Kangaroo Rats (pre-weening age; eyes still closed) became
visible at the second entrance of the collapsed burrow, with one
pup attempting to crawl out of the burrow. The adult Desert Kangaroo Rat (presumably the mother) returned to the opening of the
second burrow (Fig. 2a), grabbed the pup that was outside of the
burrow (Fig. 2b) and carried it to a third burrow opening located
3.5 m away from the second (Fig. 2c). The mother then stood out224
Herpetological Review 39(2), 2008
FIG . 1. Excavation and consumption of a nest of kangaroo rats
(Dipodomys sp.) by a foraging Gila Monster.
FIG. 2 (opposite). Sequence showing adult female Desert Kangaroo
Rat (Dipodomys deserti) attempting to rescue her pups from predation by
a Gila Monster. The Gila Monster is inside the burrow and the mother
removes one of the pups before it can be eaten.
Herpetological Review 39(2), 2008
225
side the third burrow and began a foot-drumming display (Kenagy
1976. J. Mammal. 57:781–785; Randall and Matocq 1997. Behav.
Ecol. 8:404–413.) in which she rapidly and repeatedly beat her
feet against the sand.
The remaining pups inside the second burrow were squeaking,
and the mother returned and moved quickly into the burrow with
both the remaining two visible pups and the H. suspectum. At 0807
h, the mother emerged from the second burrow (without any pups)
and moved over to the third burrow where she had left the rescued
pup. At 0820 h, the mother left the third burrow without her pup
and moved out of sight. For 20 min, the H. suspectum remained
inside the second burrow. At 0840 h, squeaking noises emanated
from the third burrow where the Desert Kangaroo Rat mother had
placed the rescued pup. The Gila Monster remained underground
in the second burrow until 0923 h, and then emerged from the
burrow (Fig. 2d) and walked to, and into, the third burrow, where
it presumably consumed the rescued pup. The H. suspectum remained inside the third burrow for ca. 30 min and then emerged
above ground. After walking a few steps, the H. suspectum licked
its face and arched its back with snout pointed upward, a posture
Gila Monsters often assume after eating large meals (pers. obs.;
Beck 2005, op. cit.). This arching posture may help force food
items down into the stomach, especially when the stomach is already full.
We were also able to verify nest predation by three other Gila
Monsters on kangaroo rats at this Nevada site on four additional
occasions. Each time, we were able to verify the genus of the prey
(Dipodomys) by observing adults leaving the nest or pups attempting to crawl out of the burrow. Most of the time we were not able
to identify Dipodomys to species, as the adults fled the nest once it
was discovered, and Gila Monsters immediately consumed the
pups. The possible kangaroo rat species in this area include D.
deserti and D. merriami (Hall 1946. Mammals of Nevada. University of California Press, Berkeley. 710 pp.), but because these
observations were made as part of a larger natural history study of
Nevada H. suspectum, we did not interfere with lizard foraging
activities to determine which of the two Dipodomys species were
involved. However, the observation of 19 June 2003 is undoubtedly a nest of D. deserti, as the kangaroo rat we observed was
large and had white hairs on the terminal end of its tail (D. merriami
are smaller and have black terminal tail hairs).
These six observations suggest that kangaroo rats can constitute an important part of the diet for certain populations of Gila
Monsters. Our study site had a considerable amount of sand dunes
and Creosote Bush (Larrea tridentata), both of which are appropriate habitat elements for Dipodomys (Longland and Price 1991.
Ecology 72:2261–2273; Schroder 1987. Ecology 68:1071–1083).
Most previous ecological studies of Gila Monsters have been conducted at sites lacking sand dunes (Beck 2005, op. cit.), hence
Dipodomys might not be available prey to those populations. Additionally, our Nevada study site lacks conspicuous populations
of Desert Cottontail Rabbits (S. audubonii), which constitute the
most common food item at other study populations of Gila Monsters (populations in Arizona and Utah; Beck 2005, op. cit.).
We thank Ned Dochtermann and Kellie Kuhn for reading early
drafts of the manuscript. The Clark County (Nevada) Multi-species Habitat Conservation Program and the Biological Resources
Research Center at UNR provided funding.
226
Submitted by C. M. GIENGER and C. RICHARD TRACY,
Program in Ecology, Evolution, and Conservation Biology, Department of Biology, University of Nevada, Reno, Reno, Nevada
89557, USA (e-mail [CMG]: [email protected]).
Herpetological Review 39(2), 2008