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Herpetological
Review
ACTINEMYS (= EMYS) MARMORATA (Western Pond Turtle).
PREDATION
JEFFERY T. WILCOX, University of California,
Berkeley, Blue Oak Ranch Reserve, 23100 Alum Rock Falls Rd.,
San Jose, California 95127, USA; e-mail: jtwilcox@berkeley.
edu.
Herpetological Review 41(2), 2010212
TESTUDINES — TURTLES
ACTINEMYS (= EMYS) MARMORATA (Western Pond Turtle).
PREDATION. The Western Pond Turtle is in decline over 80%
of its historical range in California, primarily due to habitat loss
and competition from introduced species (Stebbins 2003. A Field
Guide to Western Reptiles and Amphibians, 3rd ed., Houghton
Miffl in, Boston, Massachusetts). Wild pigs (Sus scrofa), hybrids
of feral domestic pigs and European wild boars, are a purposely
introduced species rapidly increasing in range and population
throughout the state (Waithman et al. 1999. J. Wild. Manag.
63[1]: 298–308). Wild pigs are omnivores, and forage for and
consume a wide variety of vertebrate prey (Wilcox and Van Vuren
2009. J Mammal. 90[1]:114–118). Worldwide, feral pigs have
been documented to consume eggs and young of four chelonian
species, including Green Sea Turtles (Chelonia mydas), giant
tortoises (Chelonoidis nigra [=Geochelone elephantopus]) in the
Galapagos (Coblentz and Baber 1987. J. Appl. Ecol. 24:103–118),
Spur-thighed Tortoises (Testudo spp.) in Sardinia (Corbett [ed.],
1989. The Conservation of European Reptiles and Amphibians.
C. Helms, London), and Texas Tortoises (Gopherus berlandieri)
(Taylor and Hellgren 1997. Southwest. Nat. 42[1]:33–39). I report
below on the fi rst documented case of wild pig predation on the
eggs of Western Pond Turtles.
Early on the morning of 21 October 2003, I went to check the
progress of a pond-draining project at Cabin Pond (37.37953°N,
121.73146°W) on the Blue Oak Ranch in Santa Clara County,
California, USA. In the heat and aridity of the California autumn,
the moist basin of the receding pond had attracted wild pigs.
Overnight, they had wallowed in the deep mud and thoroughly
rooted for earthworms in the moist soil around the shallow end of
the pond basin. Three pigs were in the basin when I arrived. Two
were rooting beyond where the soil moisture ended in the grass-
covered uplands on the slope, ca. 20 m above and beyond the pond.
The pigs bolted for cover as soon as they saw me. Moments later,
while inspecting the area, I discovered remnants of three turtle
eggs in the mounds of soil and dried plant matter where the pigs
had rooted. There was a moist, viscous fl uid still clinging to the
inside of the shells. In the hard earth just below the mound of soil
that held the eggs was an elliptical depression about 5 cm across
and 3 cm wide. The exact dimensions were diffi cult to judge due
to the rooted conditions.
A number of morphological and behavioral traits make wild pigs
ideal predators of turtle nests and eggs. Nest sites of Western Pond
Turtles typically are within close proximity of streams and ponds;
riparian areas are likewise favored resting and foraging places for
wild pigs. Turtle nests typically are no more than 8 cm in depth
(Rathbun et al 1992. Southwest. Nat. 37[3]:319–324); wild pigs
often root to depths much greater than 8 cm, depending on the soil
depth and moisture. Wild pigs’ excellent sense of smell may help
them detect turtle nests (though they also may encounter turtle eggs
in the course of routine rooting). Pigs also have remarkable oral
dexterity, which allows them, for example, to detect and swallow
earthworms without ingesting soil and may allow them to do the
same with small eggs.
In the incident described above, wild pigs were interrupted in
the act of predating on turtle eggs—but wild pigs normally swal-
low their prey whole, leaving no evidence on the ground. The
behavior observed serendipitously on the Blue Oak Ranch may
therefore happen more often than is witnessed. The spread of wild
pigs throughout California could be exacerbating the decline of
Western Pond Turtles where these species occur together.
Submitted by JEFFERY T. WILCOX, University of California,
Berkeley, Blue Oak Ranch Reserve, 23100 Alum Rock Falls Rd.,
San Jose, California 95127, USA; e-mail: jtwilcox@berkeley.
edu.