(Wiegmann, 1828)
Mexican giant musk turtle
Recognition
The elongated (to 37.9 cm), oval adult carapace has three strongly developed longitudinal keels throughout life. Its medial keel extends along all five vertebrals, and the dorsolateral keels are also more extensive than in S. salvinii, running along the entire length of the pleurals. The five vertebrals are usually longer than broad in adults. The posterior marginals are unserrated and somewhat flared, and the 9th through 11th are elevated over the preceding eight, with the 11th highest of all. In appearance, the carapace of S. triporcatus seems to be longer and more elevated than that of S. salvinii. The carapace is brown with yellow seams, dark radiations and spots. The plastron is small and cross shaped with a movable hinge between the pectoral and abdominal scutes. Its anterior plastral lobe is shorter than the posterior lobe (the opposite of S. salvinii), and its sides are almost parallel. The posterior lobe is narrow and pointed to the rear, without an anal notch. Gular and humeral scutes are absent; the plastral formula is: fem > pect > an >< abd. Abdominal scutes are about as broad as long; femorals are narrow. The bridge is broad, extending over 25% of the plastral length. Large axillary and inguinal scutes are present. Both bridge and plastron are yellow, sometimes with dark seams. The head is large, broad across the temples, and has a projecting snout and an upper jaw that is not hooked or only slightly so. The head is yellowish to olive with numerous well-marked dark reticulations that extend onto the jaws. There are two chin barbels. Limbs and tail are grayish brown, and two rows of conical tubercles occur on the tail. The toes are webbed.
Males have long, thick tails and roughened patches of scales (vinculae) on their thighs and crura. The female tail is short and she lacks the roughened scales on the hindleg.
Distribution
Staurotypus triporcatus ranges, at elevations of usually less than 300 m, in Gulf and Caribbean drainages from central Veracruz southward through the base of the Yucatán Peninsula, Belize, and Guatemala to northwestern Honduras.
Habitat
This turtle lives in slow-moving waterways such as lakes, weedy marshes, and the lagoons of large rivers.
Natural History
Holman (1964) reported an unsuccessful mating attempt. The male was mounted on the posterior part of the female's carapace and was trying to grasp her lateral keels with his forefeet. His hindlegs and tail were thrust under her carapace.
Clutches contain ellipsoidal, brittle-shelled eggs measuring about 35-44 x 21-26 mm (Ewert, 1979; Goode, 1991). Twenty-seven clutches laid at the Columbus Zoo, Ohio averaged 11.07 (1-16) eggs (Goode, 1991). These females used resources equivalent to 11.42% of their body mass to produce an average of 29.36 young per year. Nesting occurred in August-January, March-April, and June, and up to four clutches were produced by a female in a single year. Hatchlings have carapace lengths of 30-35 mm.
Staurotypus triporcatus is a voracious predator, capturing and eating many types of small invertebrates (aquatic insects, worms, clams, snails, crustaceans); Vogt and Guzman (1988) found evidence suggesting it is a mollusk specialist in some microhabitats. Vertebrates consumed include fish, amphibians (mostly tadpoles), smaller turtles, and mammals (Pritchard, 1979; Vogt and Guzman, 1988). Holman (1964) observed that during feeding a captive would not pursue live fish; when taking food it moved its head slowly and involved much hyoid action both in drawing in and swallowing. Surprisingly, Vogt and Guzman (1988) also found that S. triporcatus consumes much more plant material (seeds) than was previously assumed.
This species, like its congener S. salvinii, has an evil temper and can inflict a severe bite on a careless handler.
IUCN Red List Status (1996)
Lower risk: near threatened.