Kinosternon sonoriense

LeConte, 1854
Sonoran mud turtle

This medium-sized (to 17.5 cm), dark mud turtle has an elongated, tricarinate carapace with an unserrated posterior margin. Some individuals have three well-developed keels, but these may be lower in others, or non-existent in very old turtles. In adults, the first four vertebrals are longer than broad, but the 5th (which is posteriorly flared) is usually broader than long. The 1st is anteriorly flared and usually touches the 2nd marginals. Posterior marginals are flared, and the 10th is higher than the others. The carapace is olive brown to dark brown with dark seams. The hinged plastron is large and capable of almost closing completely; at best, it is only slightly notched posteriorly. The plastral formula is: abd > an > gul >< hum >< fem > pect (but see under geographic variation). Axillary-inguinal contact occurs on the bridge. The plastron is yellow to brown (sometimes mottled) with dark seams; the bridge usually brown. The head is moderate in size with a slightly projecting snout and a hooked upper jaw. There are three to four pairs of relatively long chin or neck barbels, and the rostral scale is not bifurcated posteriorly. Gray, mottled skin occurs on head, neck, and limbs. The jaws are cream colored and may be dark flecked.
Males have concave plastra; long, thick, spine-tipped tails; vinculae; and are shorter (to 15.5 cm) than females (to 17.5 cm). Females have short tails and flat plastra.

Kinosternon sonoriense ranges from the lower Colorado and Gila rivers in Arizona and New Mexico southward to the Rio Yaqui basin of the Continental Divide, and eastward through the Rio Casas Grandes basin of northwestern Chihuahua (Iverson, 1981a).

Geographic Variation
According to Iverson (1981a), two subspecies exist. Kinosternon sonoriense sonoriense LeConte, 1854, the Sonoran mud turtle, is found in New Mexico, Arizona, Sonora, and western Chihuahua. It has a long interanal seam (19.5% of carapace length in both sexes), a 1st vertebral scute of medium width (24.4% of carapace length in males, 25.5% in females), and a relatively wide gular scute (20.0% of carapace length in males, 19.4% in females). K. s. longifemorale Iverson, 1981a, the Sonoyta mud turtle, is known only from the Rio Sonoyta basin in Arizona and Sonora, Mexico. It has a short interanal seam (14.4% of carapace length in males, 18.5% in females), a long interfemoral seam (12.8% of carapace length in males, females 13.5%), a wide 1st vertebral scute (28.9% of carapace length in males, 28.8% in females), and a narrow gular scute (17.7% of carapace length in males, 17.8% in females).

The Sonoran mud turtle occurs at elevations to 2,042 m, in permanent streams, creeks, ditches, ponds, springs, and waterholes, usually in woodlands; only rarely does it enter temporary bodies of water. In a study in the Chiricahua Mountains of southeastern Arizona, 90% of 580 individuals were captured in stock tanks, 5% in stream pools, and 5% on land (Van Loben Sels et al., 1997). Individuals occasionally migrate from one waterbody to another, as indicated by their presence in farm ponds as far as 8 km from permanent water (Degenhardt and Christiansen, 1974).

Natural History
On the basis of sperm in the vas deferens, Hulse (1982) found that males matured at 5-6 years of age and carapace lengths of 76-82 mm. Van Loben Sels et al. (1997) found no sexual dimorphism in age classes 5-10 years. At two sites of higher elevation, maturity was delayed, and the smallest mature male was 91 mm and eight years old. Testes reach their maximum size in June and then retain this size through August (Hulse, 1982). Testicular regression begins in September and the testes probably reach minimum size in January or early February. Females mature at carapace length greater that 93 mm (Hulse, 1982). The youngest gravid female found by van Loben Sels et al. (1997) was five years old and had a carapace length of 106 mm. According to Hulse (1982) the female ovarian cycle begins in March when follicles begin to enlarge as yolking occurs. Rapid growth continues until May when follicles reach ovulation follicle size (13-15 mm diameter). By August these large follicles have disappeared. Some 10.0-12.9 mm follicles may persist until December, but after this follicles begin to decrease to less than 10 mm through February.
Hulse (1982) observed captive males courting in March and wild matings in April. Iverson (1981a) observed mating in the field on 4 May, but copulation probably also occurs earlier in the spring.
Nesting takes place from May to September (but probably only into July at higher elevations); 1-4 clutches (Rosen, 1987) of 1-11 eggs may be laid a year (Van Loben Sels et al., 1997; John B. Iverson, pers. comm.). Clutch size is positively correlated with female body size. The white, elliptical eggs (28.0-35.0 x 13.8-19.0 mm) have hard shells. Hatching occurs in August.
Hatchlings (22-28 mm) have a low, broad, medial keel and two elongated lateral keels. The upper posterior edge of each marginal often has a black smudge, and the 10th marginal is not noticeably elevated. Their plastra are cream colored with a dark central blotch extending outward along the seams. Hatchlings may also have a pair of yellow lateral head stripes.
Kinosternon sonoriense is omnivorous, eating algae and higher aquatic plants as well as insects, snails, crustaceans, worms, fish, and amphibians. It is predominantly nocturnal. Unlike many of its congeners, K. sonoriense and its sister taxon K. hirtipes are not known to aestivate terrestrially (Iverson et al., 1991).

IUCN Red List Status (1996)
Vulnerable (A1c).