Secondary Sexual Differences and Notes on the Mud Turtle, Kinosternon Subrubum in Northern Indiana

Abstract

Anatomical differences in the sexes of the musk turtle, Sternothereus odoratus, similar to those of the mud turtle have been recorded by Hallowell (1856), Agassiz (1857), and Hay (1892) who found that the male had a much larger and heavier tail, bearing a nail at the tip; that its plastron wa's concave, and that it bore patches of horny scales on thigh and lower leg. Risley (1930) basing his conclusions on a study of 60 specimens adds, again for the male; a relatively longer plastron; wider head; more skin between plastral scutes; plastral scutes smaller and no difference in color or length of carapace. This paper on the mud turtle is based on the study of 21 females and 14 males, all mature, taken in Jasper, Pulaski, Starke and Porter Counties, Indiana. Several of the sexual differences mentioned above for the musk turtle apply also to the species under discussion. Thus the male mud turtle has a long, thick, fleshy tail bearing a heavy chisel-like nail at the Hay noted that the male of Sternotherus odoratus carried the tail coiled at the tip. It is believed that the tail tips of males of this genus have an important sexual usefulness as a grasping or prying organ in copulation, and that mutilation cannot be risked by allowing the tail to drag behind as is done by those turtles to which the tip is of no vital importance. The female mud turtle bears an extremely short, spiny tail ending in a slender pointed spike. The concavity of the plastron in the male is more apparent than real. The fore and hind lobes of the plastron of the male may be depressed, the surface being as flat as in the female. The patches of horny scales in opposition occur also on the male. It is conjectured by Risley that these horny patches are used for grasping the shield of the female during copulation. DeSola (1931) says that each of these is a stridulatory organ, producing sounds like the chirp of crickets. The following differences added by Risley agree with this species: The male has a longer plastron and wider head. The following differences, not mentioned as occurring in the musk turtle, are found in the present species: In males the iris is occasionally white and frequently has a narrow light rim. The iris of the female is usually brown, but occasionally has a narrow light rim. The larger and heavier mandibles of the male are usually spotted or striped with dark brown; those of the female are usually horn yellow. The head of the male is about 2 mm. deeper and wider than that of female. The anterior a'spect of the carapace shows a lobed arch, the central lobe being for the accommodation of the large head, and this (798)