North American Recent Soft-shelled Turtles (Family Trionychidae) Part 24

The seasonal reproductive potential is perhaps less in northern populations (averaging 20 eggs per clutch and only one clutch per season) than in southern populations (averaging about 10 eggs per clutch, but three clutches per season). Larger females deposit more eggs than smaller females. Eggs laid in northern lat.i.tudes are slightly smaller than those laid farther south. In any lat.i.tude the incubation period probably is at least 60 days. Hatchlings presumably leave nests at dusk, nighttime or dawn, and may winter over in eggs or nests.

Man is a great enemy of softsh.e.l.ls. Predation on eggs probably accounts for most mortality. Physical conditions of the environment (overcrowding of nest sites, inadequate hibernation sites) and probably some kinds of parasitism contribute to mortality. Softsh.e.l.ls are eaten locally and sometimes appear in the market of large cities, but over most of their range, there probably is no general demand and no special efforts are made to capture them. Fish, mostly minnows, comprise a small proportion of the diet. There is no evidence that softsh.e.l.ls are active predators on any kind of fish, but their known food habits suggest that they compete with game fishes for food. Softsh.e.l.ls are scavengers.

Fossil material was not studied in detail. The fossil softsh.e.l.ls indicate a more widespread, former distribution. Some osteological characters and their variation in the living species are mentioned as an aid to future workers concerned with an a.s.say of fossil remains. Fossils occur in marine, brackish and fresh-water deposits, and many are much larger than the living species; the oldest American fossils are of Upper Cretaceous age.

The interrelations.h.i.+ps of the living species and subspecies suggest that the species _spinifer_, _ater_, and _muticus_ are derivatives of a _ferox_-like ancestor, and that they differentiated in North America; most differentiation occurs in southwestern Texas and northern Mexico where characters of some populations indicate alliance with _ferox_. It is hypothesized that aridity in the late Tertiary effected specific differentiation by the modification and isolation of aquatic habitats.

Pluvial periods in the Pleistocene provided for confluence of aquatic habitats and expansion of geographic ranges, and coupled with physiographic changes, conceivably caused or enhanced some of the subspecific variation.

LITERATURE CITED

References marked with an asterisk were not seen by the author.

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1791. Travels through North and South Carolina, Georgia, east and west Florida, the Cherokee country, the extensive territories of the Muscogulges, or Creek Confederacy, and the country of the Chactaws; containing an account of the soil and natural productions of those regions, together with observations on the manners of the Indians. First Edition, Philadelphia, x.x.xiv+522 pp., 8 pls. (including frontispiece), 1 map.

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BERGOUNIOUX, M. F.-M.