Ecology of the Aquatic Box Turtle, Terrapene coahuila (Chelonia, Emydidae), in Northern Mexico

Abstract

An ecological study of the Coahuilan Box Turtle, Terrapene coahuila, was undertaken between December 1964 and November 1967 in its natural habitat on the northern Mexican Plateau. The species is endemic to an intermontane basin of the Chihuahuan Desert near Cuatro Ciénegas, Coahuila. Its geographic range consists of disjunct populations in an area that does not exceed 800 km2. Preferred habitats of T. coahuila in the area studied are small, north-south trending, spring-fed marshes, characterized by mud bottoms, shallow water, and dense aquatic vegetation, principally Chara spp., Eleocharis rostellata, and Scirpus olneyi. Marshes are distinct aquatic communities surrounded by desert grassland and shrub communities. Ecological adaptations of this species more closely resemble those of other North American aquatic turtles than of terrestrial members of its own genus. Population densities, foraging behavior, food habits, and thermal relationships best exemplify the aquatic mode of life of T. coahuila. T. coahuila remains active throughout the year except for short periods of environmental extremes. Mating occurs from September to June and appears concentrated in spring. Copulating pairs of box turtles were found in October, November, December, and April, frequently in shallow water. The ovarian cycle appears to be intermediate between the lengthy cycle of tropical emydid species and the compressed cycle of northern species caused by cold weather. Follicle enlargement occurs between late August and early April when ovulation begins. Egg laying begins in May and continues to September. Complements of 2 or 3 eggs are produced most frequently. An estimated half of the females can produce second clutches, and about one-third may deposit three sets annually. These females produce a mean of 6.8 eggs/female per season, a higher reproductive potential than in certain northern populations of Terrapene. Coahuilan box turtles forage in shallow water, with the carapace usually above the surface and dry, and the head extended underwater. T. coahuila is opportunistic and omnivorous, feeding extensively on aquatic plants (Eleocharis) and insects (stratiomyid fly larvae, beetles, hemipterans, dragonfly nymphs). Foraging behavior and food habits of T. coahuila are comparable to other aquatic or semiaquatic emydids (Chrysemys picta, Clemmys muhlenbergi). Cloacal temperatures of T. coahuila active in marshes closely approximate water temperatures at all seasons, as is generally true for most other aquatic turtles while in water. There is wide seasonal and daily variation in the mean cloacal temperature of active turtles, so no single optimum temperature within the activity range is reported. In summer, when water temperatures can exceed tolerable levels, activity occurs mainly in early morning, late afternoon, and at night. Marsh bottoms provide a cool refuge into which a turtle can burrow to avoid potentially harmful midday surface temperatures. Most T. coahuila presumably undergo temporary states of winter inactivity, although some remain active in water despite low air temperatures. In December, when several individuals on land had cloacal temperatures elevated well above air temperatures, basking was indicated. Most T. coahuila remain within a given marsh for relatively long periods, but about 20% of recaptured turtles had moved longer distances, possibly overland, from one marsh to another. Within marshes, movements between successive points of capture averaged about 13 m. Box turtles move in a sinuous fashion over mats of Chara and around sedge tussocks. Individuals seemed socially tolerant of others in nature, and were occasionally close together; no aggressive encounters were observed. Three mark-recapture census techniques were used to estimate the population size in the study area. Population densities ranged from 54 to 63 adult turtles per marsh acre (133-156/ha). T. coahuila occurs in relatively higher numbers and is restricted to smaller areas of activity than its terrestrial congeners, T. carolina and T. ornata. Its population density is more comparable to certain aquatic species, such as Chrysemys picta and Pseudemys scripta. Although populations of the Coahuilan Box Turtle are relatively dense in many marsh communities in the Cuatro Ciénegas basin, T. coahuila can be considered a rare species by virtue of its restricted aquatic habitat. Destruction of marshes by draining and excessive collecting of specimens clearly represents threats to the turtle's existence. Terrapene coahuila should be obtained, therefore, only by those seriously investigating its biology.