Thursday, October 14, 2010

Western Cottonmouth Survival Rates and Use of Space


Cottonmouths have always been of interest because they are the only pit viper that has been able to exploit aquatic environments. Why this is so, is a puzzle. Look at virtually all other lineages of snakes and you find they almost always contain species, or groups of species, that have adapted to water. The most prominent exceptions are the scolecophidians (blind snakes, threadsnakes and dawn snakes) and the vipers. The scolecophidians may be so specialized for burrowing and eating social insects that invading aquatic habitats is not an option for them. But why the vipers have not invaded the water is more obscure. Only the cottonmouth exploits aquatic resources and seems to spend a substantial amount of time in the water, but even it quite terrestrial and not completely adapted to water. Populations that hibernate do so in terrestrial situations, the snakes often thermoregulate out of the water, and they use a variety of terrestrial habitats, albeit usually near water. Two recent papers reveal parts of its life history. Rose et al.(2010a) estimate the survival of the western cottonmouth (Agkistrodon piscivorus leucostoma) in central Texas using the Cormack-Jolly-Seber Model that accounts for delectability. They studied the annual probability of survival of the western cottonmouths at Honey Creek, Comal County, Texas for 11 years. Honey Creek is a spring-fed stream flowing into the Guadalupe River. Cottonmouths can be secretive have a low probability of detection (0.12) and the study produced relatively small sample (n = 51). However, the estimate of survival was reasonably precise (coefficient of variation was 4%). The study time included multiple floods and droughts, and therefore, represents a relatively wide range of conditions to which western cottonmouths are exposed at this locality. The results suggested adult snakes have a 0.81 probability of survival in any given year, an annual survival rate similar to that reported for other pit vipers. In a second paper, Rose et al. (2010b) looked at the Honey Creek cottonmouth population to see how it used space along the stream. They made 57 searches along a 1564 m study site and marked 39 mature snakes, 14 subadult snakes, and 4 juveniles. Recapture frequency did not differ between sexes, but females outnumbered males (2.3:1) and adults were recaptured more frequently than juveniles. Distances between captures were less than predicted if distributions were random, and distances did not vary with number of times captured or time between captures. Most snakes were sedentary, but a few individuals made long distant movements. At least some the snakes were displaced by flooding but returned after the water subsided.

A Western Cottonmouth from Johnson Co., Il. JCM
Rose, F. L., T. R. Simpson, J. R. Ott, R. W. Manning, and J. Martin. 2010a. Survival of Western Cottonmouths (Agkistrodon piscivorus leucostoma) in A Pulsing Environment. The Southwestern Naturalist 55:11-15.

Rose, F. L., T. R. Simpson, J. R. Ott, and R.W. Manning. 2010b. Use of Space by Western Cottonmouths (Agkistrodon piscivorus) Inhabiting a Variable–Flow Stream. The Southwestern Naturalist 55(2):160-166.

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