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.
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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.