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What is the length of a snake? Contemporary Herpetology. 2008(2): 1-3 click here for pdf
What is the
length of a snake?
The way that herpetologists have traditionally measured live snakes is by stretching them on a ruler and recording the total length (TL). However, due to the thin constitution of the snake, the large number of intervertebral joints and slim muscular mass of most snakes it is easier to stretch a snake more than it is to stretch any other vertebrate. The result of this is that the length we record of a snake is influenced by how much we stretch the animal. Stretching the animal as much as we can is perhaps a precise way to measure the length of a specimen but it might not correspond to the actual length of a live animal. Furthermore, it may seriously injure a live snake. Other methods consist of placing the snakes in a clear plexiglass box and pressing the snake with a soft material (e. g., rubber foam) against a clear surface. Measuring the length of the snake may be done by following the its body with a string (Frye 1991). This method, though, is restricted to small animals that can be placed in a box and no indications of accuracy of the technique are given. In this contribution we propose an alternative way of measuring snakes that is more accurate than stretching the animals on a ruler. We further analyze the precision of this method by using a sample of measures taken from wild populations of green anacondas (Eunectes murinus) with a large range of sizes.
To record naturalist measure of the length of the animal we muzzled the snake with a sock and tape (Rivas et al. 1995) followed a imaginary middle line of the body from head to tail with a string and then measured the length of the string by laying it loosely on a ruler (Figure 1). This allowed us to record the actual length of the animal regardless of its position and without having to stretch it. A total of 82 newborn and 42 stillborn from 14 wild caught pregnant females were measured for this study. Three measurements of each animal were slightly different due to errors caused by the snake struggling and moving from under the string. Thus calculated the average of the three measures.
We also recorded the TL of each snake in the sample using the conventional method of stretching them on a ruler. We used a sign test to compare both measures of each animal. We divided the measurements obtained with the stretching method by the measurements obtained with the string method in order to calculate a relationship between the two measures. In order to analyze the changes of this relationship in respect to size, we used the mass as an independent measure of the size of the animal. We performed a Spearman correlation test between the variables. The use of stillborn in this study was to remove the effect of the error introduced by the struggle of the animal; allowing us to to determine the actual TL of the animal.
Another sample of 68 animals from a wild population (ranging from 84.7 cm to 494.7 cm TL) was measured independently by three people. All of the animals were measured by one of two researchers who had three years of experience performing the procedure, and by two people well instructed in the technique but without much previous experience. Thirteen of additional individuals were measured by the two researchers that had previous experience.
We calculated the coefficient of variation (CV) on the three measures collected on animals from the wild to study the changes on the precision of the measurement of snakes of different sizes. The CV was calculated by dividing the mean by the standard deviation (see formula in Sokal and Braumanm 1980) and provides a measurement of the variance in units of the mean, so it is not dependent on the absolute value of the variable measured. This is especially important when dealing with variables that vary in a wide range of values. All statistical analysis were made with SSPS 8.0.
The string technique described here is comparable to using it with the squeeze box except that it can be used on larger animals that cannot fit in a box or that cannot be pinned and restrained allowing broader applicability. Measurements taken with the string were consistently shorter than measurements with the ruler (Z= 6.82; p < 0.000; Table 1). The quotient among the measurements is smaller in larger animals (r = –0.362; p< 0.001; Figure 2) which suggests that smaller animals are being significantly stretched when measured on a ruler.
All the measurements estimate a unique parameter: "the size of the neonate". However, measurements from the two methods using stillborn snakes were more disparate than measurements on live individuals (Table 1). Measurements of stillborn snakes with the ruler were the largest of all and the measurements of stillborn with the string were the shortest of all (Table 1). An ANOVA shows significant difference between the measurements of all the groups (F = 70.47; df = 3; p<0.0001). We used only stillborn animals that were completely formed and whose cause of death was most likely due to dystocia of the female or other problems at the end of the gestation (Ross and Marzec 1990). We believe that the size of the stillborns was not significantly different than the size of live neonates, which is supported by the fact that there was no significant difference in mass (t= 1.252; df = 120; p = 0.21; Table 1). Thus the difference in the measurements of the live babies and the stillborns are most likely due the struggling of live animals.
If we assume that the "real" length of the animal is the length when it is relaxed, and not struggling or being over-stretched (as is usually the case when most other vertebrates are measured), then the length of the stillborn measured with the string should be more realistic estimate. However, the data suggest that this method is not error-free.
Repeated measurements collected with the string on the same wild-caught animals showed a relatively high variance. The average variance in animals around 80 cm was 0.514 cm and the maximum was up to 2.35 cm. It was clear while processing calmer animals that the repeated measures on them were more similar than measures of more active animals. In animals that struggled a lot, the first measurement tended to be the most different. After the process was done once on the individual, it tended to calm down.
The struggle of the animal during the measurement can potentially influence the repeatability of the measure. The data collected by experienced vs inexperienced researchers were significantly different (Z= -3.13; p< 0.002), where inexperienced researchers consistently obtained shorter measurements than experienced ones. The data collected by the two experienced researchers were consistent with each other and were not significantly different in a Wilcoxon sign test (z= -0.27; p< 0.79).
The variance of the measurements changed with the size of the animal being measured (Figure 2). Between the size of 2 to 3 meters the variance was particularly high, mostly due to a few animals that had a very high CV (Figure 3). This might be a consequence of the higher level of struggling found in some smaller animals. The smallest animals can be easily subdued during the process and the measures are more consistent with each other (but see below). Beyond a certain size the snakes are stronger and some of them are able to put up more of a struggle, which decreases the precision of the measurements. Larger animals are calmer and although they could make the measuring much harder they tended to be easier to measure consistently (Figure 4). However, the CV was high in all the smaller sizes and decreased after three meters. Thus, the lower variance found in Figure 2 for smaller sizes is probably an artifact of smaller values. The first measurement of each animal tended to be more different than the following two; this was especially true in medium-sized animals. Larger animals are only females and the medium-sized ones are mostly males so some differences in the behavior of each sex could be involved in this trend. However, the effects of size vs sex could not be tested because adult males are always smaller and females are typically larger (Rivas 1999).
Stretching a snake apparently has a considerable effect on the measurements collected on the length of the snake. Smaller animals seem to provide less resistance to being stretched than larger ones, thus studies involving measuring animals among several size classes must consider this issue. This method is not different in theory from the method of the squeeze box (Frye 1991) but this has a much broader application to snakes of larger sizes. The squeeze box method is most often used to measure animals that are difficult to handle such as venomous snakes or very small animals. Here we suggest that this technique should be used in a more generalized manner since it provides a more accurate measurement of the size of the animals. The size of newborn anacondas is within what is considered a relatively small snake. Herpetologists have traditionally known that measuring large snakes is problematic but animals smaller than 1.3 meters have been considered to be "small animals" in which the standard measuring techniques are reliable; we have shown that this is not the case. Stretching the animal on a ruler is less time consuming and in some situations it might seem appropriate. However, the degree that the animal is stretched can be influenced by the size and behavior of the animal, or even the mood of the researcher! Measuring the animals with a string is a more reliable method especially if it is done by people properly trained in the technique. Research involving mark and recapture, or growth studies must consider these issues.
Acknowledgements: We thank The Wildlife Conservation Society
and The National Geographic Society for the financial support of this research.
We also thank COVEGAN and Estacion Biologica Hato El
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Table
1 Total length of neonate green anacondas measured
by stretching them on a ruler and by following their midbody line with a
string. Lengths are the mean of three independient measurements of each snake.
|
|
Length Ruler (cm) |
Length String (cm) |
Mass (g) |
N |
|
Live |
79.72 |
77.57 |
228.11 |
82 |
|
Stillborn |
85.12 |
76.0 |
225.54 |
42 |
Figure 1 Measuring
technique streching the string over the back of the anaconda to asses its
length.
Figure 2. Scatter plot of ontogenetic change of the quotient
between the measures of neonate anacondas obtained stretching them on a ruler
and follwing the midline of the body with a string.
Notice how the relationship between the two measures changes with the size (r =
–0.362 p< 0.001; n= 124).
Figure 3. Size related change of variance of three
measurements of SVL obtained from each wild-caught anaconda using a string to
follow the midle line of the body.
Figure 4 Relationship on the coefficient of variation from
three measurements on the same individual wild-caugth anaconda measured with
the string. Notice the decrease in larger sizes.