New published research: Examining indigo snake growth

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Measuring indigo snake growth
Measuring an Eastern Indigo Snake on a survey in southeastern Georgia. – Houston Chandler

Growth and body size are fundamental traits that impact a variety of evolutionary and ecological processes across many groups of animals. For example, larger snakes can consume larger and more diverse prey items and eat similarly-sized prey items faster than their smaller counterparts (Shine 1991). Many studies have also documented positive effects of body size on important demographic parameters, including survival and fecundity. Larger individuals typically live longer, while larger females typically produce more juveniles. The upper limits of body size in snakes are ultimately limited by evolutionary constraints, but there is reason to believe that a fast growth rate could convey significant benefits to individuals, which may ultimately translate into population-level processes.

Understanding factors that impact indigo snake body size and growth is particularly relevant because of ongoing reintroduction efforts. In an attempt to reestablish populations, indigo snakes are bred in captivity and released at reintroduction sites. During this captive period, their diets, environments, and activity levels are all considerably different than experienced in the wild. Furthermore, it is notoriously difficult to monitor post-release individuals (especially without some type of radio telemetry), and growth metrics could provide a small glimpse into how snakes are faring after release. Thus, we set out to conduct an analysis of growth and body size using as much data as possible from wild, captive, and reintroduced populations of indigo snakes. We also sought to examine how measurement error impacted reported body size measurements in indigo snakes, which can be challenging to effectively measure. This error can make individuals appear to shrink between capture occasions (Madsen and Shine 2001). We then used the results of our analyses to offer suggestions for future research needs associated with the reintroduction program. The results of this work were recently published in the journal Herpetologica (Chandler et al. 2023). I highlight some of the results of this research below.

Distribution of body size data for indigo snakes measured in wild, captive, and reintroduced populations. Growth increments refer to the change in size recorded between successive capture events. – Houston Chandler

For this project, we first compiled data from many partners involved in both reintroduction efforts and in monitoring wild populations. This included measurements of indigo snake length and mass from individuals captured in the wild, reared in captive facilities, and captured after release at reintroduction sites. Combining so many different data sources is not without challenges. For starters, the methods used to measure body length are variable between projects. When dealing with a large number of data sources, there was also a substantial amount of cleaning and checking for common errors that needed to occur prior to analysis. Some missing measurements also made it necessary to estimate snout–vent length (the distance from the tip of the snout to the cloaca) from total length in certain cases. Ultimately, we were able to use a large dataset to fit Von Bertalanffy Growth Models for indigo snakes, which is one of the most common methods used to assess growth.

Our combined dataset contained 5,073 occasions where an indigo snake had at least one length or weight measurement recorded. The dataset included data from 1,503 individual snakes (1–26 measurement occasions per individual). We found that there was a strong positive relationship between snout–vent length and total length, regardless of sex. Snout–vent length typically accounts for approximately 80% of an individual’s total length.

Relationship between snout–vent length and total length in indigo snakes. – Houston Chandler

Using our data, we were able to fit a growth model for indigo snakes. This model confirmed some well-known aspects of indigo snake biology (e.g., male snakes reach much bigger sizes than female snakes but take longer to reach maximum size; Stevenson et al. 2009). Surprisingly, we found evidence that indigo snakes at reintroduction sites tended to reach their maximum size faster than snakes in captive or wild populations. We are unsure of the mechanism causing this difference in growth between populations. Some possible explanations include reduced competition at reintroduction sites, long-lasting benefits of an early life in captivity, or potentially biased recapture data that favors snakes that perform well at reintroduction sites. No matter the mechanism involved this result suggests that at least some indigo snakes at reintroduction sites experience elevated growth rates, potentially benefiting the population.

Growth curves for indigo snakes in wild (A), captive (B), and reintroduced (C) populations. Solid lines are mean values, while shaded regions represent 95% credible intervals. The horizontal gray line represents the largest indigo snake recorded in wild populations in this study. – Houston Chandler

Both our raw data and modeling results highlighted that there is frequent error associated with measuring indigo snakes. For example, there were 194 instances of snakes with negative growth increments between successive captures (approximately 9% of all growth increments). Mean estimated measurement error for all snakes was 4 cm, accounting for approximately 4.1% of the average female snout–vent length and 3.9% of the average male snout–vent length. Measurement error was also variable across projects, ranging from 3–9 cm. Unsurprisingly, measurement errors were smallest in captivity where snakes are of smaller size or commonly measured using a squeeze box. However, these errors represented a larger percentage of snake body size, on average, because of the high number of juvenile snakes measured in captivity. Overall, the frequency of error in this dataset suggests that it may be worth considering additional methodologies when measuring indigo snakes.

The indigo snake reintroduction program is one of the longest running reintroduction efforts to aid the conservation of an imperiled snake. This work highlights the utility of collecting data alongside such efforts, which can be used to answer important questions. Furthermore, we recommend increased standardization and creation of a centralized database to support future efforts. Finally, the analysis described here was conducted at a relatively course scale. Additional work is needed to better understand how various landscape factors may influence indigo snake growth in wild and reintroduced populations.

This work is part of our ongoing efforts to develop range-wide conservation and planning tools for indigo snakes. Growth metrics, while interesting in their own right, are a critical component of developing population models for indigo snakes. The results of this work will continue to be incorporated into future population modeling efforts.

The full publication is available here.

 

Literature Cited

Chandler, H.C., D. Steen, J. Blue, J.E. Bogan, M.R. Bolt, T. Brady, D.R. Breininger, J. Buening, M. Elliott, J. Godwin, C. Guyer, R.L. Hill, M. Hoffman, N.L. Hyslop, C.L. Jenkins, C. Lechowicz, M. Moore, R.A. Moulis, S. Piccolomini, R. Redmond, F.H. Snow, B.S. Stegenga, D.J. Stevenson, J. Stiles, S. Stiles, M. Wallace, J. Waters, M. Wines, and J.M. Bauder. 2023. Evaluating growth rates of captive, wild, and reintroduced populations of the imperiled Eastern Indigo Snake (Drymarchon couperi). Herpetologica 79:220–230.

Madsen, T., and R. Shine. 2001b. Do snakes shrink? Oikos 92:187–188.

Shine, R. 1991. Why do larger snakes eat larger prey items? Functional Ecology 5:493–502.

Stevenson, D. J., K. M. Enge, L. D. Carlile, K. J. Dyer, T. M. Norton, N. L. Hyslop, and R. A. Kiltie. 2009. An Eastern Indigo Snake (Drymarchon couperi) mark-recapture study in southeastern Georgia. Herpetological Conservation and Biology 4:30–42.