Climatic and geographic predictors of life history variation in Eastern Massasauga (Sistrurus catenatus): A range-wide synthesis

Elucidating how life history traits vary geographically is important to understanding variation in population dynamics. Because many aspects of ectotherm life history are climate-dependent, geographic variation in climate is expected to have a large impact on population dynamics through effects on annual survival, body size, growth rate, age at first reproduction, size-fecundity relationship, and reproductive frequency. The Eastern Massasauga (Sistrurus catenatus) is a small, imperiled North American rattlesnake with a distribution centered on the Great Lakes region, where lake effects strongly influence local conditions. To address Eastern Massasauga life history data gaps, we compiled data from 47 study sites representing 38 counties across the range. We used multimodel inference and general linear models with geographic coordinates and annual climate normals as explanatory variables to clarify patterns of variation in life history traits. We found strong evidence for geographic variation in six of nine life history variables. Adult female snout-vent length and neonate mass increased with increasing mean annual precipitation. Litter size decreased with increasing mean temperature, and the size-fecundity relationship and growth prior to first hibernation both increased with increasing latitude. The proportion of gravid females also increased with increasing latitude, but this relationship may be the result of geographically varying detection bias. Our results provide insights into ectotherm life history variation and fill critical data gaps, which will inform Eastern Massasauga conservation efforts by improving biological realism for models of population viability and climate change

Locations of Eastern Massasauga study sites (counties/districts shaded black) and the approximate historic range of the Eastern Massasauga (gray shading, from http://www.iucnredlist.org/).

<p>County and district codes: IA = Bremer, IA; IL.1 = Clinton, IL; IL.2 = DuPage, IL; IL.3 = Cook/ Lake, IL; IL.4 = Piatt, IL; IL.5 = Warren, IL, IL.6 = Will, IL; IN.1 = Hendricks, IN; IN.2 = LaGrange, IN; IN.3 = Marshall, IN; MI.1 = Barry, MI; MI.2 = Cass, MI; MI.3 = Kalkaska, MI; MI.4 = Lenawee, MI; MI.5 = Oakland, MI; MI.6 = Van Buren, MI; MI.7 = Washtenaw, MI; NY.1 = Genesee, NY; NY.2 = Onondaga, NY; OH.1 = Ashtabula, OH; OH.2 = Champaign, OH; OH.3 = Clark, OH; OH.4 = Greene, OH; OH.5 = Greene/ Warren, OH; OH.6 = Hardin, OH; OH.7 = Trumball, OH; OH.8 = Wyandot, OH; ONT.1 = Bruce, ONT; ONT.2 = Essex, ONT; ONT.3 = Muskoka, ONT; ONT.4 = Beausoliel Island, ONT; ONT.5 = Parry Sound District (1995–1996), ONT; ONT.6 = Parry Sound District (1992–2009), ONT; ONT.7 = Regional Municipality of Niagara, ONT; PA = Butler/ Venango, PA; WI.1 = Buffalo, WI; WI.2 = Juneau/ Monroe, WI. Reprinted and modified from [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0172011#pone.0172011.ref150" target="_blank">150</a>] under a CC BY license, with permission from [Collin P. Jaeger], original copyright [2016] (See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0172011#pone.0172011.s003" target="_blank">S3 File</a>).</p

Snout‐vent length plotted against capture day‐of‐year of Eastern Massasaugas captured during 2006–2014 in Cass County, Michigan.

<p>Seven distinct age classes are evident: age 0 (blue) includes recently born animals prior to their first hibernation; age 1 (gray) includes animals captured following their first hibernation, but prior to their second hibernation; age 2 (black) includes animals captured following their second hibernation, but prior to their third hibernation, and so on; age 3 (white); age 4 (orange); age 5 (red); age 6 (yellow). Age 1 begins to overlap with age 2 in August (~ DOY 225).</p

Sources for Eastern Massasauga life history information including locality and study years.

<p>Sources for Eastern Massasauga life history information including locality and study years.</p

Relationship between latitude (untransformed) and age-zero annual growth as explained by the top-ranked model using AIC_c (Table 3).

<p>The shaded area represents the smoothed 95% CI using t-based approximations. County and district abbreviations are as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0172011#pone.0172011.g001" target="_blank">Fig 1</a>.</p

Relationship between latitude (untransformed) and size–fecundity (natural log back-transformed) as explained by the top-ranked model using AIC_c (Table 3).

<p>Female size was held constant at 55.2 cm SVL based on the average size of adult females in Cass County, Michigan. The shaded area represents the smoothed 95% CI using t-based approximations. County and district abbreviations are as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0172011#pone.0172011.g001" target="_blank">Fig 1</a>. The image of dam and offspring was taken within minutes of parturition in Cass County, Michigan (Photograph credit, E. T. Hileman).</p

Candidate model sets for life history variables.

<p>Bolded 95% confidence intervals exclude zero and therefore indicate the standardized effect size for a given explanatory variable is informative. For models with two predictor variables, the standardized effect size and 95% CI for the first and second variable are in the first and second row associated with that model. Model abbreviations are the same as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0172011#pone.0172011.t002" target="_blank">Table 2</a>.</p

Relationship between mean annual precipitation (untransformed) and neonate mass as explained by the top-ranked model using AIC_c (Table 3).

<p>The shaded area represents the smoothed 95% CI using t-based approximations. County and district abbreviations are as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0172011#pone.0172011.g001" target="_blank">Fig 1</a>.</p

(A) Number of study sites, number of explanatory variables per model, and number of candidate models possible and (B) specific candidate models used in analyses of Eastern Massasauga life history response variables.

<p>(A) Number of study sites, number of explanatory variables per model, and number of candidate models possible and (B) specific candidate models used in analyses of Eastern Massasauga life history response variables.</p

Relationship between mean annual precipitation (untransformed) and adult female size as explained by the top-ranked model using AIC_c (Table 3).

<p>The shaded area represents the smoothed 95% CI using t-based approximations. County and district abbreviations are as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0172011#pone.0172011.g001" target="_blank">Fig 1</a>.</p