Geographic Variation in Somatic Growth Rate of Wood Turtles (Glyptemys insculpta)
Life history traits, such as body size and age at maturity, display intraspecific variation across a species' geographic range. Previous studies that examined body size variation among conspecific populations of turtles found that body size generally increases with increasing latitude, with the exception of Wood Turtles (Glyptemys insculpta) and Spotted Turtles (Clemmys guttata), which follow a non-linear relationship with larger body sizes at the two range extremes. Similarly, growth rates are influenced by climatic variables and should display geographic patterns reflective of different environments. The objectives of our study were to a) quantify somatic growth in a northern population of Wood Turtles, and b) determine if the observed geographic variation in body size was the result of interpopulation variation in somatic growth rates. In the northern population, as turtles approached maturity, somatic growth increments varied between the sexes such that on average males grew to larger mean carapace lengths than females within the same number of growing periods, likely indicating the point at which energy is reallocated from growth to reproduction. Populations in the south had relatively higher somatic growth rates, grew to smaller mean carapace lengths, and attained sexual maturity earlier than those at the northern extreme; this pattern was related to the number of frost-free days and temperature. Understanding variation in species' life history traits is critical to understanding changes in population demography, which is important when managing populations that are at risk of extinction.
Mean relative carapace length, determined using the Sergeev method (Sergeev, 1937), for male (n = 12; open squares) and female (n = 15; solid circles) Wood Turtles (Glyptemys insculpta) in a population in the Sudbury District, Ontario. Each growth ring indicates approximate age up to 20 growth rings (Greaves and Litzgus, 2009).
Regression analysis of mean relative carapace length (mm) and somatic growth increments (mm) for male (open squares, dashed line; R2 = 0.33) and female (solid circles, solid line; R2 = 0.59) Wood Turtles (Glyptemys insculpta) in a population in the Sudbury District, Ontario. Carapace lengths and somatic growth increments were determined using the Sergeev (1937) method.
Polynomial regression analyses showing the relationship between somatic growth rate and latitude for female (solid circles; R2 = 0.95) and male (open squares; R2 = 0.97) Wood Turtles (Glyptemys insculpta) across the species' geographic range. Somatic growth rate (mm/month) is represented by the slope of the line in a linear regression of relative growth rate and carapace length (not shown). Negative values on the y-axis indicate decreasing relative growth rate with increasing carapace length.
Comparison of somatic growth rate (mm/month), as represented by the slope of the line in a linear regression of relative growth rate and carapace length (not shown), to (A) the number of frost-free days, (B) the mean active season temperature, and (C) the mean annual temperature for female (solid circles, solid line) and male (open squares, dashed line) Wood Turtles (Glyptemys insculpta) in the five study populations. Negative values on the y-axis indicate decreasing relative growth rate with increasing carapace length.
Comparison of somatic growth rate (mm/month), as represented by the slope of the line in a linear regression of relative growth rate and carapace length (not shown), to the mean carapace length of female and male Wood Turtles (Glyptemys insculpta) in the five study populations. Negative values on the y-axis indicate decreasing relative growth rate with increasing carapace length.
Contributor Notes
Associate Editor: W. L. Smith.