Summary
Sexual bimaturation, an intersexual difference in age at maturity, is a consequence of sexual size dimorphism (SSD). Sexual bimaturation arises through intersexual differences in growth trajectories. In theory, differences in growth trajectories should bias the operational sex ratio in favour of the early-maturing sex. In addition, in animals with sexual bimaturation, the late-maturing sex always maintains a lower intrinsic rate of growth (k) that may be linked to the metabolic cost of growth. We studied growth, its relation to the operational sex ratio, and its metabolic cost in the extremely size-dimorphic northern map turtle. We found that females take twice as long as males to reach maturity and that females maintain higher absolute rates of growth but have a lower k. The estimated operational sex ratio was even in our study population, and estimates of annual mortality were similar between adult males and females. Based on respirometry, we found no evidence that fast-growing females incur a metabolic cost compared to non-growing males.
Methodology
We conducted this study from 2003 to 2007 at the Queen’s University Biological Station (44° 34' n, 76° 19' w) approximately 100 km south of Ottawa, Ontario, Canada. We captured northern map turtles in Lake Opinicon with basking traps, by snorkelling, and by surveying nesting areas. Captured turtles were brought back to the laboratory where we measured maximum plastron length using a forestry calliper (± 0.5 mm). We marked turtles captured for the first time by drilling a unique combination of small holes in their marginal scutes. Turtles were released at their capture location within 24 h. We determined sex based on external sexual characteristics. Males have long tails and their cloacal openings are located past the posterior margin of the carapace. In contrast, females have short tails and their cloacal openings are at the margin of the carapace. In addition, females have wider heads than males (Bulté, Irschick & Blouin-Demers, 2008).