Embryo Development and Sex Ratios in the Red-Footed Tortoise (Chelonoidis carbonarius) at Masculinizing Temperatures

Juliana Gaviria-Hernández, Vivian P. Páez, D. M. Ramírez, Claudia P Ceballos
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Abstract

Abstract. Incubation temperature is a factor that can affect several traits in turtles such as body size, growth, shape, and sex in species with temperature-dependent sex determination. A clear understanding of these effects is particularly important in threatened species such as the red-footed tortoise (Chelonoidis carbonarius), classified in Colombia as Vulnerable mainly due to the capture of wild individuals to be kept as pets. A previous study on the effect of incubation temperature on sex determination concluded that constant temperatures of 31°C and 33°C are lethal to C. carbonarius embryos and that a temperature of 29°C produces 100% females. Although this showed that C. carbonarius has a temperature-based sex determination mechanism, its full reaction norm is still unknown. To fill in the gaps, we incubated 160 eggs from 47 nests at constant temperatures of 24°C, 26°C, and 28°C. The hatching success rates were 55%, 53%, and 60%, respectively, with extremely long incubation periods of 213, 164, and 138 d. Of the 75 neonates obtained, 58 reached 8 mo of age, at which time 45% (n = 26) were sexed. The sexual proportions (% males) obtained were 88.9%, 60%, and 0% from the 24°C, 26°C, and 28°C treatments. From the calculated thermal reaction norm, we estimated that the pivotal temperature is 26.05°C (95% CI = 25.31–26.69) and the transitional temperature range is from 24.34°C to 27.77°C. Incubation temperature also affected incubation period and hatchling body size and growth. Additionally, we found a significant effect of female body size and mass on egg size and mass, supporting a fecundity selection hypothesis of body size. Finally, we inspected hatchlings for sexual shape dimorphism using geometric morphometrics. We found significant differences in anal notch and plastron shape at 7 d of age, but only differences for anal notch at 1 and 2 mo of age. However, the statistical signal was weak and the results varied with age; thus this sexing technique was unreliable. Given the low critical thermal maxima for embryo survivorship (31°C or less), together with a 100% feminizing temperature of 27.9°C, global climate change coupled with deforestation constitute imminent threats to this species due to demographic impacts such as low recruitment rates and skewed primary sex ratios.
雄性化温度下红足龟的胚胎发育和性别比
摘要孵化温度是一个可以影响海龟的几个特征的因素,如身体大小、生长、形状和性别,在温度依赖性别决定的物种中。对这些影响的清晰认识对濒危物种尤其重要,如红足龟(Chelonoidis carbonarius),在哥伦比亚被列为易危物种,主要是因为捕获野生个体作为宠物饲养。先前一项关于孵化温度对性别决定影响的研究得出结论,恒定温度为31°C和33°C对炭疽弧菌胚胎是致命的,29°C的温度产生100%的雌性。虽然这表明炭疽弧菌具有以温度为基础的性别决定机制,但其完整的反应规范尚不清楚。为了填补这一空白,我们在24°C、26°C和28°C的恒定温度下,从47个巢中孵化了160个蛋。孵化成功率分别为55%、53%和60%,孵化期极长,分别为213、164和138 d。获得的75只新生儿中,58只达到8月龄,其中45% (n = 26)为性别。24°C、26°C和28°C处理获得的性别比例(%雄性)分别为88.9%、60%和0%。根据计算的热反应范数,我们估计关键温度为26.05℃(95% CI = 25.31-26.69),过渡温度范围为24.34 ~ 27.77℃。孵育温度对孵育期、幼体大小和生长均有影响。此外,我们发现雌性体型和质量对卵的大小和质量有显著影响,支持了体型的繁殖力选择假说。最后,我们使用几何形态计量学检查了幼体的性别形状二态性。我们发现在7日龄时肛门切口和板形有显著差异,但在1月龄和2月龄时肛门切口只有显著差异。然而,统计信号较弱,结果随年龄而变化;因此,这种性别鉴定技术是不可靠的。考虑到胚胎存活的临界热最大值较低(31°C或更低),加上100%雌性化温度为27.9°C,全球气候变化加上森林砍伐对该物种构成了迫在眉睫的威胁,这是由于人口统计学影响,如低招募率和初级性别比例失衡。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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