Optimal egg size in a suboptimal environment: reproductive ecology of female Sonora mud turtles (Kinosternon sonoriense) in central Arizona, USA

in Amphibia-Reptilia
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We studied the reproductive ecology of female Sonora mud turtles (Kinosternon sonoriense) at Montezuma Well, a chemically-challenging natural wetland in central Arizona, USA. Females matured between 115.5 and 125 mm carapace length (CL) and 36-54% produced eggs each year. Eggs were detected in X-radiographs from 23 April-28 September (2007-2008) and the highest proportion (56%) of adult females with eggs occurred in June and July. Clutch frequency was rarely more than once per year. Clutch size was weakly correlated with body size, ranged from 1-8 (mean = 4.96) and did not differ significantly between years. X-ray egg width ranged from 17.8-21.7 mm (mean 19.4 mm) and varied more among clutches than within. Mean X-ray egg width of a clutch did not vary significantly with CL of females, although X-ray pelvic aperture width increased with CL. We observed no evidence of a morphological constraint on egg width. In addition, greater variation in clutch size, relative to egg width, suggests that egg size is optimized in this hydrologically stable but chemically-challenging habitat. We suggest that the diversity of architectures exhibited by the turtle pelvis, and their associated lack of correspondence to taxonomic or behavioral groupings, explains some of the variation observed in egg size of turtles.

Optimal egg size in a suboptimal environment: reproductive ecology of female Sonora mud turtles (Kinosternon sonoriense) in central Arizona, USA

in Amphibia-Reptilia



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    Responses of egg width to pelvic aperture width (shown by solid line scenarios) in various turtle species. Line labels refer to the line immediately above. The dashed line shows the expected relationship between carapace length and pelvic aperture width. The unconstrained/not optimized response suggests a constraint due to something other than pelvic aperture width since egg size increases with body size but the largest egg is still smaller than the smallest pelvic aperture (refer to text for details).

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    The relationship between clutch size and carapace length in Kinosternon sonoriense from Montezuma Well, Arizona, USA. The data include one female that produced a clutch of eggs in both years of the study. Including all log10 transformed data results in a regression equation where log10 clutch size = −6.269 + 3.243(log10 CL). Removing the two outliers (females that produced a single egg clutch) results in the relationship log10 clutch size = −6.042 + 3.161(log10 CL).

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    Relationship between carapace length and both mean X-ray egg width (MXREW) (solid circles) and X-ray pelvic aperture width (XRPAW) (open circles) in Kinosternon sonoriense from Montezuma Well, Arizona, USA. Linear smoothing functions are shown with accompanying 95% confidence intervals. XRPAW = 12.632 + 0.085(CL). MXREW = 19.631 − 0.001(CL).

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