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Correlates of Reproductive Output in Turtles (Order Testudines)

John B. Iverson
Herpetological Monographs
Vol. 6 (1992), pp. 25-42
Published by: Herpetologists' League
DOI: 10.2307/1466960
Stable URL: http://www.jstor.org/stable/1466960
Page Count: 18
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Correlates of Reproductive Output in Turtles (Order Testudines)
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Abstract

The effects of body size, latitude, diet, habitat, age at maturity, and annual clutch frequency on individual and total annual clutch mass (clutch mass × clutch frequency) were examined based on data from 51 populations of turtles representing 35 species and eight families. Because body mass was correlated with all of these traits and with egg mass and clutch size, its effects on reproductive output were removed by ANCOVA, ANOVA, or partial correlation analysis. Individual clutch mass, adjusted for body size, was correlated most strongly (positively) with latitude, although diet also had a weak effect (higher in carnivorous species). Body size-adjusted total annual clutch mass was correlated most strongly with a decrease in average age at maturity. Age at maturity adjusted for body size was not correlated with latitude. Annual clutch frequency was correlated negatively with age at maturity, latitude, and size-adjusted clutch mass, and positively with size-adjusted annual clutch mass. Habitat (i.e., marine, freshwater, or terrestrial) had no detectable effect on size-adjusted clutch mass or size-adjusted annual clutch mass. These results suggest a model that explains the trade-offs among individual clutch mass, clutch frequency, and annual clutch mass in turtles. Species of turtles in high latitudes have shorter nesting seasons, and therefore lower clutch frequencies, and higher size-adjusted individual clutch mass. However, latitude has no effect on size-adjusted annual clutch mass or its negative correlate, age at maturity. Because age at maturity is generally correlated with longevity (i.e., survivorship), variation in the latter may be the primary evolutionary determinant of variation in annual reproductive output in turtles. On the other hand, annual reproductive output may simply be related to the rate at which a turtle acquires its resources, with slow rates resulting in slow growth, delayed maturity, and reduced energy available for annual reproductive output.

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