Sexual dimorphism, particularly sexual size dimorphism has been observed in a large number of animal taxa (Shine 1989, Blanckenhorn 2005). Two main hypotheses, both first suggested by Darwin in 1874, have been proposed to explain the evolution of sexual size dimorphism. The first hypothesis proposes that sexual selection causes the observed differences and predicts that the relationship between body size and reproductive success differs between sexes. The result is selection favouring different body sizes at adulthood. This has proven to be an easily testable prediction and has been explored in many animal groups (reviewed by Andersson 1994 as cited by Pearson 2002). Among the most common examples is the degree to which males are larger than females in mating systems that involve male-male combat. There exists an important correlation between the intensity of male-male combat and the degree to which males exceed females in adult body size (e.g. Trivers 1976).
The second hypothesis proposes that ecological causes play an important role in the evolution of sexual size dimorphism, which may lead to sexes exploiting different ecological niches (Slatkin 1984, Shine 1989). Since ecological niches are often difficult to describe, Shine (1989) proposed the use of trophic structures (e.g. jaw width or length) as a tool to compare niche partitioning between sexes. If niche divergence has occurred, particularly through diet specialization, sexual differences in trophic structures may be a good indicator of specialization. When these structures are not sexually selected they become good candidates to test this hypothesis. Furthermore, to eliminate the simple effect of larger body sizes between the sexes, it is also important for trophic structures to be more dimorphic than body size (Shine 1989, 1991, Thom et al. 2004). Using these parameters, much evidence for the ecological causes of sexual size dimorphism has been put forth (see e.g. Shine 1991, Temeles et al. 2000, Shetty and Shine 2001, Pearson et al. 2002, Thom 2004). Snakes have been used as a particularly good example of this phenomenon since they are gape-limited predators and maximum ingestible prey size is limited by the size of the snake’s head (Shine 1991).
Trophic differences can be interesting to examine in turtles since they are also, in their own way, gape limited. Many turtles are limited by the crushing strength of their jaws, which has been demonstrated through a strong correlation between prey type and head width (Lindeman 2000). Mollusc specialist, like the common map turtle (Graptemys geographica) have much wider heads than other species that specialize on softer prey such as insect larvae and algae (Lindeman 2000). In addition, they exhibit extreme female-biased sexual size dimorphism, where trophic structures are more dimorphic than body size (Lindeman 2000, Bulté and Blouin-Demers, unpublished data) (see Fig.1). There is no overlap of body size at adulthood but juvenile females do overlap in size with adult males, making them an interesting species with which to test for ecological causes of sexual size dimorphism.
In accordance with the ecological hypothesis we expect males and females to specialize on different prey types and different prey size. We can make the following predictions: prey occurrence should differ between the sexes, body size should be positively related to mollusc prey size within each sex, body size should not be related to non-mollusc prey type within each sex, residual head width (after the removal of the effect of body size) should be positively related to prey size and juvenile females should consume larger prey than same-sized males.
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