Contributions to Zoology, 71 (4) (2002)Luca Luiselli; Godfrey C. Akani; Claudia Corti; Francesco M. Angelici: Is sexual size dimorphism in relative head size correlated with intersexual dietary divergence in West African forest cobras, Naja melanoleuca?

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This study demonstrates that in Naja melanoleuca, although females tend to have larger heads than males at the same body size (being not larger in terms of absolute body size, see Luiselli & Angelici, 2000; Luiselli et al., 2002), the two sexes are nearly identical as for dietary habits, both if we consider prey type or prey size. Thus, this study presents some evidence that pronounced sexual dimorphism in head size does not always reflect the presence of any important dietary divergence in snakes, and that the generality of previous conclusions (e.g. see Shine, 1986) should always be tested for before accepting it with unknown species.

With regard to the studied case, it is difficult to devise any plausible evolutionary scenario for the observed pattern of a significant intersexual divergence in relative head size but not in dietary habits. Traditional theory (e.g. see Houston & Shine, 1993) suggests that sexual size dimorphism in snakes result either from trophic niche divergence, or from sexual selection (respectively, through selection of larger litter sizes (and thus body sizes) of females, or larger body sizes of males (to win more sexual combats). But, with regard to Naja melanoleuca, the problem seems a lot more complex. In this species, both the occurrence of male-male combats during the mating season (dry season, from December to February) and the usual positive relationships between female size and litter size have been observed (Luiselli & Angelici, 2000). Moreover, Naja melanoleuca is an elapid, and superiority based on absolute size is a common pattern of ritual combats among male elapids (e.g. see Shine et al., 1981). Thus, the lack of a significant sexual dimorphism in absolute body size (Luiselli & Angelici, 2000; Luiselli et al., 2002) is likely dependent on the equilibrium in the interaction of two antagonist evolutive forces: sexual selection towards larger males (to win more combats), and natural selection towards larger females (to produce larger litters). Shine (1978) has observed the same pattern in several snake species with concurrently occurring male-male combats and positive relationships between female size and litter size.

But, why do females have larger heads than males at the same body length? Given that the snakes swallow the whole prey, the simplest solution would be that females have larger heads in order to swallow larger preys. But, in fact, they preyed upon organisms of size nearly identical as that of prey eaten by males (Luiselli et al., 2002), and moreover both sexes preyed upon very little prey in comparison to their own body size (Luiselli et al., 2002). Therefore, we doubt whether this simple solution may be true in the case studied. A possible alternative explanation for the observed pattern is that the larger head of females is a “ghost” of a past condition, i.e. it reflects an adaptation towards eating on larger prey that is not longer occurring at the present time. However, this hypothesis is not testable on the basis of the data in our hands. As alternative, we suggest that larger head size in females may be a result of the selection via male choice of mating. In other snakes (e.g., Natrix natrix, see Luiselli, 1996), the males mate preferably with the largest female available, when a choice is possible. We suggest that male cobras may choose the female with the largest head, although our hypothesis is still entirely tentative and is going to be tested by us in the years to come, with a specifically designed experimental procedure.

In any case, it is evident that cobras may be very useful models to study the ecological attributes of sexual dimorphism, as well as many other aspects of snake ecology (e.g. see Luiselli & Angelici, 2000; Luiselli, 2001, 2002).