2.1 To eat or to mate? sexual cannibalism in mantodea and arachnid  (Page 11/15)

Gould’s spandrel: a non-adaptionist hypothesis

Though experimental results appear to support a sexual selection hypothesis, some researchers present an alternate interpretation. Female spiders may consume smaller males because, quite simply, it is easier to do so (Wilder and Rypstra 2008). Though tiny males may not be nutritionally substantive, it is possible that female predatory behavior is ingrained and non-discriminatory (a female is willing to catch any prey around). In size dimorphic species, sexual cannibalism may have arisen incidentally from the condition of sexes’ size divergence.

The non-adaptionist hypothesis is composed of a simple argument:

1. High levels of size dimorphism is an attribute of typical predator-prey interactions. Sexually cannibalistic spider species often display high sexual size dimorphism (Wilder and Rypstra 2008)
2. There is no feasible explanation for how sexual cannibalism could have selected for high sexual size dimorphism. If females consistently selected against smaller males, there would be lower dimorphism than spider species exhibit. Instead, sexual size dimorphism must have evolved first, driven by external factors. Fecundity selection favored greater female size because massive females could produce bigger ootheca, and more surviving offspring. Conversely, scramble competition promoted smaller size among males.
3. Since size dimorphism is conducive to predator-prey interaction (see premise 1), sexual cannibalism emerged subsequently (Wilder and Rypstra 2008).

Thus, the non-adaptionist hypothesis argues that the phylogenic distribution of sexual cannibalism in some spiders is a by-product of selection for sexual size dimorphism in other contexts. Stephen J. Gould terms such incidental cross-trait associations “spandrels,” after an architectural term for the space between two arches (Gould 1997). In a spandrel , a behavior may not be beneficial at all, but inevitably arises out of external survival pressures.

A corollary to the “spandrel” explanation is another, “spillover” hypothesis. According to some researchers, sexual cannibalism is a consequence of generalized female aggressive behavior. The trait of female rapacity is favorable for juveniles, promoting their survival in a stage that demands high food consumption. Rapacity is less important after females attain maximum size and sexual maturity, but the aggressive trait cannot be extricated by that point (even if it drives away a fecund female’s potential mates). Thus, sexual cannibalism is non-adaptive, but sustained because rapacity helps juveniles survive (Arnqvist and Henriksson 1997).

Discussion question:

Conclusions and future directions

Several well-developed hypotheses arise to explain the peculiar and dramatic behavior of sexual cannibalism. Models based on fitness economics, sexual selection, and non-adaptive “spandrels” or “spillovers” provide viable descriptions of cannibals’ evolution. These various hypotheses are not mutually exclusive, and may apply differently to species based on organisms’ unique morphology and environmental pressures. A common vein in several hypotheses is that sexual cannibalism is a manifestation of conflict of interest between the sexes. Intersexual conflict is succinctly expressed by cost/benefit models for males and females, in which parameters for the benefit of cannibalism contrast. This conveys the divergent nature of reproductive behaviors. Sexual cannibalism demonstrates that, even when organisms have the common goal of producing quality offspring, they interact through selfish and often antagonistic tactics.