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Discussion question:

Do female redbacks benefit from being monopolized by the male? What are the costs and benefits of polyandry in this situation?

Sexual cannibalism and sexual selection

Though these economic models are enticingly straightforward, they still do not explain all incidences of sexual cannibalism. Sexual size dimorphic species (prominently spiders) pose a problem when males are not complicit. Because their males are too small to provide a nutritionally substantive meal for females, these species elude the foraging strategy hypothesis. Consuming diminutive males may even be costly to the female if she neglects hunting larger food items while busy consuming her tiny mate (Elgar 1991). Since many male spiders are not willing self-sacrificers like the redback, cannibalism appears to be unfavorable to male fitness as well (Wilder and Rypstra 2008). How can non-complicit sexual cannibalism in mate size dimorphic couples be considered adaptive?

Relationship between sexual cannibalism incidence, sexual size dimorphism, and fecundity in arachnid and mantis species (modified from Barry et al. 2008).
chart 2

One hypothesis for this quandary claims that the cannibalistic behavior is an extreme form of mate selection, in which females reject miniscule, undesirable males by consuming them. According to the sexual selection hypothesis, the female is not driven to cannibalism by a proximate, nutrition-related cause, but instead by the function of ultimately beneficial sexual selection. Females may prefer larger males because body size reflects foraging skill: males who are able to obtain higher mass may be more capable of constructing and placing their webs, as well as capturing trapped prey (Wilder and Rypstra 2008b). If foraging abilities are heritable, then females can use male size as an indicator of the fitness of her offspring. Though sexual size dimorphic females may not obtain survival benefit from cannibalism, they use is as a mechanism of choosing males to sire fitter sons. This strategy is only costly if females are too selective and cannibalize all potential mates.

Testing the sexual selection hypothesis

In order for the argument to be supported, pre-insemination (non-mating) cannibalism in sexual size dimorphic species should target smaller males (Elgar and Nash 1999). One means of testing the hypothesis is through observation within a species , wherein researchers observe mating interactions involving same-species males with different levels of secondary sexual characteristics . Wilder and Rypstra monitored Hogna helluo (wolf spider) males in 90-minute mating trials within a laboratory setting, and found that larger male Hogna helluo were less frequent victims of sexual cannibalism (Wilder and Rypstra 2008b). In another investigation, Araneus diadematus females (1.89 female/male body length ratio) preferred to mate with, not consume, larger males (Elgar and Nash 1999). Furthermore, Schizocoas ocreata virgin females (1.26 female/male body length ratio) attacked smaller males of poorer body condition that displayed small, asymmetrical tufts of bristles, as opposed to voluminous tufts (Persons and Uetz 2005). Experimental evidence has consistently suggested that, in species with high sexual size dimorphism, predatory mating behavior targets smaller males. Thus, sexual cannibalism may indeed serve as a mechanism of sexual selection favoring larger males.

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Source:  OpenStax, Mockingbird tales: readings in animal behavior. OpenStax CNX. Jan 12, 2011 Download for free at http://cnx.org/content/col11211/1.5
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