Evolution of cannibalism – why do insects do it?
As mentioned previously, the most common and well understood form of cannibalism observed in insects is a sexual one. Hence, this form of cannibalism has attracted the greatest number of theories in regard to its evolution. Initial investigations into the evolution of sexual cannibalism attempted to explain the conditions under which the process would provide benefits (in terms of fitness) to both sexes (Thornhill 1976). In the case of males, sexual cannibalism could perhaps have evolved as an extreme type of paternal investment. If a male's sacrifice significantly enhances the quality or quantity of his progeny, sexual cannibalism could be an adaptive male strategy, rather than the result of a conflict between the partners (Parker 1979). One model predicted that sexual cannibalism would be favoured by natural selection when it significantly increased the number or viability of eggs fertilized by the cannibalized male's sperm (when the expected number of matings over the lifetime is low). An obvious drawback with this model is that it applies only to situations in which cannibalism occurs prior to sperm transfer. Perhaps because of this, and because of the anecdotal nature of many parental investment theories, many researchers reject theories of this kind. Even in species in which cannibalism is known to increase the number and/or viability of offspring (including mantids) males approach females with caution and retreat hastily after the completion of mating (Suttle 1999). Further, many males which belong to sexually cannibalistic species use diverse strategies to avoid being eaten by a female. If cannibalism is such an adaptive and ‘beneficial’ form of investment, why is this so? Furthermore, in many model organisms, sexual cannibalism occurs before the transfer of sperm to the female, thus barring the cannibalized male from any fitness advantage (Elgar 1992). Again, why would this be the case if cannibalism is apparently so beneficial?
The answer to this question might come by examining the other advantages for the male of a species other than ‘parental investment’. Participation in cannibalism may result in two major paternity advantages for the species: firstly, females that devour their mates are less likely to mate on another occasion (hence the cannibalised/first male is more likely to have his lineage continued), and secondly, cannibalized males copulate longer and fertilize roughly twice the amount of eggs than those not killed during copulation (further ensuring his genetic involvement). Furthermore, the cost of cannibalism may often be very low for these males, as their sexual organs may become detached during copulation and remain inside the female prior to the event, leaving the male mortally wounded and basically ‘useless’ (Andrade 1996). Although these theories were derived from an arachnid model organism, they nevertheless have much relevance to other arthropod classes, such as Insecta.
Despite the relatively sound nature of the argument regarding male-based benefits as the basis for cannibalism, it is unlikely that adaptive male participation is a ‘general model’ for the evolution of sexual cannibalism. This is largely because of the frequency of pre-mating sexual cannibalism (Suttle 1999). One popular theory for the presence of this phenomenon is as a mechanism of mate rejection, or female choice (Elgar & Nash, 1988). Basically, females would benefit from mating with larger males so as to produce larger offspring, assuming that large body size indicates superior and heritable foraging abilities, and size, strength, etc. Females should cannibalize smaller males to increase fecundity and improve maternal health, resulting in a flow on to offspring health etc. While this model was developed based on a study of a garden spider, it is also applicable to species in which females cannibalize males before sperm transfer (Elgar & Nash, 1988).
Yet another model which attempts to explain cannibalism, again from the female perspective, suggests that cannibalism can evolve simply through foraging considerations (Newman & Elgar, 1991). According to this "Economic" model, the two penultimate factors for the evolution of sexual cannibalism are (1) the number of males encountered and (2) the mass of food obtained from other prey. Females are predicted to cannibalize males when they encounter many males and the mean food intake rate from other prey items is low. When males are less abundant or less mobile, and alternative prey items are more plentiful, cannibalism should decrease. Hence whether a courting male is accepted as a mate or cannibalized hinges on his value to the female as a mate versus his value to her as a food source.
Many more theories exist in regard to sexual cannibalism, many of which are not supported by empirical/experimental evidence. Furthermore, many model organisms used to support these theories do not belong to class Insecta, and are very specific to those organisms. While some of the theories outlined above are based on non-Insect arthropods, they nevertheless remain relevant as the patterns described are also observed in insects.
The answer to this question might come by examining the other advantages for the male of a species other than ‘parental investment’. Participation in cannibalism may result in two major paternity advantages for the species: firstly, females that devour their mates are less likely to mate on another occasion (hence the cannibalised/first male is more likely to have his lineage continued), and secondly, cannibalized males copulate longer and fertilize roughly twice the amount of eggs than those not killed during copulation (further ensuring his genetic involvement). Furthermore, the cost of cannibalism may often be very low for these males, as their sexual organs may become detached during copulation and remain inside the female prior to the event, leaving the male mortally wounded and basically ‘useless’ (Andrade 1996). Although these theories were derived from an arachnid model organism, they nevertheless have much relevance to other arthropod classes, such as Insecta.
Despite the relatively sound nature of the argument regarding male-based benefits as the basis for cannibalism, it is unlikely that adaptive male participation is a ‘general model’ for the evolution of sexual cannibalism. This is largely because of the frequency of pre-mating sexual cannibalism (Suttle 1999). One popular theory for the presence of this phenomenon is as a mechanism of mate rejection, or female choice (Elgar & Nash, 1988). Basically, females would benefit from mating with larger males so as to produce larger offspring, assuming that large body size indicates superior and heritable foraging abilities, and size, strength, etc. Females should cannibalize smaller males to increase fecundity and improve maternal health, resulting in a flow on to offspring health etc. While this model was developed based on a study of a garden spider, it is also applicable to species in which females cannibalize males before sperm transfer (Elgar & Nash, 1988).
Yet another model which attempts to explain cannibalism, again from the female perspective, suggests that cannibalism can evolve simply through foraging considerations (Newman & Elgar, 1991). According to this "Economic" model, the two penultimate factors for the evolution of sexual cannibalism are (1) the number of males encountered and (2) the mass of food obtained from other prey. Females are predicted to cannibalize males when they encounter many males and the mean food intake rate from other prey items is low. When males are less abundant or less mobile, and alternative prey items are more plentiful, cannibalism should decrease. Hence whether a courting male is accepted as a mate or cannibalized hinges on his value to the female as a mate versus his value to her as a food source.
Many more theories exist in regard to sexual cannibalism, many of which are not supported by empirical/experimental evidence. Furthermore, many model organisms used to support these theories do not belong to class Insecta, and are very specific to those organisms. While some of the theories outlined above are based on non-Insect arthropods, they nevertheless remain relevant as the patterns described are also observed in insects.