Phylogeny

 

Sexual cannibalism has arisen many times in arachnids, insects and amphipods.  This raises the question of what phylogenetic factors are associated with the evolution of sexual cannibalism. Some factors that seem to be involved in the emergence of mate cannibalism include feeding mode, sexual size dimorphism, and mating behavior. This section will focus on how these factors could have been involved in the evolution of sexual cannibalism in mantids.

Controversy

First, it is necessary to address the controversy surrounding mantid sexual cannibalism.  In the 1980’s there was concern that sexual cannibalism only occurred in a lab setting, as few researchers were able (or even tried) to verify its existence in the field.  There have been several studies since then that have confirmed that indeed sexual cannibalism does occur in the wild, albeit at lower frequency than in the laboratory (Maxwell et al., 2010). There are 2,200 species of mantids and it is not known how many of these perform sexual cannibalism either in the wild or in a lab setting.   Sexual cannibalism has been observed in over 20 species and confirmed in the wild for a least four of these(Maxwell et al., 2010).  From a sample of 33 of the commonly studied species, about 76% have been observed to cannibalize their mates in a lab setting (Prete, 1999). However, it is important to keep in mind that many males mate with females unharmed and that sexual cannibalism is influenced by many conditions.

 

Phylogenenetic tree of the phylum Arthropoda. Red stars indicate classes or orders in which sexual cannibalism has been observed to occur.

Image Citation: Assembled from Tree of Life

 

 

Feeding Mode

90% of the 90 sexually cannibalistic species are predatory mantids.  When sexual cannibalism is seen in nonpredatory species, the males are consumed but usually not killed.  This is not to say that all predatory species are also sexually cannibalistic but rather that most sexually cannibalistic species are also predatory (Wilder et al., 2009).

 

Picture showing a predatory praying mantis catching a humming bird.

Image Citation: http://blogs.nationalgeographic.com/blogs/news/chiefeditor/2009/09/mantis-catches-hummingbird-pic.html

 

Sexual Size Dimorphism

For mammals and birds, males are usually larger than females, most likely due to male-male competition.  However, in invertebrates, the females tend to be larger than the males. This could be due to selection for larger clutch sizes produced by larger females as well as decreased juvenile mortality of small versus large males.  This sexual dimorphism is particularly noticible in mantid and can even make it difficult to identify if males and females are of the same species (Prete, 1999). 

It has been predicted that species with higher degrees of size dimorphism between the sexes will exhibit higher frequencies of cannibalism based on the observation that females have to be strong enough to overpower the males (Prete, 1999). However, the evidence for this theory has been controversial.  There are many examples in which a female paired with a much smaller male will attack more often and be more successful in cannibalizing the male than a female paired with a male of similar or larger size. However, there appear several benefits to being a small male. One is that smaller males are more likely to be able to approach the female undetected.  Another benefit is that smaller males have less nutrients and it may not benefit the female to cannibalize them. As such, it appears that sexual cannibalism is more common in species with larger sexual dimorphisms. (Wilder et al., 2009).

 

Picture showing sexual dimorphism in a copulating pair of Stagmomantis limbata (Prete 1999)

 

Mating Behavior

Other important considerations regarding whether sexual cannibalism is even possible are: the male position during copulation and mating behaviors.  Male mantids have the option to mount the female dorsally or ventrally. Males that mount ventrally are much more likely than males that mount dorsally to be cannibalized because they are brought in closer contact to the female’s head.  In praying mantids, some males’ mount dorsally and then fly away after mating, thus potentially saving themselves from female attack.  Another consideration is number of matings, as increasing the number of matings a male has to partake in also increases his chances of being eaten. (Wilder et al., 2009)