REDBACK SEX: CANNIBALISM BUT NOT ‘COERCION’

Photo: Robert Downie

It has been known for some time that mature female redback spiders (Latrodectus hasseltii, above) cannibalise male partners during mating, but it has recently been discovered that males also engage in damaging mating tactics. Adult male redbacks mate with immature females (in their final juvenile instar), tearing the exoskeleton that covers the female’s reproductive tract in order to gain access to her newly developed, concealed genitalia. When males mate with mature females, prolonged vibratory courtship is typical, as males that attempt copulation early are killed by females before mating is complete (premature cannibalism). When males approach immature females, they engage in reduced courtship displays but are more likely to copulate twice, thus inseminating the females’ paired sperm-storage organs. The females store the sperm through their final moult and later produce fertilised eggs. At least one-third of immature females are mated this way in the field. Some researchers consider this behaviour coercive, because females do resist these matings (which cause haemolymph bleeding) and respond with elevated deterrent behaviours, such as rapidly raising and lowering their legs, hitting the male or the web near the male, or moving the rear legs and body while the male is mounted. However, coercion is said to occur only if the behaviour reduces overall female fitness. Luciana Baruffaldi and Maydianne Andrade set out to measure whether female fitness was reduced and found that immature-mating in females was either neutral or beneficial, as it meant they did not have to attract a male after moulting. Most immature-mated females did not produce sex pheromones as adults whereas unmated adult females produce sex pheromones that attract mates and trigger courtship. Some 17% of redback females die without mating. Those that do not mate as adults show reduced longevity, dying younger than mated females. Thus, Baruffaldi and Andrade found no reproductive cost for immature-mated females in terms of longevity, fertility or fecundity, and they therefore concluded that this male mating behaviour is not coercive.

Baruffaldi L, et al. 2017. Scientific Reports 7. DOI:10.1038/s41598-017-17524-6