Terminal Investment

Much of the current research in the lab involves studies of the sexually cannibalistic Australian redback spider (Latrodectus hasselti), and its close relatives, the black widows (genus Latrodectus). Redback spiders are intriguing because males actively ‘encourage’ females to cannibalize them while they mate (= copulatory somersault, below). Unlike most other sexually cannibalistic species (e.g., praying mantids) where males attempt to escape from the female’s fangs, redback males actually ‘somersault’ onto the female’s mouthparts during copulation = male sexual sacrifice (see video)

Copulatory Somersault

Copulatory Somersault

Copulatory Somersault:

(A) Copulation begins with the male standing on the female’s abdomen. Both spiders are facing in the same direction and are ‘belly to belly’. The male has two copulatory organs (the palps) that are attached at the anterior-most part of his ‘head’ (cephalothorax). Copulation begins when one of the palps is inserted into the female’s genital opening. In most other black widow spiders, the pair copulates while in this posture.

(B) In redbacks, however, a few seconds after palp insertion, the male, using the palp as a pivot, moves into a ‘headstand’ posture.

(C) The male then quickly turns through 180 degrees, landing with his ‘back’ (the dorsal surface of the abdomen), directly above the female’s fangs. In most matings, the female begins to extrude digestive enzymes almost immediately. She also pierces the male’s abdomen with her fangs and begins to consume him while he is transferring sperm.

Diagram: Forster, LM. 1992. Australian Journal of Zoology. 40(1):1-11 . (male = black, female = outline)

Because of the unique reproductive biology of male spiders, redback males are able to transfer sperm while they are being consumed. Thus, although males give up future reproductive opportunities when they sacrifice themselves, they achieve fertilizations in their current mating. Males that are consumed reap a two-fold paternity advantage relative to males who survive.  Males are unlikely to mate more than once, even in the absence of cannibalism, because of high mortality during mate searching. 

This extreme form of male mating investment provides a unique opportunity to test sexual selection and life history theory because males are under strong selection to succeed in their single mating opportunity, but are constrained by ecology and physically dominant females. We have a good understanding of factors affecting the strength of sexual and natural selection on redback males, and can manipulate cues indicating the strength of selection in the field and laboratory.