Why these male spiders somersault into females' fangs while mating

Among Australia's redback spiders, mating includes a startling move: the much smaller male flips his body so his abdomen lands in the female's waiting fangs, then constricts his abdomen to slip free and finish mating before she can devour him. Smithsonian Magazine reports on a new study, published August 12 in the journal Biology Letters, that asked how such a strange and elaborate ritual is written into the spiders' inheritance.

The researchers took advantage of a close natural comparison. Redbacks and New Zealand's katipo spiders split only about 100,000 years ago, yet katipo males simply mate and run, while redbacks and just one other species out of 31 in the wider widow family perform the self-sacrificial somersault. By crossbreeding the two species and filming more than a hundred matings of hybrid males with infrared cameras, the team could watch which parts of the behavior the offspring inherited.

The surprise was that the two halves of the routine, the somersault and the abdominal constriction, were inherited separately: only about half the hybrid males performed both. That points to genes sitting far apart, or on different chromosomes altogether, with the somersaulting instinct apparently traceable to a single region of the spider's X chromosome. Beyond its curiosity value, the work may help conservationists confront a real problem, since invasive redbacks are threatening New Zealand's katipo through interbreeding.