Israeli Science Puzzled by Suicidal Sexual Compulsion in Widow Spiders

The male's predilection for age could be understandable if older females were less likely to eat them, but they’re not, and why do the males throw themselves into the female’s mouth?

Brown widow spider preparing to consume a meal
Johannes Henschel

That post-coitus cigarette suddenly looks healthy compared with the after-sex habit of the brown widow spider.

Male brown widows prefer to mate with older females, even though younger ones don’t eat their lover post-satisfaction and older ones do, a collaboration of Israeli researchers from Ben Gurion University, the Volcani Center and the Hebrew University of Jerusalem reported Monday in the journal Animal Behavior.

It sounds categorically awful. But it seems the arachnids feel otherwise. Not all widow species delight in female-on-male cannibalism, but in the brown widow, up to 70% of matings end in ingestion, coauthor Dr. Yael Lubin of Ben Gurion University tells Haaretz. In two other Israeli species, the Revivim widow (Latrodectus revivensis) and the white widow (Latrodectus pallidus) it only happens in about 12% to 15% of matings.

In the brown widow, “The male actually sacrifices himself to the female by somersaulting into her jaws during mating,” Lubin tells Haaretz.

So, what have we here? Boy spider meets girl spider, they do what spiders do, he leaps into her mandibles... “Males don’t seem to be behaving in their own self-interest,” the researchers observe.

Being eaten costs the males twofold: they have fewer offspring and no opportunity to mate with another female. Which they will do if given a chance, Lubin says.

The scientists also note that they found less benefit for males in mating with the "older" spiders, which they define as at least a month after their last molt into adulthood (females may live up to a year in nature and two years in lab conditions; males tend to expire after a month – guess why). “Older females were significantly less fecund than young mature females,” they observed.

Flinging oneself into the jaws of death after coitus isn’t the only weird thing about the brown widow, a cousin of the infamous black widow. Another oddity is that nobody knows where the thing is from. As Lubin explains, it’s a “cosmopolitan species, invasive in many parts of the world, occurring around human habitation, including in Israel. Its actual original distribution is unknown.”

Why, why, why?

Having established that the males cast themselves into the great void after sex, one wonders why on earth the males, which average 3 to 4 millimeters in length, would choose to couple with cougar females, which average about 8 mm in length.

To investigate this, the researchers caught brown widows around central and southern Israel. Then they gave the males the choice of approaching either mature spiders, or immature (sub-adult) ones. How the males distinguish a mature spider from an immature one is still being investigated: probably pheromones, says Lubin.

In the immature ones, a separate study done at the Volcani Institute found that males cut a hole in females’ cuticle above her genital opening, and deposit sperm. This works fine, i.e., results in offspring, if done shortly before the female has her final molt into adulthood. Anyway, the female stores the sperm until that maturation, and in this instance, she does not consume the male.

Ultimately in fact the immature brown widows are more fertile than the older ones, observation has proved, and they don't cannibalize the male. But the males scorn them anyway. What gives?

The team wondered whether the males were possibly plugging the females’ epigyne (genitals) by leaving part of their pedipalps (part of the genitals) inside the females. Some species do that, to prevent the female from productively mating with somebody else. “If this occurred more frequently with older females, that would be advantageous for the male. But that was not the case,” the researchers stated.

Clearly, confining mating behavior to immature females would benefit the male brown widow’s general fitness. By the way, the same behavior was found by Lubin and others in the Australian redback spider, and reported in Biology Letters.

Perhaps like the fruit fly, the male craves the thrill of proper ejaculation, as Bar-Ilan University reported earlier this month. Any pleasure the brown widow male feels in ejaculating remains to be studied.

Are the older females luring the males into that last coitus with the equivalent of spider perfume – more powerful pheromones? Maybe. It remains to be tested, the researchers say. It really does bear adding that the research is being supported by the Federmann Center for the Study of Rationality at Hebrew University.