Cannibalism: A Perfectly Natural History(14)

Although encounters with humans are rarely fatal (at least for the human), the same cannot be said for male redback spiders attempting to mate. In the first stage of courtship, the male approaches the female’s web and proceeds to get himself noticed (which takes a bit of doing, since he is only about one-fifth her size). He does so by bouncing his body up and down, throwing some silk around, and waving his front legs. As a point of information, while insects have six legs, spiders have eight of them, plus an additional pair of anteriorly located appendages called pedipalps. In male spiders, the pedipalps are modified for transferring sperm to the female’s body, a chore necessitated by the fact that spiders lack penises. Furthermore, there is no internal connection between the pedipalps and the testes, which are located within the abdomen. Instead, sperm is initially extruded from a furrow on the male’s abdomen into a spun receptacle called a sperm web. As a male dips his pedipalps into the pooled sperm, a pair of coiled structures called emboli and their associated muscles work like tiny turkey basters to suck up the liquid and store it until copulation.

The next phase of redback courtship begins as the male initiates repeated bouts of physical contact with his potential mate, behavior that includes tapping, probing, and nuzzling. The real heavy petting begins once the male locates the female’s epigynal opening. According to spider expert L.M. Forster, this behavior includes “nibbling, palpal boxing, and knocking, embolus stretching, push-ups, abdominal vibration, and epigynal scraping.”

By now, if the female hasn’t already eaten the male (which can put a serious dent into all of this foreplay) the spiders briefly assume “Gerhardt’s position 3.” To visualize this, picture two people in the missionary position. Now tweak the imagery a bit so that the guy is approximately the size of your favorite throw pillow. Okay, now add another eight limbs. (All right, maybe you shouldn’t picture this.)



Gerhardt’s position 3 appears to be favored by all Latrodectus species except the Australian redback, where it is abandoned immediately after the male penetrates the female’s epigynal opening with the tip of a sperm-charged embolus. At that point, things take a hard turn toward the strange. The male slowly performs a 180-degree somersault that ends with his abdomen resting against the female’s mouthparts and she immediately expresses her gratitude by vomiting enzyme-laden gut juice onto the tiny acrobat. She then begins to consume the male’s abdomen as they copulate, pausing from time to time to spit out small blobs of white matter. Upon the completion of the sex act, which takes anywhere from five to 30 minutes, the male crawls off a short distance, reportedly making repeated attempts to reel in his spent embolus “by stretching it with his forelegs and then releasing it abruptly.”

Approximately ten minutes later, rather than fretting over missing body parts or a less-than-tightly-coiled sperm applicator, the male returns to the fray, this time wielding the second embolus. The half-eaten spider then proceeds to reenact its earlier copulatory acrobatics, although this time minus some viscera. By way of a “welcome back,” the female resumes her meal, consuming more and more of the male’s abdomen. At the end of this round, though, rather than allowing him to crawl off, the female wraps her shredded partner in silk, eventually snorking up his now enzyme-liquefied innards like a spider-flavored Slurpee.

While the benefits of a risk-free meal for the redback mom-to-be are fairly obvious, one has to wonder what the hell is in it for the male? Because of this very question, the mating habits of L. hasselti have drawn interest from spider experts. The puzzled scientists determined that females that had recently eaten their mates were less receptive to the approach of subsequent suitors. Cannibalized males also copulated longer and fathered more offspring than non-cannibalized males. Ultimately, then, it seems that this rather extreme example of paternal investment optimizes the likelihood that the cannibalized dad gets to pass his genes on to a new generation.

Things get dicey, though, when trying to determine the benefits for redback males eaten before mating takes place, a situation that has also been reported in orb-weaving spiders like Araneus diadematus. Mark Elgar and zoologist David Nash worked with this species and proposed that pre-mating cannibalism allows the female to choose which male will get to inseminate her. The researchers supported their hypothesis with the observation that smaller males were eaten more often than larger—and presumably healthier—individuals. They also used modeling studies to hypothesize that pre-mating cannibalism would occur only in instances where there was no shortage of males from which to choose. Alternately, post-mating cannibalism appears to make good evolutionary sense when short-lived males are relatively few in number and have a low probability of encountering receptive females. In these instances, when a potential mate is encountered, it pays to give it your all, even if that means paying with your life.

Observations related to mantis and spider cannibalism serve to illustrate the Gary Polis generalization that among invertebrate cannibals, males get cannibalized far more frequently than females. This behavior occurs in species that exhibit sexual dimorphism, a condition in which there are anatomical differences between males and females of the same species.

Besides body size, other examples of sexual dimorphism include coloration and ornamentation, and here it’s usually the males that display bright colors, or exhibit elaborate structures like horns, frills, and crests. Essentially, these showy accouterments are used to advertise the wares of a presumably healthy male to potential mates (in other words: “If I can spend energy advertising just how beautiful I am, then I’m also a great bet to father a clutch of healthy offspring”).