Male Herennia multipuncta, more commonly known as ornamental tree trunk spiders, have a grotesque post-mating habit. After mating, males will gnaw off one or both of their sperm-transferring organs called palps. This genital self-removal is called emasculation and leaves the spider as a partial or full eunuch.
Now I know that you must be asking yourself why anything would bite off its own genitals, but there are apparent benefits for this mutilation. Research recently published from the Slovenian Academy of Sciences and Arts in Ljubljana investigated several hypotheses postulated to elucidate the benefits of this unusual behaviour.
The team collected spiders from areas such as China, Vietnam, and Taiwan for use in the study. Out of 60 mating attempts 32 were successful, with nine males chewing off one palp and 23 chewing off both.
When male spiders mate, they leave behind a small portion of the tip of the palps, which obstructs the female reproductive tract. This prevents other males from successfully mating with the female (how romantic). The male does this instinctively, ensuring the paternity of the female’s offspring.
A eunuch male is very protective of the female he mates with, and is more aggressive against other virgin males in the area. Emasculated males exhibit behaviours such as attacking the virgin spiders and shaking the web they are sitting on. Once a male has successfully done all the mating that he is physically capable of, he has nothing to lose dying in a fight against another male.
The palps are disproportionately large and heavy in these spiders, and it was proposed that with palp loss, males would have better physical agility and endurance. This increased endurance was shown by engaging spider movement using a gentle poke with a paintbrush. If a spider didn’t move after five successive touches, then it was deemed exhausted.
To make things worse for male spiders, females will eat males if they are hungry. Cannibalism is common in spiders.
Many spider species practise sexual cannibalism, where females eat their mating males before, during, or after sex. Latrodectus species are more commonly known as widow spiders, and their naming was inspired by this phenomenon.
Sexual cannibalism is common in species with sexual size dimorphism, where the female is considerably larger than the male. There are many proposed reasons for why a female would consume a potential mate. If a female is starved, she may consider a male spider as a better meal than a mate. During or after mating, a male makes a free meal and nutrients to pass on to her newly conceived offspring.
The aggressive spillover hypothesis states that females that are more aggressive in hunting prey are also more likely to consume males. Some species of female spiders, like wolf spiders, may reject unattractive mates by eating them.
Males have developed particular behaviours in response to sexual cannibalism. These behaviours include opportunistic mating and altered sexual approach, where the time of and lead-up to mating is carefully calculated by the male. This is all done to prevent becoming detected and eaten.