(Updated, 1/15/17: fixed broken links)
I wasn’t planning on writing another entry for “creepy-crawly week,” but on Saturday I saw something in the garden that could not be ignored.
As I was walking across the yard, I noticed some motion in one of the cross spider (Araneus Diadematus) webs. It was another spider, one much smaller than the resident of the web, both in span and volume, carefully flitting to the bigger spider, engaging a little, and then moving back, while also strumming on the web with its front legs*. Based on this behavior I realized that it must be the male spider trying to mate with the female. It’s not an easy task, given that spiders will eat pretty much anything they can catch.
Knowing that the mating act for some spiders is a cannibalistic one — the female eats the male after mating — I had a great title for the post picked out, something along the lines of “Finding love in the garden, followed by death.”
Males approach females with caution in order to avoid being eaten. During copulation, males embrace the female’s abdomen; sperm is transferred by the insertion of one of the male’s palps. The male departs after mating, and the female spends a number of days inside her retreat. She then begins to spin an egg sac or ‘cocoon’, which protects the eggs. She stays close to the cocoon for a number of days before dying.
I checked back on the spider pair a few more times on Saturday and Sunday to find that the male was still hanging around while the female was perched under a leaf, perhaps resting in “her retreat,” preparing for her egg sac spinning. Or perhaps tired of the male’s advances and web music.
One of the spiders that practices post-mating cannibalism is the Australian redback spider (a member of the black widow genus, Latrodectus). Earlier this summer, PBS’s scienceNOW had a profile of biologist Maydianne Andrade (transcript available and some related links at the bottom of the page, including an interview with Maydianne Andrade, but the full video is not available for streaming or download. It might be available on a Nova scienceNOW DVD), a patient and daring researcher who does her fieldwork at night in the Australian forest. Her quarry is the redback, one of the most poisonous spiders on the planet. One of the big questions she is investigating is “why does this spider practice cannibalism?”
Not surprisingly, Andrade’s research shows that there is some evolutionary logic to cannibalism during mating: by allowing itself to be eaten by the female, the male redback can spend just a little bit longer in the mating position, thereby improving the odds that his genes will be passed on to the next generation. And in any case, if he isn’t eaten by a female, he’ll probably be eaten by ants or starve to death (if that didn’t happen already — most males never find a female).
* In 2007, Living on Earth had a segment about spider songs, with audio available for streaming or as an MP3 download.