During sperm transfer and climax, the male redback spider does a somersault of sorts, placing his abdomen in the perfect position for the female to eat, and more often than not, she goes for it.
Whether you think it romantic or horrific, there is something captivating about this ultimate sacrifice during the moment of climax. But because of this, the female redback may remain a lonely single after sex, albeit a little less hungry.
Black widows are also members of the Latrodectus genus to which redbacks belong.
This act of sexual sacrifice, called ‘terminal investment,’ has been extensively studied by the lab of Dr. Maydianne Andrade at UTSC, where thousands of black widows — notorious for their highly potent and neurotoxic venom — share refuge.
However, if you think it romantic, perhaps the male spider’s reasoning for sacrifice may make you think again. It appears that the male redback’s terminal investment serves an evolutionary, or depending on how you look at it, selfish purpose.
Though it may seem counterproductive for a male spider to sacrifice his entire existence for just a single shot at producing progeny, there are several adaptive advantages that he gains by taking this risk.
Self-sacrifice serves to enhance male paternity both by increasing the number of eggs the male spider fertilizes and by decreasing the chance that their female partner will mate with another competitor.
Andrade’s team found that six out of nine females that cannibalized their partners refused to mate with a second male, while only one in 23 females that didn’t have the pleasure of consuming their mates did the same.
Also, the chance that the male could mate again if he escapes the fangs of his lover is meagre. Therefore, his self-sacrifice offers a way to give it all he’s got by partaking in the ultimate act of evolutionary fitness.
Though female redbacks can be violent in their courtship, they do offer some mercy to the fittest of their male counterparts.
‘Premature cannibalism’ — which occurs before copulation is complete — is much less common if the male offers courtship for over 100 minutes, a marathon of sorts for the reward of paternity. However, males that are ready for this marathon must be wary of cheating competitors that can sneak in at the finish line, disguise themselves as the winner, and avoid being prematurely eaten.
And when it comes to being eaten by their mate, size does matter. Females are less likely to prematurely cannibalize a large marathon runner than a small sprinter.
However, more importantly, a male is less likely to be eaten by his female counterpart if she is well-fed — he only offers a meal sized at one or two per cent of her body mass.
Unfortunately for him, food is typically a rare commodity for a plump female redback in her native Australian habitat, so she may well take the meal that he offers in addition to her potential future offspring.
While this extreme sacrificial gesture and its violent ending could be seen as a spider’s ultimate Valentine’s Day gift, in the end, it is neither the life of the female nor the male redback that is rewarded, but their offspring that ultimately earn the gift of life — and go on to do the same.