BiologyScience & Nature

Sex is weird: The Australian Redback Spider

CW: I only talk about spider sex, there aren’t any pictures of them for anyone with a phobia!

Hey guys, so last time I was talking about the evolution of sex (one of my favourite topics) so I thought I’d crack on with some fun case studies of where sex gets really weird. So today for a bit of a break from lots of theory which can sometimes make science more of a chore than ‘oh wow that’s odd, LETS INVESTIGATE!’ (I want to be the science Nancy Drew, so sue me). Let’s get our excitement pumping for some truly odd mating strategies which make us ask how is this adaptive? Alongside making the word paradox leap about in our minds giggling manically.

First up is the Australian Redback Spider, in the case of… SEXUAL CANNIBALISM. I love that phrase, biologists get with the drama and crack out the shock factor for only the best weird biological phenomena. Yet these spiders do exactly what it says on the tin, the females eat the males during copulation. However, the weirdest bit is the males aren’t against this at all, instead they back flip into the females waiting jaws. Yeah, you heard that right. Enjoy the image for a while for it is truly bizarre and freaky.

Do you want an explanation? Of course you do my fierce Nancy Drew’s or Abbie Mill’s or (insert cool investigative lady here). Well lets explain sexual selection first, it is the theory we turn to when a behaviour/trait cannot be explained under the action of natural selection, it is brought out to explain bizarre traits which can be attributed to increasing reproductive success and not survival. As it turns out for the male Redback spider it is adaptive for him to sacrifice himself into his lady love’s jaw’s. It’s adaptive specifically because of that dastardly thing called sexual competition and not to mention simple life history traits.
Let’s take a look at the life style of our wee Redbacked male, weighing in at 1-2% of the size of the females and living for only 2-4 months when compared to female’s 2 years (under lab conditions). They might not even survive that long as when travelling between female webs they have an 80% chance of mortality (poor lil dudes) and on top of that their reproductive organ always breaks off inside the female during copulation. Male Redbacks really do have a hard time.

Okay so I’ve made it pretty clear that being a male Redback sucks, so how on earth is leaping into the jaws of death adaptive? Surely it’s just the cherry on top of these poor guys lot in life? Well it turns out that presenting his abdomen to the female during copulation means that if he is the first lucky chap to mate with her she is less likely to remate. If she doesn’t remate then the male’s sperm will not be competing with another male’s sperm to fertilise her eggs and so produce more offspring. It has been found that the chance of females mating again is significantly more likely if she doesn’t cannibalise the first male she mates with.

What about if she does remate? How can presenting yourself as a tasty snack be beneficial then? Well for males which mated with already mated females, being eaten increased the duration of copulation, in some cases from 14 minutes to 30 minutes. By mating for longer males simply transfer more sperm, increasing the number of eggs they will fertilise, with the majority of eggs being fertilised by him if he sacrifices himself compared to only half the eggs if he survives.

So it is a combination of the fierce sexual competition between males for mates and the low likelihood of survival to a second mating which is suggested to have driven this odd behaviour. For the male Redbacked Spider it is indeed a better option to let his partner eat him in order to produce more young carrying his own genes.

Ah sexual selection is weird and awful in the best way.

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Floatingmanatee also known as Alice, is a certified Zoologist living in Britain and has an unhealthy love of weird creatures. She's bisexual, a lover of books, TV, film and anything else that tells a story. Generally she's either angry or sad or both about the state of the world, but watching youtube videos of cute animals makes her feel better.

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