Baby Please Eat Me

In our eyes, we think of a nuptial gift in the form of an engagement ring or wedding ring. And although presenting your partner with a nuptial gift is commonplace in many animal species, some spiders take it one step further.

During mating the male meets his end when the female eats him. That’s right, eats him.

This grisly occurrence is called sexual cannibalism.

What’s even stranger is that the males of these spider species appear to actually initiate sexual cannibalism.

An interesting case is of our very own Australian redback spider. During sex the male does a little somersault which places his abdomen over the female’s mouthparts, practically begging the female to eat him. She does so, during the act itself.

 

What the hell, right?

It seems so extreme that the males would actually want to be eaten. But there may be a good reason for this.

There are thought to be two benefits that male spiders receive for sacrificing themselves. The first is that the cannibalised male gets to mate with the female for longer and fertilise more eggs than those that survive mating. The females are also more likely to reject other suitors after consuming their first mate.

Unfortunately for redback males this all depends on how hungry the female is, as she won’t eat him if she’s full… better luck next time.

So apparently, in some species, sex is worth dying for.

 

But why would such a gruesome occurrence evolve in the first place?

Sexual cannibalism appears to have more commonly evolved in species where there are more males than females (male biased sex ratios), and where females are much larger than males (size dimorphism).

3347980953_5539e071dc_o
Sexual cannibalism and sexual dimorphism go hand-in-hand for the grass cross spider. Image Credit: H. K. Tang [CC BY-NC-ND 2.0] via Flickr
The female garden spider is also much bigger than the male, and usually enjoys him as a snack after mating. Image Credit: Les Chatfield [CC BY 2.0] via Flickr
The female garden spider is also much bigger than the male, and usually enjoys him as a snack after mating. Image Credit: Les Chatfield [CC BY 2.0] via Flickr
 

 

 

 

 

 

 

 

 

 

 

 

In some species, such as dark fishing spiders, the male always sacrifices himself, in obligate male death. Though, a lot of the time males can choose between life and death. The wasp spider is a good example of this, as the male can escape before the female has a chance to eat him. However, he can only do this during his first mating, as he will always sacrifice himself during his second mating.

 

The question is: what strategy is most beneficial?

This question is still being disputed, as many factors seem to influence the occurrence of sexual cannibalism. On the one hand, the male can mate with the female for longer if he chooses to hang around, while on the other, he can mate again if he chooses to leave early.

Another large uncertainty is why the female would want to eat her mate in the first place.

Although hunger is an immediate factor in some species, such as the redback spider, it doesn’t seem to be a factor in other species. The evolutionary advantage for females is also not apparent, as sexual cannibalism does not appear to increase female size or reproductive ability.

What IS known is that the bigger the size difference between males and females, the more likely it is that the males will be cannibalised.

So we don’t really know for sure why sexual cannibalism evolved in spiders, although we do have an idea as to why it would be beneficial to males.

Though personally, I’d much rather stick to an engagement ring.


11 Responses to “Baby Please Eat Me”

  1. Asher Trama says:

    Hi Olivia,

    It is known that the female gets some nutrition from the male, however, it is still debated as to what evolutionary benefit this has.
    It is known that when the male is cannibalised, the offspring are healthier, so this may be the reason for it. It is in the females interest to have ‘fitter’ offspring.

    The males receive two benefits for sacrificing themselves:
    1) The cannibalised male gets to mate with the female for longer and fertilise more eggs than those that survive mating.
    2) The females are more likely to reject consequent males after consuming their first mate.

    So the male is thought to have reduced competition with other males, and have more offspring if he sacrifices himself.

    Remember though, this evolved individually for each species, so no two species are the same, and the reasons for this behaviour may vary between species.

  2. Olivia Campbell says:

    Hey Asher,

    Im a little bit confused, is eating the male significantly important to the nutrition of the female? So the offspring have a better chance of surviving? What drives the males to offer themselves up? It seems very counter intuitive but very interesting!

  3. Asher Trama says:

    A very nice insight sogandh.

    The strategy of the Malabar spider (Nephilengys malabarensis), as used in your example is similar to that of the wasp spider, that I referenced in my article.

    Wasp spider males have two pedipalps (sperm transfer apparatus), and they snap off the first after their first mating, so that they can mate again. However, at least for the wasp spiders, voluntary cannibalism still appears to be the most beneficial to paternity advantage, as they will choose to do this instead of snapping off their second pedipalp (they wouldn’t be able to mate a third time no matter what).
    http://onlinelibrary.wiley.com/doi/10.1002/ece3.372/pdf

    The Malabar spider is interesting in that the males have seemingly evolved this strategy so that they can guard the female to reduce sperm competition with other males. It seems the Malabar spider has been able to bypass voluntary sexual cannibalism altogether.
    Though I wonder if this strategy is actually better than if they were cannibalised after the second mating.

    At least with the golden orb-web spider, the males (who also have two pedipalps) have higher paternal output when cannibalised than if they snap off their pedipalp, like the wasp spider. Unlike the wasp spider though, they don’t always sacrifice themselves after the second mating.

    There are many different strategies. However, many of these don’t seem to be an evolutionary stable strategy, so there are usually a mix of strategies in one species, with one being the most common.

  4. sogandh says:

    In my undergraduate course we did animal behavior and although I can’t answer how this evolved, I do know that in order to find out how this trait started you would have to trace it back to its evolutionary history and figure out what pathways would have lead to this trait (one of Timberg’s four important questions when trying to understand behavior and evolution- maybe there is a study on this, i’m just getting this from old notes). However, this specific trait would have developed due to the animals survival and reproductive success. It has been found that eaten males will fertilize more than uneaten ones. So due to natural selection the eaten males will be passing on the desired specific trait to offspring and to future generations,whilst the uneaten males don’t get to mate and die without passing any of their own specific traits(The favored trait is kept and the rest is weeded out).

    So the male allows the female to eat him as that will increase her chances of becoming fertilized giving him paternity advantage over the offspring.

    http://www.bbc.com/news/science-environment-16816344
    http://beheco.oxfordjournals.org/content/12/5/547.full

    It might be interesting to also know that males have found ways to counteract being eaten. When the male mates with the female they are likely to break off their sperm transfer apparatus (within the female) so she can keep being fertilized by him only and to stop any other males from coming along and trying to also fertilize that female.

    http://newshub.nus.edu.sg/headlines/0212/spider_02Feb12.php

  5. Asher Trama says:

    Thanks Ashton 🙂

    I think it would have started by accident. It is even possible that female aggression evolved first – and aided female survival, maybe by increasing foraging ability?
    Then males who were less able to escape/ less inclined to escape had more offspring.

    Unfortunately though, evolution takes more than a generation. It would have taken many (and I mean likely hundreds-thousands or more) generations for the fittest males (in this case those who aid cannibalism) to become the most common type in the species.

    Though, we will never know for sure how sexual cannibalism evolved, it’s a great mystery!

    What I find even more interesting is that this tactic evolved independently in each species. So males who aided sexual cannibalism were ‘fitter’ than males who didn’t in many individual cases! Cool, hey?

  6. Ashton Dickerson says:

    Really interesting Asher! I find it so crazy that behaviours such as these evolve. I mean, how did it all start? At what point was there a male spider that thought, “Heyyy, maybe if I let her eat me I’ll be better off in the long run”?!

    I suppose it all started by accident? Maybe some particularly aggressive females ate their partners without the males consent and had significantly more of their offspring as a result.. but surely it wouldn’t just take the one generation for such behaviour to develop?

  7. Asher Trama says:

    Thanks ebyers 🙂 Animal behaviour is so fascinating isn’t it. I could watch animals all day, and I wouldn’t get bored.

    I am too sagca. I wonder if there’s other reasons for their behaviour that we haven’t thought of. I think the race to have higher genetic output is a big factor, but we still have a lot to learn!

  8. sagca says:

    Interesting subject.
    I’m curious about the purpose of this behavior; it’s factor in the natural role of the spiders. Since most if the behavior of any kind of animals seems to have a specific role in the entire ecosystem, I think it is quite likely that sexual cannibalism has also has a secret what yet is to be discovered…

    Nature is amazing.

    Oh and I also think i’ll stick with the engagement ring.

  9. ebyers says:

    Great Article Asher! I think you did an excellent job taking on board all the things Jenny said. The title, intro, conclusion and subheadings were all really solid. On another note, I can’t get over how strange animal behaviour can be!

  10. Asher Trama says:

    It really can, the weird and wonderful products of evolution never cease to amaze me!

  11. crosato says:

    Very interesting! Evolution really can lead to some weird stuff sometimes…
    I think I would have to agree about preferring the engagement ring!