Male Frog Extracts and Fertilises Eggs From Dead Female

For a small Amazonian frog called Rhinella proboscidea, death is no impediment to sex. The males form huge mating balls in which dozens of individuals compete to fertilise a female. These competitions are so intense, and the combined males so heavy, that the poor female sometimes drowns in the struggle.

But for the males, that’s not a deal-breaker. Thiago Izzo from Brazil’s National Institute of Amazonian Research has found that the males can force the eggs from the bodies of the deceased female, and fertilise them. It’s a unique strategy and one that effectively involves sexual reproduction with a dead partner. Izzo calls “functional necrophilia”.

R.proboscidea is a small frog that looks like a dead leaf, right down to its pointed snout, its brown colour and the central white ‘vein’ running down its back. But its camouflage breaks down when it’s time to mate. Hundreds of males gather at breeding sites for just two to three days and when any female shows up, there’s intense competition for her attention. This strategy is called “explosive breeding” and it’s as violent as it sounds. Males wrestle for mating rights, and will try to displace any rivals that have actually found a female. The result is a large mating ball with a female at its bottom. She often drowns.

In the deadpan style of academics, Izzo writes that “such occurrences are obviously detrimental to females”. You don’t say.

He has seen the aftermath of the carnage first-hand, having found several explosive breeding sites in Brazil’s Adolfo Ducke Forest Reserve between 2001 and 2005. The first time, he found around 100 males and 20 dead females. The second time: 50 males and 5 dead females. But when Izzo dissected the females, he couldn’t find any eggs inside them. Where had they gone?

Izzo found the answer when he saw a male grasping the body of a dead female and rhythmically squeezing the sides of her belly. Out popped her eggs, like beads on a jelly-coated string.

The male on top squeezes eggs out of the dead female below. From Izzo et al, 2013. Journal of Natural History

The male on top squeezes eggs out of the dead female below. From Izzo et al, 2013. Journal of Natural History

Izzo saw the same behaviour again and again. On one occasion, the male pushed his dead partner around the pond, “apparently to avoid other males”. The eggs that emerge are quickly fertilised—Izzo kept an eye on them and saw that they eventually developed into embryos.

There have been many other cases of animals having sex with the dead, including several frogs and an ameiva lizard that tried to mate with a road-killed female. On Scott’s Antarctic expedition of 1910-1913, George Murray Levick saw a male Adelie penguin trying to have sex with a dead female, an act of “astonishing depravity” that he removed from his paper on the penguin’s behaviour.

And of course, there’s the now legendary case of homosexual necrophilia between two mallards—one living and one dead by window. That incident led to an IgNobel award for its discoverer, the inauguration of Dead Duck Day, and an academic paper with the keywords: “homosexuality, necrophilia, non-consensual copulation, mallard”.

All of these incidents were regarded as mistakes, but the behaviour of R.proboscidea is anything but. Izzo expects that other explosive breeders—and there are many frogs that do this—might also rely on functional necrophilia.

This behaviour clearly benefits the male. He doesn’t waste the huge amount of energy that he just spent on wrestling with other suitors. It would be a wasted effort, in any case—R.proboscidea males outnumber the females by ten to one, so the odds of finding and mating with another partner are pretty low. But he doesn’t have to. He can still foster a new generation of frogs.

There might even be some benefits for the female. She too gets a post-mortem chance of passing her genes to future frogs despite the unfortunate side effect of, er, being drowned by a ball of violent males. Silver lining!

Reference: Izzo, Rodrigues, Menin, Lima & Magnusson. 2013. Functional necrophilia: a profitable anuran reproductive strategy? Journal of Natural History

Hat tip to Olivia Solon for alerting me to this story

There are 19 Comments. Add Yours.

  1. Lena
    February 26, 2013

    These frogs just won’t take no for an answer! :)

  2. Dan J. Andrews
    February 26, 2013

    First exploding duck genitals, then poop-exploding face mites, now Kermit the necrophiliac frog. Depending on a reader’s viewpoint, you are either ruining nature or making it totally flipping awesome (this reader is in the latter category, of course).

  3. jonathan
    February 26, 2013

    I think the caption under the picture is wrong. The paper was published in 2012, not 2013.

  4. Richelle Nicole
    February 27, 2013

    This is beyond amazing! Such a defying act of thwarting an obstacle and continuing forth with procreation…what an amazing species :)

  5. Christina
    February 27, 2013

    “R.proboscidea males outnumber the females by ten to one” That’s rather unusual! What causes that lopsided sex ratio? Is it something like Wolbachia, but in reverse?

  6. Brian
    February 27, 2013

    Maybe the males outnumber the females by ten to one because they kill all the females when they try to have sex with them.

  7. Caitlyn Johnston
    February 27, 2013

    The paragraph “There might even be some benefits for the female. She too gets a post-mortem chance of passing her genes to future frogs despite the unfortunate side effect of, er, being drowned by a ball of violent males. Silver lining!”
    … is totaly mysogynistic!! There is nothing beneficial to a female about being killed in a orgy. What is wrong with you???

  8. Madhusudan
    February 27, 2013

    I’m curious about that sex ratio as well. Why is it so lopsided? I’m also rather worried about other violent effects of lopsided sex ratios – for humans, in places like India where life is already tough and violent for women!

    But what an incredible story this is. Seriously: W. T. F. Evolution?!

  9. Caitlyn Johnston
    February 27, 2013

    Clearly, it’s because they keep killing off the females.

  10. Christopher Moore
    February 27, 2013

    Not surprising, but REALLY cool!

    For those of you interested in sex ratios, the original paper has no quantitative foundation. I bet it’d be an interesting problem if you have the mathematical tools.

  11. Caitlyn Johnston
    February 27, 2013

    The sex ratio thing could easily be solved by breeding them in captivity and only releasing females. We (humans) could flood the population with females so they aren’t so gruesomely massacred and the males aren’t so desperately psychotic.

  12. Ed Yong
    February 27, 2013

    Okay several things.

    1) Misogyny. The last paragraph is meant to be read in a deadpan, tongue-in-cheek way. Also, these are frogs. And yes, in a situation where the female has a high chance of dying, she gets reproductive benefits from having her eggs fertilised post-mortem. Just as, for example, this male spider ( gets reproductive benefits from ripping his penis off.

    2) The sex ratio thing – it’s really not that unusual! All sorts of factors contribute to massively skewed sex ratios in the wild, including parasites, predators, differing investment by parents, differing competition between the sexes.

    The bizarre thing is mapping human values onto this (“psychotic”??), seeing skewed sex ratios as a problem to be fixed, and actually suggesting that we deliberately skew the sex ratios back to what we think is appropriate.

  13. amphiox
    February 27, 2013

    WIth respect to trying to actively meddle in the natural biology of amphibians, does anyone recall the Australian Bullfrog?

  14. W.Benson
    March 1, 2013

    Izzo is probably looking at the operational sex ratio: the number of males ready to mate when gravid females show up at mating sites. Females spawn all their eggs rather quickly and, if they survive, return to the forest; males stick around and mob each female as she arrives. In two or three nights, as many females may arrive individually as there are males waiting. Also, there is no guarantee that all females will be ready to mate in a given mating session. This would further skew the operational sex ratio even if males and females were equally numerous.
    That said, it seems reasonable to assume that female deaths further skew the operational sex ratio towards males.

  15. kcurtain
    March 1, 2013

    I’m not sure why these findings are surprising at all. Many male frogs squeeze females during amplexus to either encourage the female to initiate ovipositing or to physically push the eggs out of her. The males probably have no idea that the females are dead – they are just following their normal instinct to grab a female in amplexus and squeeze. To identify this as if it is some sort of newly discovered behavior is a stretch. And naming it functional necrophilia? This is external fertilization we’re talking about! In frogs! It’s not as if they’re picking corpses on purpose.

  16. Kevin McCurdy
    March 3, 2013

    Where to begin…

    As far as this behavior contributing to a skewed sex ratio in the general population, did you read the article? The article states that males are found dead at a rate of 10:1 to 20:1 over that of females after these mating events. If anything, this should skew the population towards females. Something else is going on here.

    I, too, am fascinated by the overlay of equity and equality, uniquely human ideals, on biological processes.

  17. Sola Oludiji
    March 16, 2013

    I guess Never say die”. the species must continue

  18. Malcolm
    May 19, 2013

    I cant see any benefit for this species, because these females would have only one chance of passing on their genes, whereas females of other species could lay multiple batch of eggs.

  19. Allie
    November 10, 2013

    And I thought humans were awful.

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