When fertile, females of this species wait outside the nest each night waiting for a flying male to show up. When a male flies in, he courts her a bit and then they mate. After the successful coupling, the two remain stuck together while the female drags her suitor inside the nest. She then curls around and bites the males abdomen in half, leaving the male to die while she walks off with his genitals and the lower half of his body. Since this seems like a rather strange thing to do, two researchers (Moonin and Peeters) decided to investigate whether this amputation acted as a rather gruesome copulation plug preventing other males from mating. They found that other males were unable to mate while the severed body of their rival was blocking access and that by the time the female removed the male pieces about a half hour later she was no longer interested in sex. The female ants were never observed mating again.

While this may at first glance seem pretty detrimental to the male, males have little chance of finding another fertile female and the payoff for a successful mating is offspring for the entire life of the female so the male is well rewarded in the evolutionary sense although he doesn’t look too happy in this sketch of the female about to separate him from his lower half.

The sketches in the article were interesting but I had been hoping for pictures. Luckily, Monnin (the lead author) has a website covering his many studies of the queenless ant and includes the pictures the paper’s sketches were based off of and many other cool pictures.

If you were wondering what exactly is going on in the picture above, the biologists Allard, Gobin, Ito, Tsuji and Billen set out to answer your question. They decided to catch a similar species of ants in the act, cut a thin section out of the center of them and see how everything was connecting.

Their caption left me looking for a dictionary so I’ve inserted definitions in []’s:

Longitudinal section through copulating pair of Diacamma sp. showing male aedeagus [the whole male reproductive parts on the end of the abdomen] inserted inside female genital tract, and spermatophore (SP) [a packet of sperm] inside oviduct [passage from ovaries to outside]. Arrow points at strand connecting spermatophore to male gonopore [hole where sperm comes out]. Note cross section of sting (S), which is directed sideways by female during copulation. Scale bar: 1 mm.
Fig. 2. Schematical representation of fig. 1.
Fig. 3. Detail showing volsellar digitus (D) [bottom part of claspers for holding onto female] and cuspis (C) [top part of claspers for holding onto female] of male clasping the last sternite [the tail end of the abdomen] of female (arrow) during copulation. Scale bar: 100 um.
B: bursa copulatrix [insect vagina]; C: cuspis; D: digitus; P: inflated penis; PV: penis valves; S: sting; SP: spermatophore; VS: vesicula seminalis [sperm storage].

You may be wondering how they could cut such a thin slice out of a pair of ants. It turns out they fix the ants in place by submerging them in a clear plastic and then cut a tiny section through them with a double bladed saw. Reminds me of undergrad, when I had to do something similar when sectioning fish ear bones for aging.

If you were still curious, they also extracted the ant penis. I’m not too sure what the purpose of that was but if a trivia question ever asks how many lobes an ant penis has, we’ll now know the answer is 2.

The letters correspond to P: inflated bilobe penis; PA: paramere [case for penis when not in use]; PV: penis valve.

References

Monnin T. & Peeters C. 1998. Monogyny and regulation of worker mating in the queenless ant Dinoponera quadriceps Animal Behaviour. 55:299–306

Allard, D., Gobin, B., Ito, F. Tsuji, K. & Billen, J. 2002. Sperm transfer in the Japanese queenless ant Diacamma sp. Netherlands Journal of Zoology. 52:77-86

When females of a species mate with several males, evolution can develop some pretty weird adaptations in males competing to fertilize the egg. More numerous, faster or cooperative sperm can improve the chances of their sperm reaching the egg first. Another alternative is to try to prevent other males from mating with that female.

As a way to interfere with other males, many species develop sticky semen to form copulatory plugs blocking other sperm access to the egg. I had heard of this before but what I didn’t know was that some insects have taken this method to an extreme and actually leaves pieces of their genitals behind in the female. These pieces had been shown to protect the males’ sperm against competition by blocking rivals from mating with the female. Similar genital breakage had been observed in spiders but since spider females like to eat their mates and males have two penis-like organs, called pedipalps, scientists debated whether a spider sacrificing one of his genitalia could be a sort of quick release to allow the males to live another day and mate again.

So the authors of this paper (Nessler, Uhl and Schneider) set about solving this question. They figured that if genital damage was a survival tactic then virgin males, who have a second pair of genitals in reserve, should damage their pedipalps more often than males who had already lost one genitalia and have nothing to gain (evolutionarily speaking) by surviving with no genitalia. On the other hand, if broken genitals function as copulation blockers then duration or probability of mating should decrease after a female mates and is stuck with male genital pieces inside her.

Virgin males did appear to be trying harder to escape post-copulatory predation with virgin males cut and running after about 6 seconds and one-genitalia males sticking it out for a lengthy 25 seconds. This resulted in 35% of virgins escaping while only 5% of their more seasoned counterparts avoided post-coital dining. Yet even with these differences in behavior, the researchers still found that genital damage was not significantly different between virgin and one-pedipalp males. This suggests that sacrificing one’s genitals does not make one more likely to escape a cannibalistic female.

On the other hand, the researchers found that males mating with females containing pieces of genitalia copulated about half as long and were 75% less likely to break off their genitals as males mating with either virgin or genitalia-free mated females. Since longer matings increase the chance of that males sperm being successful, genital breakage appears to be a form of copulation plug that increases the evolutionary fitness of that male.

For the curious, the article also included a picture of a) female spiders, b) male pedipalp genitalia, c) a piece of male genitalia stuck in a female, and d) a close-up view of the male pedipalp showing the part that breaks off.

I was surprised to see how small the broken piece of pedipalp was. It’s kind of amazing that losing that little piece of tip results in sterility for that genitalia and blocks other males. Then again, I suppose any genital breakage is too much genital breakage.

References

S. H. Nessler, G. Uhl and J. M. Schneider. 2007. Genital damage in the orb-web spider Argiope bruennichi (Araneae: Araneidae) increases paternity success Behavioral Ecology 18:174-181

I had always pictured sperm as little tadpoles but it turns out that rodent sperm actually have some rather nasty looking hooks on their heads. Scientists were not sure what the purpose of these hooks was. Some guessed they were just for aerodynamics while others had observed sperm using their hooks to link up and form cooperative groups.

This paper theorized that sperm cooperation would occur more in mice and rats where the sperm of different fathers had to compete against each other. In rodents where females were promiscuous, sperm from the same father, since they are on average 50% genetically identical (the same as brothers), would be evolutionarily selected to cooperate with each other. Promiscuous females present many problems to males trying to pass along their genetic material. One other ways that males compete is by volume of semen and number of sperm (the more players in the game, the better the chance of winning). This means that males in species where many males mate with one female will tend to be selected for big testicles. The authors of this study used testicle size as an index of female promiscuity and found that in more sexed-up species sperm are in fact hookier and more cooperative.

To back up their results, the scientists measured the speed of sperm trains vs. individual sperms. They found that in rats sperm trains are about 30% faster but in mice the sperm trains are slower. They guess that sperm trains also provide greater thrust to help move through viscous fluid so mice sperm may still perform better in trains.

A pretty cool study and a nice quick read at only 3 pages. Before today, I had no idea sperm cooperated. That’s why I like biology, there’s always something surprising and interesting to learn yet it’s all still tied together by evolution. Now if you’re anything like me, you probably want to see one of these sperm conga lines. I know I would have been pretty disappointed if the article didn’t have any pictures. So here’s the money shot:

If pictures aren’t enough, there’s also a video.

References:

By Hook or by Crook? Morphometry, Competition and Cooperation in Rodent Sperm. S. Immler, H.D.M. Moore, W.G. Breed, T.R. Birkhead. PLoS. 2007.