Burying Beetles Play for Both Teams

Since Bruce Bagemihl‘s wonderful book Biological Exuberance (and Ricky Gervais’s stage adaptation of it), there’s no denying the fact that homosexuality is everywhere in the animal kingdom. Not as the occasional embarassing mistake, but often as a standard part of the sexual repertoire. Clearly, homosexuality has evolved in many species, but the question is, how and why? Since evolution works via success in spreading one’s genes, how could a behaviour that is not primarily geared towards leaving offspring, be advantageous?

The crucial word here is “directly”. Perhaps in most cases homosexuality itself is not *directly* advantageous, but it may be indirectly so. One example of this has just been published in the journal Biology Letters. Katharina Engel and colleagues of the University of Ulm, Germany, studied the burying beetle Nicrophorus vespilloides. This is a common beetle in Europe and Asia, known for its habit of interring dead animals (small birds or rodents, usually) and then tending the buried cadaver as food for their young. (In fact, they are one of the few kinds on insect where the parents care for the young larvae till they are grown up.)

In the scramble around a newly-discovered corpse, where many beetles will vie for the right to take possession of the corpse, male beetles face great risks but also great opportunities: if a male manages to outcompete rivals *and* fertilize the female that eventually is going to lay its eggs on the dead mouse, it will have reached its reproductive goal. And, as Engel and colleagues discovered, whether these males will engage in any gayness, will depend on how frantic the scramble around the mouse is.

They conducted two experiments.

In the first one, they placed males in cages with zero, one, or three females. After several weeks, they introduced such males to other males to see whether they would fancy them. The beetles that had been housed with females would never mount another male, but the males that had been housed in celibacy, would be more so inclined. So, the researchers say, males that “think” females are few and far between, won’t let a potential female get away, even if the female is actually a male. In other words, they can no longer afford to check first.

In the second experiment, they kept males in isolation for 60 days and then gave some a dead mouse and some not. Then, they introduced a male “lover”. As it turned out, the males-with-a-mouse were less likely to try to mate with the new male than the males-without-a-mouse. The explanation in this case is that the other male will be more inclined to fight the starry-eyed male if there’s food around that’s worth fighting for. So in this case, mounting a potential female and running the risk that it turns out to be a male, is a wise choice only if there’s little risk that the male (if in fact it turns out to be one) will bite back.

A heterosexual copulation in the burying beetle Nicrophorus vespilloides (male on top). Photo by Heiko Bellmann (from the Biology Letters paper--supplementary information)

A (heterosexual) copulation in the burying beetle Nicrophorus vespilloides (male on top). Photo by Heiko Bellmann (from the Biology Letters paper–supplementary information)

In conclusion, in male burying beetles, homosexuality seems to be an option born out of the risk of missing out on a heterosexual encounter with a real female.

Stay tuned for more discoveries from the realm of biological exuberance.

Get that phallus out of my face!

The “widow periwinkle” Littoraria melanostoma is a handsome little periwinkle living in mangroves along the coasts of East and Southeast Asia. Unlike many landsnails (which are usually hermaphrodite, that is, male and female at the same time), these estuarine snails come in separate males and females. Unhindered by any detailed knownledge of mollusks, one might assume that in sexual and other matters alike, snails are generally sluggish and docile. This is not generally the case, and also not in the widow periwinkle, as shown by Terence Ng and Gray Williams of the University of Hong-Kong in a paper just published in the Journal of Molluscan Studies.

Ng and Williams discovered that females, when they are mounted by a male, often use their snout to push the probing phallus away, time and time again, until the male gets so exhausted that he gives up:

In the video, the female is above, the male below; the female’s snout is seen pushing away the male’s phallus, which repeatedly appears out of the male shell beneath her.

The researchers tried to understand why a female would reject males so often (in about 90% of all mating attempts). They confronted large and small females with large and small males, but in all combinations the females were equally likely to push away a male’s penis, so it was not that the male’s weight and size was a reason to reject him. Also, they deprived females of sex for several months and yet found that after this long celibacy, the females still only accepted one out of ten suitors.

Long story short: the researchers still don’t understand why females give so many males the cold…ehm…snout. But one thing is clear: widow periwinkles are no pushovers.

It’s official: genitals *do* evolve faster!

Want to tell one species from another (identical-looking) one? Check their genitals! That’s the rule of thumb applied by taxonomists worldwide. In fact, many an identification manual, be it for cicadas, spiders, or snakes, often supplies little more than detailed images of the genitalia to assist in the identification of species that look very similar on the outside. My book Nature’s Nether Regions is all about why this would be so–which evolutionary processes are responsible for the breakneck speed by which genitals evolve and change their shape and function all the time. But surprisingly, nobody had ever actually measured the evolutionary rates of genitals and compared these with those of other organs. Nobody, that is, until Julia Klaczko and colleagues of Harvard University did so and reported on it in a new paper in Journal of Zoology last week.

What Klaczko and colleagues did was take a closer look at Anolis lizards. Almost 400 species of these colorful lizards are known, and especially the Caribbean species are famous among evolutionary biologists for adapting rapidly in body shape to their local environment. The researchers picked 25 Anolis species. For each species they selected 30 specimens and took measurements from their legs, dewlaps (a colourful skin fold under the chin) and one of their penises (like all lizards, a male anole is doubly endowed with two “hemipenes”). They then calculated how fast each of these organs evolved, by plotting the traits on the lizards’ evolutionary tree. And they found that, as expected, the genital shapes evolved faster than the other organs. About six times as fast, in fact.

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Each of these pictures shows one of the two hemipenes of five different Anolis species. The picture on the right shows the measurements that the researchers took.

The graph below shows how fast the lizards’ penis shape evolves compared with their legs and dewflap. The horizontal axis shows the evolutionary time separating two species, and the vertical axis shows the difference in organ shape. The line that climbs the fastest shows the genital evolution; the other lines (which climb slower) are for dewlap and limbs.

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A video (in Portuguese) on Julia Klaczko research:

The dangers of display

Sex is a risky business. Courtship, display, combat in rivalry, and copulation are all behaviors that expose the protagonists to predators. For example, bats home in on the ripples produced by calling túngara frogs, as research by Wouter Halfwerk and colleagues showed earlier this year. And it is no surprise that acts of copulation often take place in hidden sites, where the animals’ natural enemies cannot make use of the fact that they can catch them with their trousers down, as it were. The same also applies to males that are engaged in rivalry. In capercaillie grouse, for example, the males often engage in one-to-one territorial defenses. The fact that they are completely absorbed by this reproductively essential behavior, is often abused by the birds’ enemies, which include hunters, but also eagles, as this striking video shows:

Oooh, Mr. spider, you tickle me so!

Manipulative experiments are all the rage in animal sex studies. After all, one of the most coveted research outcomes is direct, experimental evidence that this or that sexual feature has an impact on what biologists call “fitness”, that is, an animal’s sexual success, measured in the currency of number of descendants. It all started off with studies in birds, already several decades ago, where researchers would clip tail feathers of swallows, add dots to the chests of grouse, or even stick colourful hats on finch heads; all in an attempt to see how this would alter their sex-appeal. But in recent years, the trend to doll up (or doll down) your experimental animals has gone micro, with researchers using nano-laser guns to zap off fruit fly hairs, for example.

Before and after: the top row shows male and female Leucauge mariana spiders au naturel; the bottom row shows them after the researchers applied their razors to their faces.

Before and after: the top row shows male and female Leucauge mariana spiders au naturel; the bottom row shows them after the researchers applied their razors to their faces.

In a study appearing ahead of print in the journal Behavioural Ecology and Sociobiology, three spider researchers describe how they shaved the faces of spiders to see how this would affect their sex appeal. And I mean literally shaved: they fashioned miniature razor blades and removed the facial hair from the large jaws of female spiders of the Central American species Leucauge mariana. In these tetragnathid spiders, the couple locks jaws during copulation and while the male transfers his sperm to the female, he moves his jaws in such a way that they tickle the hairs on the female. After mating is over, and if the female has not shooed the male off in mid-mating, she sometimes produces a glue-like substance to cover her vagina with; a sign that she is going to use this particular male’s sperm for fertilizing her eggs and will not entertain any other suitors for a while.

What the researchers found is that this tickling during mating is crucial for the female’s inclination to allow the mating to continue and also for her decision to produce the post-coital sperm plug. A shaved female, who no longer has hairs with which to register the male’s tickling, much more often would interrupt copulation by pushing the male away, or would refuse to apply a sperm plug after mating. Also she would be more inclined to mate with a subsequent male. And shaving off the hairs from the male’s jaws would, by and large, have the same effects, since he no longer has the tools to tickle the female with.

In other words: in Leucauge mariana, a good tickle during sex makes a male irresistible. And he gets the offspring the prove it.

Species spawned by special sperm?

When I wrote Nature’s Nether Regions, I struggled a lot with sperm and ova. Should I include the evolution of sex cells (or “gametes”) in the book or not? In the end, I did so, but only minimally. After all, the evolution of the multitude of shapes and traits, forms and functions of sperm cells and egg cells is a mer à boire just as complex as that of genital organs themselves. All the sexual selection forces that mold the shape of penises and vaginas do their tricks all over again when confronted with the cells that are channelled by these genitals.

A new article in this week’s PLoS Biology gives us a peek into the amazing world of gamete evolution. Janice Ting of the University of Toronto and co-authors studied that workhorse of developmental biologists, the tiny nematode worm Caenorhabditis. They discovered that, when they tried crossing one Caenorhabditis species with another, not ony did they not get any offspring, but also the female would be the worse for wear afterwards: she often died shortly after the deed. As it turned out, this was due to the sperm cells she had received from the other species’ male. Ting and colleagues saw how these had broken out of the uterus and were wreaking havoc with the female’s internal organs.

The reason probably is that that worm species with such rogue sperm was a sexually reproducing one, whose sperm has been continuously evolving to outcompete other males’ sperm (with the female reproductive system keeping up, evolutionarily, by bolstering the organ walls and the like). The species to which the hapless female belonged, however, was a species that mostly reproduced as a hermaphrodite (that is, it is male and female at the same time), by self-fertilizing–in nematodes evolution causes repeated shifts between sexual and hermaphroditic species. Since in such species there is not so much competition among the sperm of different males, these species have not been adapted to withstand the onslaught of aggressive sperm cells.

If true, then this is an interesting new reason why different species often have reproductive barriers: they cannot cross-breed because their female reproductive system cannot cope with the aggressive sperm cells of the other species. In one of my previous books, Frogs, Flies & Dandelions, I describe how female tsetse flies sometimes die after mating with a male from a different part of Africa because their vaginas are simply ripped apart by those males’ genitals. This new worm study again shows how barriers between species may evolve simply because sexual strategies do not keep up with one another.Screen Shot 2014-07-28 at 8.26.25 AM