The birds and the bees

~This post describes sex in explicit terms ~

It’s springtime in Seattle which means the air is marked with the scent of warm, wet earth, the cherry trees are in full bloom, and our conversations of the goings on in the natural world are couched in the phrase “the birds and the bees”.  While its origin story is not completely clear, it’s very likely that this euphemism has been around for hundreds of years, dating back as early as 1644 in the Evelyn Diaries.  Despite the phrases’ ubiquity for discussing human sexuality, ironically very few people know much about the sex lives of the phrase’s subjects.  This is attributable largely to the fact that how these animals procreate is quite different from the mammalian systems we are familiar with.  So in the spirit of springtime let’s talk about the birds and the bees, the real story of the birds and the bees.

Birds
The most consequential departure by birds from mammalian systems is that most birds lack any kind of external sex organ. In contrast, most other kinds of animals have sex organs that are either maintained outside the body permanently, or can emerge temporarily as needed. Among birds, there are exceptions to this but fair warning you’ll probably never look at a duck in the same lighthearted way again if you pursue this line of knowledge. Which is to say of course that you absolutely should.

As far as crows and other songbirds are concerned though, sex must be conducted without penetration by a sex organ. So instead, these birds copulate by way of the “cloacal kiss”.  The cloaca is a bird’s single external vent and it’s used for both waste elimination and reproductive activities. During the breeding season, both the male and female’s vents swell.  To copulate, the male mounts the female and pushes her tail aside with his.  Once they make cloacal contact, the sperm that’s been stored in his cloaca is transferred.  It’s an awkward looking balancing act that lasts only a few seconds.  After the initial copulatory event the pair may mate again several times within minutes or over the course of weeks.

Whereas people need to consider timing, birds can successfully mate even in the absence of ovulation.  That’s because females have specialized sperm storage tubes inside their oviducts.  A female’s vagina is very picky, however, and only a very small number of the male’s sperm will reach this holding chamber.  Depending on the species, once there a female can store the living sperm for weeks or months. The exact mechanism of how the sperm are subsequently released is not entirely known, but possibilities include that it’s the mechanical pressure of the passing ovum (unfertilized eggs) that releases the sperm, or that it’s directed by hormonal changes during ovulation.¹

One additional consequence of sperm storage is that it allows for different eggs within the same clutch to be sired by unique males.  In crows for example, about 80% of a nest has been sired by the female’s permanent partner.  It is unclear how many of these extra pair copulations are either solicited by or forced on the female.

Bees
While mid-air copulations have never been confirmed in birds, honey bees do in fact mate on the wing during what’s called the mating flight.  A newly minted, week old queen will take to their air followed by thousands of male drones.  She will only mate with about 10-20 of these individuals though.  To mate, the male inserts his endophallus (bee penis) into the queen’s sting chamber.  Once he has deposited his sperm, the male detaches.  Unfortunately for the male, however, he doesn’t do so with all the parts he arrived with.  His endophallus will rip off and remain inside the female, most often killing him.  As if that wasn’t brutal enough, the next male is required to remove the previous suitor’s leftovers before being granted the ability to seal his own fate.

By the end of the mating flight, the queen will be carrying hundreds of millions of sperm, most of which will be used immediately to begin fertilizing her eggs.  Like birds, the queen has a specialized storage chamber called a spermathecal where she keeps the unused sperm.  The stored sperm from her mating flight will last up to 4 years, after which the queen’s reign will come to an end, and the cycle begins anew.

Literature cited

1. SASANAMI, T., MATSUZAKI, M., MIZUSHIMA, S., & HIYAMA, G. (2013). Sperm Storage in the Female Reproductive Tract in Birds. The Journal of Reproduction and Development, 59(4), 334–338. http://doi.org/10.1262/jrd.2013-038