It Pays to Have a Wingman

In most courtship displays or leks male birds are competing with one another, attempting to show off and prove their superiority even if they know they are not the strongest or the fittest individual there. It’s pre-programmed for them to face off. This is what was originally theorized when people watched the Lance-Tailed Manakins performing.

Lance Tailed Manakins preforming.

However, this turned out not to be the case. Instead of competing with one another, the two males were actually undergoing an elaborate courtship display in which ones individual is the alpha and the other is the beta. In other words, the alpha bird had a wing-man to help him display and successfully woo females. This was more than a little baffling as the males were not related to one another, and altruism is rare in the animal kingdom and the beta males were willingly cooperating in these dances. So what does the beta male get out of this relationship if he isn’t mating himself?

Emily DuVal (2007) was curious enough to test it, undergoing several experiments which included genetic sampling and typing of birds and offspring, observation studies of the courtship displays, as well as an alpha removal experiment where the alpha male from each courting duet was removed for a time. 

The results were quite interesting. It was hypothesized that the territory (or display site) would eventually be passed from the alpha to the beta, yet the results found that the alpha role was not passed along in a linear fashion as proposed. Instead rather, it was concluded that within the complex social network of these birds the betas may be undergoing a sort of apprenticeship, learning from the alpha so later on they themselves can have a more successful courtship display as alpha. Additionally, the alphas are determined through that same complex social structure she mentions, where the birds that are essentially the best net-workers with the most social connections to other male Manakins are the ones who end up as alpha generally.

The courtship for the Lance Tailed Manakin is still not totally understood, but it is a great example of how not all mating displays are what the initially seem as well as that males can get along when the incentives are good.

Reference

DuVal, EH 2007, ‘Adaptive Advantages of Cooperative Courtship for Subordinate Male Lance‐Tailed Manakins.’, The American Naturalist, vol. 169, no. 4, pp. 423–432.

Video: http://www.youtube.com/watch?v=2OQq5P3PLCw

A Corkscrew By Any Other Name is Still Not Romantic

For most of the behaviors I’ve discussed so far there has always been some effort and choice in the matter of individuals mating. A male develops large plumage to entice a female to choose them, both sexes evaluate one another for health and have extra-pair copulations accordingly, and so on. However, nature isn’t all that nice most of the time and the avian world is not exception to that. Some species of bird have forces copulations, wherein the females have absolutely no say in the matter and are forced to mate with whichever male is strong enough to force the copulation. Ducks, as well as several other species of waterfowl, are well known for this forced copulation mating system and as such have developed some outright WEIRD strategies for both sperm competition as well as insemination avoidance.

Now at this point some of you will know what I’m talking about, but for the rest prepared to be baffled and be warned: there are graphic duck genitalia images ahead. But they’re really cool. Because male ducks have adapted to sperm competition by having corkscrew penises. Yes, you can go back and re-read that. In a mating system where force is the only factor is achieving a mating males are in essence taking a chance that any one female that have sex with will actually become pregnant with their offspring, as every other male in the pond is also copulating with as many females as they can find. As such, many males ended up with a corkscrew penis which, in the case of the Muscovy duck, is able to evert and become fully erect in 0.36s (Brennan, Clark & Prum 2010). The high explosive rate actually helps force the penis further into the vagina, and some species of duck even have brush like appendages along their penis in order to collect and in a sense, “scrub away” competitors sperm. 

 Male Duck shown with fully extended penis. They can reach up to 40cm in length.

Now this is all very impressive in itself, but it gets even more interesting when you look at how the females of these species have adapted to thwart the males. These species of waterfowl essentially have antagonistic sexual conflict, in that the females do not wish to copulate (I don’t think I need to elaborate on why) yet are continually forced into it by aggressive males. Well, the corkscrew shape of the penis didn’t just emerge from thin air. Female duck vaginas are actually also corkscrewed in shape, just in the OPPOSITE direction. At some point in more recent evolutionary time the direction of the vaginal geometry flipped, making it more difficult for the males to successfully fertilize the females. The same study done by Brennan et al on the explosive time also looked into the effectiveness of the penetration of those males into different shapes. They used glass tubes to see how far the penis was able to reach in what must have been the oddest experiment done to date.

They tested a straight tube, a counterclockwise spirals which matched the male penis chirality, and finally they tested a clockwise spiral and a 135˚ bend. The last two mimicked actual vaginal geometry found in Muscovy ducks, and they found that the penis was far less successful in reaching any great depth into those tubes compared with the straight and counterclockwise spirals. This supports the idea that difference in corkscrew direction coevolved between the two sexes of many waterfowl species. The rather abused ladies had found a way to fight back. 
 

Example of oppositely spiraled genitals as well as the tubes used.

What’s more, in some species of duck females have even been found to have several “dead ends” in their vaginal tracks. Enfolds of the vaginal lining provide a sort of diversion to the penis, which can get re-directed into pockets that lead nowhere and store the sperm away from any of the actual eggs. This prevents fertilization while allowing the male to believe that he has successfully fertilized that particular female and move along.

So there you have it. Nature is not always pretty, but when organisms are faced with extreme challenges they will adapt, even if it’s to defend against their own species. And some of those adaptations can be pretty screwy!

References

Brennan, PLR, Clark, CJ & Prum, RO 2010, ‘Explosive eversion and functional morphology of the duck penis supports sexual conflict in waterfowl genitalia’, Proceedings of the Royal Society B: Biological Sciences, vol. 277, no. 1686, pp. 1309–1314.

http://www.kiwiblog.co.nz/2012/10/the_ducks_will_get_you_in_the_end.html

http://blogs.discovermagazine.com/notrocketscience/2009/12/22/ballistic-penises-and-corkscrew-vaginas-the-sexual-battles-of-ducks/

Timing is Everything

While there is a huge diversity of behaviors birds use during mating, it is important to remember that in many cases there are outside factors that can affect them that may not be obvious at first. For instance, Bearhop et al (2006) have fairly recently done some interesting work on migration. Specifically, how the migration of the European Blackcaps may end up undergoing speciation in the long run.

The European Blackcaps are a species of European passerines, a fairly common little bird found across Eurasia and northern Africa, have historically bred in south central Europe. However, instead of migrating to Iberia and Northern Africa for their overwintering grounds a rapidly increases number of the birds are showing up in the British Isles. And as time goes on, it seems that they are staying.

Bearhop and his colleges used a stable isotope technique for tracking these birds across their migration distances, something that had previously been quite difficult as Iberia and the British Isles are a fair distance away from one another and these birds are rather small. Lots of room for error and lost birds. However the stable isotope technique allowed them for the first time to get accurate, traced results and what they found could have some interesting long term implications.

They found that there is a genetic basis for the new migratory pattern displayed by the birds overwintering in the British Isles yet still returning to the south central Europe breeding grounds. And their numbers are growing, hinting that there may be some benefit towards fitness to change up the overwinter areas.

European Blackcap at a feeding table.

There are several reasons why the birds heading to the UK and Ireland may be experiencing higher fitness. The level of urbanization is quite high, presenting easy access to food from feeders, rubbish bins, as well as exotic plant species which fruit and flower more frequently throughout the year. The shorter migration distance is less energetically exhausting, and some have even theorized that the colder climate better prepares the birds for the early breeding ground climate upon their return. However, what may be the most important observation is that critical photoperiods during juvenile development that trigger gonad development occur in northern climates about 10 days earlier than the ancestral breeding ground.

That change shifts the development and breeding cycle of the birds up by several days, triggering migration back south sooner than those in Iberia return north. What we’re seeing is assortive mating being driven by a genetically driven behavioral change. This could, over time, isolate the species by migration pattern, and eventually even lead to speciation as the change is genetically driven.

So there you have it, a gene change for migration behavior may lead the European Blackcap to speciate sometime in the future. A time that may not even bee that far away really, as the migration change of birds towards the British Isles has rapidly occurred just over the past 60 years or so!

References

Bearhop, S, Fiedler, W, Furness, RW, Votier, SC, Waldron, S, Newton, J, Bowen, GJ, Berthold, P & Farnsworth, K 2005, ‘Assortative Mating as a Mechanism for Rapid Evolution of a Migratory Divide’, Science, vol. 310, no. 5747, pp. 502–504.

Picture: http://www.burdr.com/2009/12/blackcaps-adapting-to-a-new-migratory-path/