Dance for me

July 21, 2011 § 1 Comment

Ah, courtship. That crazy time when you’ll do almost anything to show off for your potential mate: drink too much, fight with rivals, play chicken with cars, and generally behave in ways that make you shudder in later life.  The courtship rituals of suburbia are complex enough, but they pale in comparison to the behaviors some animals show.  Why do these rituals evolve?  Darwin hypothesized that both sex-specific ornamentation, such as the tail of the peacock (bling, if you will), and elaborate courtship displays could be explained by selection of preferred mates by “choosy” females.  The female gets to be choosy because she puts far more effort and risk into generating the progeny, so males compete to have their genes benefit from all that work.

Ever since Darwin, this idea has been discussed, debated and elaborated.  One of the factors females seem to be evaluating in courtship displays is the stamina of the potential mate.  The notion is that displays that involve large investments of energy (like leg-waving in spiders, or the calls of frogs) reflect the “vigor” of the individual.  Thus the idea that courtship displays evolved to let the female identify the healthiest males, usually the ones with the best genes to pass on to her progeny.  But that seems unsatisfying as an explanation of the most elaborate courtship displays: the complexity of the acrobatics in these rituals goes far beyond a brute test of strength.   The idea that skill, not just stamina, matters for mate choice has been the subject of inconclusive discussions for decades. A recent study of the mating dance of the golden-collared manakin (Barske et al. 2011,  Female choice for male motor skills.  Proc. Roy. Soc B doi: 10.1098/rspb.2011.0382) now offers evidence that, at least in this one species, skill in performing the detailed choreography of the courtship display matters a lot.

Male golden-collared manakins are small birds with gold throats; they mate in a highly skewed fashion (a few males mate very frequently, the others rarely) and the males don’t share the work of raising the chicks.  The courtship display seems likely to be unusually important under these conditions — there’s no other basis on which the female can choose between one male and another.  And indeed, when you watch a male golden-collared manakin perform its courtship dance you definitely get the impression that he feels that what he’s doing is crucially important.  The courting and male-male competition takes place in a location known as a lek.  Each male clears a courting arena, and when the area is appropriately cleared he starts jumping repeatedly from tree to tree, snapping his wings together in midair.  When he lands on the tree, he briefly displays his golden throat, pointing it towards the center of his courting area (where the female will sit if she’s interested), in a move called “beard up”.  Sometimes he produces a series of wingsnaps so close together you can’t separate them, called a rollsnap.  These are birds that are trying very, very hard… to do something.

(Data from Barske et al. 2011).

Barske et al. observed and took high-speed video recordings of several male golden-collared manakins during the mating season, and analyzed how their performance correlated with mating success.  It turns out that these choosy females are indeed very choosy.  Not only do they care about issues such as the frequency of wing snaps (an energy-intensive part of the choreography), but they also care about the performance of specific moves.  The “beard up” move turns out to be particularly important: the faster the male completes the move, the better his chance of mating.  Barske et al. compare this “beard up” move in the choreography to the controlled touch-down of a gymnast at the end of a routine, which is difficult enough that even the best athletes sometimes stumble.  The difference in “beard up” time between a successful male and an unsuccessful one can be less than a tenth of a second.  It seems that the females are watching this move very carefully indeed.

Returning to the question of vigor, the authors wondered just how challenging a workout these displays actually are.  To find out, they telemetrically recorded the heartbeats of birds during a performance, using tiny transmitters that weigh about 1 gram.  When not courting, a male golden-collared manakin’s heart rate is around 550 beats per minute; the average heart rate of a male in the middle of a courtship display is almost double that, at 1060 beats per minute.  The highest heart rates were recorded during wingsnap production, reaching as high as 1370 beats per minute.  Wow.  (For comparison, most people have a resting heart rate of 60-100 beats per minute.)  These peak heart rates are as high as those of hummingbirds in hovering flight, the fastest heart rates known for any animal.  Birds may not need to buy flowers and engagement rings, but courting is still (metabolically speaking) a very expensive process. Male manakins actually lose about 20% of their body weight during the roughly 6 weeks of courtship activity.

Barske et al. also analyzed various aspects of male morphology, including the size of the golden collar, or beard.  None of the measurements they made showed a significant correlation with mating success.  So what is the point of the collar?  One theory of the evolution of sexual displays holds that the courtship dances are a way of showing off male ornamentation, and you could rationalize the “beard up” maneuver this way: the motion could be designed to maximize the female’s view of the collar.  But you can also turn this idea on its head: perhaps, at least in some cases, the ornament is a way of drawing attention to the dance moves.  There’s evidence that this might be what’s happening here: phylogenetic analysis [PDF] suggests that the elaborate courtship dance evolved earlier than the collar. If rapid completion of “beard up” is especially important as a determinant of female choice, having a golden flash to emphasize the speed of the move could be an advantage.

So, although courtship dances may look bizarre, they provide a stringent review of the health, stamina and neuromuscular control of individual males, thus allowing the female to get a good look at the quality of the genes her kids are getting.  Quite sensible, really.

Barske J, Schlinger BA, Wikelski M, & Fusani L (2011). Female choice for male motor skills. Proceedings. Biological sciences / The Royal Society PMID: 21508030

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