It’s a familiar site on campus here during the first week of class: packs of jaywalkers moving in tight co-ordination, in sync with the flow of oncoming cars. From traffic lights and power grids to stereo sound and cinema, synchrony is so common in our environment that we usually only notice it when it fails. Not so with nature: the examples of synchrony in living things tend to be much more surprising to people studying animal behaviour.
Group courtship displays are a classic example. Think of chorusing songbirds in the morning or calling frogs gathered around a pool of water at night. Readers of my blog on peacock field work might be familiar with lek-mating birds gathered around a clearing to wait for females. Peacock train displays also tend to happen in sync. One traditional explanation for these co-ordinated displays is that, by synchronizing their most conspicuous behaviour, animals might gain some protection from predation1. Another possibility is constructive interference: co-ordinated timing might allow a pair of animals to spread the message farther than either one could on its own2. Two innovative new studies on animal courtship have added to this list. The first, on firefly displays, shows that synchrony might help insects recognize members of their own species by getting rid of visual clutter.
Fireflies are members of the beetle order Coleoptera that attract potential mates with flashes of light. Males of the family Lampyridae turn their bodies into beacons through a chemical reaction involving the enzyme luciferase, in a special light-emitting organ in their abdomens. There are hundreds of firefly species that use this strategy. In places where multiple species overlap, the potential for confusion is great, since the flash patterns of different species can easily disrupt one another. Fireflies seem to use synchrony to deal with this. In dense tropical forests, the results can be fantastic: large groups of insects flashing in perfect synchrony in the trees, as co-ordinated as a professional orchestra.
To test the idea that male synchrony helps females recognize and locate their own species, Andrew Moiseff and Jonathan Copeland studied the firefly Photinus carolinus from the eastern United States3. They brought females into the lab, and after about 10 pm or so when firefly courtship typically begins, the researchers tried to seduce the insects with flashing LED lights. They presented females with a set of 8 LEDs programmed to flash in the same pattern as males but with various degrees of synchrony, ranging from perfect co-ordination to large phase delays between the lights. Each female in the experiment saw the same light show: a bout of perfectly synchronized flashes, followed by near-synchronous and asynchronous displays. To assess female receptivity, Moiseff and Copeland looked for the typical female “doublet-flash” responses, figuring that females who talked back were probably interested.
The result was that the fireflies were much more likely to respond to perfect or near-perfect synchrony, whereas they almost never responded to the out-of-phase lights. Copeland and Moiseff argue that this evidence goes beyond simply showing that females prefer a well co-ordinated symphony. They point out that in the forest, males are in flight and moving around while they flash. As a result, females have to take in information from a large area in order to recognize the male flash patterns. This sets up the problem of clutter: the larger the area a female attends, the harder it is to sort out the patterns of all of the flashers in her field of view. Synchrony allows males to group together in one area while preserving the ability of females to recognize the signal pattern of their own species.
Another recent study of synchronized courtship in fiddler crabs put robots to work on the question. These small crabs use their claws to communicate. Males have one oversized and often brightly coloured claw that they wave and gesture at females and rival males, and a smaller claw they use for feeding. In several fiddler species, males seem to co-ordinate their claw waves with those of their neighbours, a phenomenon that Pat Backwell and her collaborators demonstrated in 1998 using video recordings of clustering males4. But are the fiddler males helping one another attract females, like fireflies, or is something else going on?
Leeann Reaney, Pat Backwell and colleagues used robotic male crabs and the Australian fiddler Uca mjoebergi to test the idea that this synchrony is more of a competition5. Their reasoning goes like this: if females can be won over by the first male they see, males should compete to be first. According to theory, this kind of escalating battle over timing should eventually end in synchrony6.
Reaney and her coauthors gave females a choice using two pairs of robotic male crabs. As in the firefly study, each virtual male signaled in a realistic way. The experimenters wanted to see how female crabs would respond to the synchrony of the waving robot pairs.
Unlike fireflies, the crabs were not particularly attracted to perfect synchrony. Instead, when given the choice between a synchronous and asynchronous pair, females tended to approach the asynchronous robots. Specifically, they went for the asynchronous leader – effectively the lead signal out of four possible robot choices. This was true regardless of small changes in the length of delay between wave patterns.
These results suggest that the crab behaviour is not co-operative. Fiddler crab synchrony is the result of males jockeying for position to be the first one a female sees. On the beach this precedence effect is not always obvious, since the crabs have reached a sort of tie or timing equilibrium. It took the ingenious use of robots to uncover the phenomenon. Reaney and coauthors suggest that there may be a simple biological constraint that causes females to fixate on lead males, such as a limit to how quickly they can perceive and process each claw wave.
Animal synchronies occur in many other contexts. The communal banded mongoose has synchronous birthdays, with over half of the females in a colony giving birth in the same night. Like jaywalking undergrads, this synchrony provides the mongoose pups with some protection, since pups born before the mass birthday are more likely to succumb to infanticide whereas those born later often have a hard time competing with older littermates for food7.
I suspect courtship synchrony has other functions as well, beyond the ones established in fireflies and fiddler crabs. For example, the male birds of many species of manakin will dance in tightly co-ordinated groups to impress females. In the long-tailed manakin, males pair up to put on a choreographed show involving a sequence of calls, leaps and bounds on their dancing perch. Two males might be better than one at bringing in the audience, but what if synchrony itself is the message? Maybe females are judging males based on their ability to dance in time with one another. In birds that can live as long as 10-15 years, this could be a good indication of experience and the number of years spent practicing, and perhaps of male quality as well. If history is any guide, inventive behaviourists will surely come up with more ways to demonstrate the benefits of animal synchrony.
- Greenfield. 1994. Annual Review of Ecology and Systematics. 25: 97-126.
- Otte. 1974. Annual Review of Ecology and Systematics. 5: 385-417.
- Moiseff and Copeland. 2010. Science. 329: 181.
- Backwell et al. 1998. Nature 391: 31-32.
- Reaney et al. 2008. Current Biology 18: 62-63.
- Greenfield and Roizen. 1993. Nature 364: 618-620.
- Hodge et al. 2010. Biology Letters.