Furious about eyespotsRoslyn | April 25, 2011
I think I flubbed an interview this week. My supervisor Bob and I just published a paper that is getting some press, because it addresses a recent controversy about the peacock’s train1. Eager for the interview with Nature News, I wasn’t exactly prepared with good lines for the reporter to go on – and I wonder if that’s why he had to pump up our story as a “furious debate”2.
In truth, most of the “debate” played out in a flurry of news articles back in 2008. That was when Mariko Takahashi and her colleagues in Tokyo and Kanagawa published the fruits of their exhaustive 7-year study of the peafowl at the Izu Cactus Park in Shizuoka, Japan3. I’ve never met Takahashi, although I did meet her supervisor and one other player in this story at a conference back then, and all were quite friendly. But the title of Takahashi’s 2008 paper, “Peahens do not prefer peacocks with more elaborate trains” was a direct jab at an earlier one, “Peahens prefer peacocks with more elaborate trains”, by Marion Petrie in the UK4. Takahashi and her coauthors had the difficult task of proving a negative – and they did it pretty convincingly, with the aid of a much more extensive data set than anyone had gathered before with this species. The upshot? For a peacock in Japan, having a bigger train ornament doesn’t necessarily win you any favours with the ladies.
Bigger in terms of the number of eyespots visible in the ornament during courtship, that is; males have about 150 on average, each on the end of a single feather. The results of the Japanese study were in direct contradiction to Marion Petrie’s earlier work as well as some recent studies of peafowl in France suggesting that eyespot number is often correlated with male mating success4,5. What’s more, in the 1990s Petrie had confirmed the causal effect of eyespots by showing that you could alter a male’s fate just by removing about 20 of them6.
Taken at the Los Angeles Arboretum in 2009. Photo by Roslyn Dakin.
The Japanese team proposed a rather bold new hypothesis. Perhaps the cumbersome, ridiculous train ornament is obsolete – a relic of sexual selection past, no longer used by females in quite the same way as it was when it first evolved3.
This was taken up with gusto by the news media. Check out the headlines: “Peacock feathers: That’s so last year”, “Have peacock tails lost their sexual allure?”, “Peacock feathers fail to impress the ladies”. Amusingly, this last article was also published with the title, “Female peacocks not impressed by male feathers” by Discovery News7-10. Males could probably be forgiven for striking out with those elusive female peacocks, since they don’t actually exist.
Headlines aside, Takahashi’s interpretation is somewhat of a concern. Here’s why: creationists picked up on this story too11.
I’ve written about this before – some “cdesign proponentsists” are pretty into peacocks, no doubt because these birds are closely tied to Darwin in the story of his development of sexual selection theory. The result was that Takahashi’s study was cited as an example of Darwin’s theory falsified: “This means that numerous textbooks and web sites need to be revised”11. Never mind all the other research by Marion Petrie, Adeline Loyau and others showing that messing with peacock eyespots can affect females in a lot of ways: whether or not they will mate with a male, the amount of resources they subsequently invest in their eggs, even the sex ratio of the brood of chicks produced as a result6,12-13.
The general defense has been to tear the Takahashi paper apart14-15. I’ll admit, the first time I read the study, I struggled to find something wrong with it too. I had just finished my first field season repeating Marion Petrie’s eyespot removal experiment, so I knew for a fact that cutting off a bunch of eyespots decreases male mating success. I had seen it happen. But at this point, I only had data from a small number of natural-looking males. Over the next few months of 2008, I accumulated more from my first season in California, and I started to realize something. Natural eyespot number wasn’t correlated with mating success at my field sites, just as the Japanese group had reported – but this didn’t necessarily contradict the experimental effect that I’d seen in 2007. The reason is that it all depends on the range in question. In the birds I’ve studied, and in the Japanese population, there just aren’t that many males at the low end for eyespot number walking around. In other words, you tend not to see birds that look like they’re missing a whack of train feathers by natural causes.
Although the females in our study did in fact discriminate against males with very few eyespots, there aren’t enough of them to detect this effect without experimental manipulation in the North American populations1. I wouldn’t be surprised if something similar is going on in Japan. Of course, we still don’t know what goes on in the wild, but we showed how this works in feral populations by combining data from several previous studies in the UK and France with our North American studies1. On the whole, it looks like the relationship between eyespot number and mating success can be described as a threshold function – either you’ve got it or you don’t – but there is no relationship between these two variables when you break it down to only the high or low end males.
My problem with the “debate” is many of the criticisms attack the methods used in Takahashi’s study, often incorrectly. The most obvious quibble? The sample size was too small. Even though they had seen a couple hundred copulations over the course of 7 years, it was all just “the same small population”14. Or, maybe there wasn’t enough skew in mating success to see what was really going on – or the Japanese birds were too inbred, causing them to lack sufficient variation in eyespot number to detect an effect14-15. Maybe the Japanese females are just copying each other and not truly choosing based on male traits14. And if they are, the Japanese researchers might have failed to detect it because they used different observation methods – so the results of these different studies simply cannot be compared15.
All legitimate concerns, but they don’t hold up in this case. First of all, the size of the Japanese population at 70-100 birds is on par with previous studies. Why is this enough to detect an effect in some populations but not others? I would argue that the fact that this was a long-term study where the same observation methods were used consistently in “the same small population” can only add to our understanding of how mate choice works in this species.
Here’s another: over at the Panda’s Thumb, Dave Wisker wrote that the Japanese group did not observe any copulations, and that they merely estimated male mating success based how often females visited different males15. He’s wrong – they measured both – and they went further than previous studies by analysing actual mating data as well as female visits to be sure they weren’t missing anything3.
Wisker and a commenter on the Nature News story also mistakenly confuse variation among males in eyespot number with genetic variation for the development of this trait. In actual fact, we have no idea how genes contribute to differences in eyespot number in any of these studies. In our new paper, though, we showed that feral males steadily lose eyespots throughout the season – suggesting that the variation we saw had more to do with feather loss than growth and development1. This could be completely due to environmental causes. Or it could be due to genes that interact with the process of maintaining the large train ornament over the season. For the purpose of the mate choice question, it doesn’t matter: females discriminate against low eyespot number males – just not very often in some places.
A lot of these criticisms hinge on eyespot number interacting with a difference between study sites. Say, for instance, that Takahashi and I failed to find a positive result because female mate choice copying obscured the effect of eyespot number14. For this to work, the copiers at our field sites would have to be preferentially copying females who preferred males at the low end of the natural range – which still needs some explanation. The copying objection is really just another way of saying that maybe the females in different places are doing things differently; on average, copying should just amplify whichever preferences are already there. Perhaps females in different places do vary in their taste in males, and I certainly agree that Takahashi and her coauthors should have emphasized this as an explanation for their 2008 results3. It is at least as likely, if not more so, than their obsolete hypothesis. But we’ve now shown that this needn’t be the case: the major difference is probably just the range of males available for choosing1.
Overall, I like the Nature News story. It provides a nice summary of the background, except for a few minor details. If you’re worried about the males I messed with for the experiment, don’t be. No train feathers were plucked in the name of science (I used scissors to clip them, no more painful than a haircut). On a related note, don’t feel bad for the males suffering decreased mating success, either, since in any given year most peacocks don’t mate at all.
The article ends with a nice quote from Marion Petrie, saying that eyespot number probably means more to scientists than it does to peahens2. In other words, we can only measure things based on our perceptually biased interpretations of them, and hope that eventually, we’ll get closer to what’s truly going on. Then Petrie delivers the bad news: “At the end of the day, we will never know what peahens are looking at and how they select their mates. You can’t ask them.”2
Granted, I’ve often wished I could ask them, especially when they’re slinking around on the leks looking completely uninterested. But I disagree with her interpretation. The ability to verbalize why you think you made some decision doesn’t necessarily bring us any closer to understanding the cognition that was actually involved. If it was that easy, we’d have a really good handle on the human mind by now. Fortunately, though, we can work out the details of perception and decision making by studying the mechanisms and behaviours involved – it just takes a whole lot of work in the field and in the lab. Otherwise, what would be the point of any of this research, including Petrie’s?
- Dakin, R. and Montgomerie, R. 2011. Animal Behaviour. In press.
- Callaway, C. 2011. Nature News. 18 April 2011.
- Takahashi et al. 2008. Animal Behaviour 75: 1209-1219.
- Petrie, M. et al. 1991. Animal Behaviour 41: 323-331.
- Loyau et al. 2005. Ethology 111: 810-820.
- Petrie, M. and Halliday, T. 1994. Behavioral Ecology and Sociobiology 35: 213-217.
- Morell, V. 2008. Science NOW. 31 March 2008.
- Barras, C. 2008. New Scientist. 27 March 2008.
- Viegas, J. 2008. ABC Science. 27 March 2008.
- Viegas, J. 2008. Discovery News. 27 March 2008.
- Tyler, D. 2008. Science Literature. 01 April 2008.
- Loyau et al. 2007. Behavioral Ecology 18: 1123-1131.
- Pike, T. W. and Petrie, M. 2005. Biology Letters 1: 204-207.
- Loyau et al. 2008. Animal Behaviour 76: e5-e9.
- Wisker, 2008. D. Panda’s Thumb. 18 October 2008.