Sunday, September 14, 2008

Indian peacock and peahen Pavo cristatus

I am an amateur naturalist trying to discover something about all the lifeforms in my garden.

No, despite appearances to the contrary I have not abandoned my blog. I hope one or two of you may still be visiting.

Some time ago you'll recall I was pleased and surprised to be visited by a white budgerigar. It seems that the larger exotic birds are not to be outdone however, as around six-thirty one morning in recent July, I was delighted to encounter a couple of Peafowl pecking around on my lawn. I hastily grabbed my camera and achieved the not-very-good photo (left) before they fled over the wall. I assume my birds came from a local farm or some country estate, but I do not know from which.

A few minutes internet searching informed me that my birds are Indian Peafowl (Pavo cristatus), as opposed to the endangered Green Peafowl (Pavo muticas) from Java and Burma. Males of the latter species can be identified by a tall, spiky crest on their heads.

Much the most famous feature of peafowl is of course the peacock's fantastic fan tail (sadly mine didn't display). Ever since Darwin, naturalists have been fascinated to understand what possible evolutionary purpose it can serve to sport such an unwieldy appendage; The benefit of carrying around a large iridesceant sign, seemingly pefect for announcing to predators "Here I am! Eat me!", isn't obvious!

My quest to understand a little more led me to an excellent review paper by one R. Gadakar (here). In brief, my understanding is as follows: As a Peahen, one approach to selecting a mate is to choose those males who most strongly advertise their health and vitality with a large fan. So far so good, but the question this poses is whether the extravagant fan we see today has arisen because at some point in history females selected to breed with males with a slightly longer tail, which led to male-offspring with slightly longer tails, from which later females again selected the longest...i.e. whether today's fan is a result of 'run-away selection' (in which case the fan is beautiful but it is not, per se, an honest indicator of the male's health). Or whether an alternative hypothesis holds i.e. the so-called Zahavi's handicap idea, which states that any male able to carry around such a huge encumbrance and yet still survive (evade predators etc.) must somehow be truly fit and strong i.e. the tail is not only beautiful but also honest indicator of the fitness of the male.

I have failed to think up a simple way to restate this (indeed, as the paper above says, the trend in biology these days is to get away from the vagary of mere words and express things in terms of mathematical models where possible), but a (very incomplete) analogy might be following: some female humans prefer their men well muscled, as a sign of their vitality. For the humble male there are then two routes to getting-the-girl: One, a regime of healthy food and physical exercise. Two, steroids and plastic surgery! The latter may achieve the visible appearance of the 'body beautiful', but it could hardly be called an honest statement of the chap's health and fitness. By contrast, in seeking to achieve the rippling musculature beloved of the ladies, the former approach requires males to accept the 'handicap' of limited diet and punishing gym sessions, but in the long run this 'handcap' is actually likely to positively benefit both their own and their offspring's health.

The existence of this, the (Zahavi's) handicap hypothesis out there in nature however was controversial amongst evolutionary biologists when it was first suggested, and therefore it has been important to find ways to test it. Which brings us back to peacocks: Glass-jawed weaklings beneath steroid-inflated shells, or supermen in both looks and deeds?

Some of the definitive studies of peafowl have been performed on a feral population at Whipsnade Zoo here in the UK Firstly it is known that peahens select those peacocks with the most symmetric tail fans. High symmetry is strongly correlated with a large number of eye spots. And what have the studies at Whipsnade concluded about those males with the highest eye spot count? That they i) have better reserves of body fat ii) have higher survival rates (against loss to foxes etc.) and iii) father healthier, fatter chicks.

In short, no preening primadonna he, the male peacock really is both both beautiful and honest!

8 comments:

Jessica said...

Hurray: you're back!
Keep up the good work,
best wishes
Jessica

Anonymous said...

In a similar vein, in A Reason For Everything (2004), pp 330 - 332, Marek Kohn notes that

"The tints of the decaying leaves in an American forest are described by every one as gorgeous; yet no one supposes that these tints are of the least advantage to the trees.' Charles Darwin, The Descent of Man

One of Bill Hamilton's last bold ideas, published a year after his death, was that the colours of autumn leaves are signals urging insects to go and pick on somebody else. Having previously suggested that the bright colours of birds' plumage might be a signal of good health, indicating the ability to resist parasites, Hamilton conjectured that the same message might be broadcast in autumn leaves. One tree may manage to stage a more spectacular display than another, thereby inducing potential insect parasites to choose its more pallid rival.

This is a deal between parties with opposing interests in which both sides' interests are served. The tree will not have to expend any more resources in resisting attack, and may avoid hazards such as viral infection which might follow in the wake of even brief insect forays. The insects avoid a struggle they may well lose, and are directed towards hosts that they will find easier to colonise. ..........................

Zahavi detected this principle at work in a breathtaking range of animal species, but it fell to Hamilton to discern it in trees. To make the case, he and Brown had to demonstrate that autumn colours might be costly. They noted that much of the pigment that produces the display is lost when the leaves fall. In the Norway maple, for instance, nearly all of the green chlorophyll is reabsorbed as the leaves prepare to be shed, but no more than half of the yellow carotenoid pigment is recovered. The green is recycled; the red and yellow are largely wasted. And some trees synthesise pigments specially for the season.

Hamilton and Brown predicted that the more species of aphids that beset a species of tree, the more gorgeous would be its display. They tested the idea by going through field guides and grading different tree species' autumn colours according to the descriptions given. A tendency to yellow was found to go with a greater range of parasite species, while a tendency to red was associated with threats from aphids which exclusively patronised the species in question. These colours were interpreted as markers of `defensive commitment'.

Trees signal to aphids their commitment; aphids make their choices accordingly. This is what comes of assuming a purpose: reason reinvents a world lost with childhood, in which trees talk and the woods are full of activities that are beyond the scope of human senses. Darwinism is blamed for taking meaning from the world by making divine purpose optional. But Darwinism in much of its practice is a project to populate the world with meaning, by identifying it in as many aspects of life as possible."

(Coincidentally, a report in the Daily Telegraph of 10.09.08 mentions research by Dr Thomas Döring of Imperial College, London, published in Proceedings of the Royal Society: Biological Sciences, confirming that red leaves are less attractive to aphids than green or yellow leaves.)

R2K said...

I am still reading this page. Would you be willing to visit my microscopy blog and give me an opinion on the spiders I have found? I wonder if anyone knows the genus.

SLW said...

I'm back too, Henry, and my what exotic fauna you're discovering! Peafowl... that's one for the record books. I'll be back to absorb more...

Laura said...

Go Henry. The world needs Walloons.

Keri Dearborn said...

Good to check in with you again. Don't be too down on the male in your yard. In the fall, the males drop those long cumbersome tail covert feathers. He'll grow new ones in the spring. Amazing that they regrow these elaborate tail feathers annually. The first time I witnessed this I was very surprised. At the Los Angeles Zoo we find that our males are very visible spring through autumn. But after they loose there fancy feathers they seem to hide out. When they grow back in the spring they will all strut their stuff again. Keri in California

Henry Walloon said...

Jessica, Anonymous, R2K, SLW, Laura and Keri

Thanks all for your encouragement and apologies I've been away for a while. I will redouble my efforts to publish more frequently!

best

samia hussain said...

nice post