Thursday, June 22, 2006
Hot heads and farmyard galliforms
Yesterday Will and I spent time on the farm. I scratched lots of pigs and got chewed by a goose, but the things that interested me most were the galliforms. Like my Marwell Zoo posts (go here and here), this isn't going to be one of my standard lengthy essay-type posts: more a collection of pictures, accompanied by a tiny bit of information*.
We start with Helmeted guineafowl Numida meleagris (photo at left). Guineafowl are terrestrial foragers that eat a lot of arthopods, and they've been shown to be important controllers of ticks in areas where tick numbers are high (Duffy et al. 1992). This can make them an important weapon in the fight against Lyme disease and other tick-borne diseases. What's with the cranial crest (or casque)? Conventionally assumed to be used in display and species recognition, it may in fact play an important role in keeping the brain cool, though I don't have the relevant paper to hand and can't be bothered to go dig out the reference.
* And I don't need to tell you that I wrote that before completing the post.
I spent far more time looking at turkeys Meleagris gallopavo. Male turkeys - called stags - famously display a distensible frontal process, or snood, fleshy polyp-like lumps called caruncles, and a dewlap that extends from the lower jaw to the neck. The structures are enlarged and flushed red in display, and the snood visibly extends in an excited bird. While displaying to a female or intimidating a rival a stag raises its body feathers. They will also drag their rectrices along the ground, wearing the tips off and creating a distinctive 'shuff-shuff' noise. I'm interested in those cases where birds modify their plumage by way of behaviour, and this is a great example. Rooks Corvus frugilegus and motmots (Momotidae*) are other good examples.
* Not a typo.
It figures that the caruncles, snood and wattle have evolved as display structures. Indeed some studies (and again I don't have the references to hand) indicate that females prefer stags with the broadest heads (and thus the broadest-based snoods) and Buchholz (1995) linked the appearance of the structures with parasite loads (and thus fitness).
But, like the crest of the guineafowl, could these structures have a thermoregulatory role as well? If so, it's a negative effect, with the naked skin causing the birds to overheat at times. Males seem to suffer more from heat stress than females do: they hide in the shade on hot days more than females do, and also pant more and are less keen to flee from people on sunny days than females are. It's also been shown that exposure to direct sunlight for extended periods can reduce male fertility by about 10% in domestic bronze turkeys, so sitting in the sun on a hot day is clearly not a good idea if you're a turkey. Buchholz (1996) looked at this area but noted that more study of wild turkeys was needed.
Finally, we come to chickens Gallus gallus. With my mind on naked display structures (if you'll pardon the expression) and thermoregulation, I looked at wattles and combs anew. Might the combs and wattles of chickens, which can often be proportionally large, also have a role in temperature regulation? A positive one or a negative one?
Hens were traditionally thought to prefer those cocks that are socially dominant and in possession of large combs, and several studies provide support for this. I'm interested however in Leonard & Zanette's (1998) discovery that hens prefer males that indulge in lots of wing flapping: an audible behaviour that cocks perform when courting or intimidating other males. A high rate of wing flapping presumably indicates fitness, and cocks perform it more when sexually frustrated, apparently. Interestingly, this mate selection mechanism differs from that of Red junglefowl G. g. murghi where, according to some studies, females chose mates depending on comb size and other morphological features.
But if wing flapping is so important, and if (according to some studies) comb and wattle size isn't, why possess the big comb and wattle at all? Might they be exapted for a role in thermoregulation (having originally evolved for display)? But might they actually help cocks to shed heat (like the casque of the Helmeted guineafowl), rather than allow overheating as do the display structures of turkeys? I don't know, and again I must confess to throwing stuff out there without checking the literature first. Naughty me.
Tomorrow we submit the post-review version of the British dinosaurs MS.
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Refs - -
Buchholz, R. 1995. Female choice, parasite load and male ornamentation in the wild Turkey. Animal Behaviour 50, 929-943.
- . 1996. Thermoregulatory role of the unfeathered head and neck in male wild turkeys. The Auk 113, 310-318.
Duffy, D. C., Downer, R. & Brinkley, C. 1992. The effectiveness of Helmeted guineafowl in the control of the deer tick, the vector of Lyme disease. Wilson Bulletin 104, 342-345.
Leonard, M. L. & Zanette, L. 1998. Female mate choice and male behaviour in domestic fowl. Animal Behaviour 56, 1099-1105.