HERE鈥橲 something not to think about as you tuck into your holiday fare. A hulking turkey tom, too massive to enjoy a more natural experience, is having his abdomen expertly massaged. He soon gives up his prize: a small sample of milky-grey semen, which is added to a pool of similar offerings from other breeder toms and taken to the long row of turkey hens. Each hen is deftly upended. In goes the semen. Out, rather later, come eggs, which duly make chicks and eventually the scrumptious, sizzling slices you鈥檙e about to douse in gravy and spear onto your fork.
The wild turkey is a fleet-footed, slimline forest creature that passes its life gobbling (and also 鈥測elping鈥�, 鈥減utting鈥� and 鈥減urring鈥�, for those who thought 鈥淕obble!鈥� was all turkeys said), roosting in trees, grubbing for acorns and beetles, and occasionally attacking rural postal workers. Its black, brown and cream feathers afford it good camouflage. Its reproductive output is modest-just 12 eggs each spring (12 more, perhaps, if it loses the first clutch). A full-grown male weighs 10 kilograms at most. Ample, but hardly ostentatious. As for collecting semen: go ahead, just try it.
Compare that with man鈥檚 creation-the snow-white, 30-kilogram domestic turkey tom, heaving his ballast of snow-white breast meat before him as he waddles about as best he can. Or the domestic turkey hen-slimmer, yes, but able to lay a cool 120 eggs in a 27-week reproductive marathon. How did this strange bird come to be, and where is modern science taking it?
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The turkey鈥檚 origins are shadowy. Centuries ago, Native Americans domesticated it. The Spaniards took it to Europe. Settlers brought it back to America again. Despite this convoluted history, it was not until the 1950s that the birds were bred for ivory feathers (Western consumers prefer a breast devoid of black, hairy, down remnants), and even more recently that serious breeding for bulk began.
Unwitting selection
There was a time when breeding focused on size and speedy development, and precious little on the growth of the bones that support all that heft, resulting in bow-legged birds that were barely able to walk. But Karl Nestor, a University of Wisconsin turkey geneticist with 38 years of breeding under his belt, showed it didn鈥檛 have to be that way. Nestor selected not just for bulk but for wide, sturdy leg bones and walking ability, measured somewhat subjectively on a scale of 1 to 5. Professional breeders have done likewise, and today鈥檚 turkeys, though hardly graceful, have a better gait than their forebears.
Classical breeding requires that in each generation you select the creatures with the most desirable characteristics to produce the next generation. You don鈥檛 need to know anything about the genes you鈥檙e selecting, or the physiology that you鈥檙e changing. So does anyone know why turkeys now grow so big?
The best insights come from chickens, not turkeys, but the principles are probably the same. Evolutionary biologist Jared Diamond of the University of California in Los Angeles and his colleague Sue Jackson were so interested in knowing how the broiler chicken developed, they spent long hours weighing and measuring guts and other body parts, and compared these measures with those of the chicken鈥檚 ancestor, the svelte, fleet-of-wing wild jungle fowl.
The brain, they found, is smaller in the broiler. It makes sense. You don鈥檛 need to be astute and alert if you鈥檙e cooped up in a pen all your life, and brains are energetically expensive. Legs are thinner and lighter in the broiler, too, which also makes sense. Sturdy legs aren鈥檛 important if you鈥檙e not doing much moving. Thus, without knowing what they were doing, breeders selecting for bulk chose animals in which energy was shunted away from 鈥渦nimportant鈥� body parts towards 鈥渋mportant鈥� ones-namely, the meat.
And that鈥檚 not all. Broilers consume lots more feed-there鈥檚 a good chance that greediness has been inadvertently selected for. The guts have risen to this challenge. Gram for gram, the broiler鈥檚 guts absorb nutrients such as glucose and amino acids less efficiently, not more. But the bird still absorbs food more effectively, as its small intestine is nearly three times more massive than that of the jungle fowl. Selecting for bulk, breeders unwittingly selected for big, fat digestive organs.
Like chickens, domestic turkeys have smaller brains than their wild relatives; these are, after all, creatures that can drown themselves by staring up too long at the rain. And their guts are more massive, says James Croom of North Carolina State University in Raleigh. Still, turkeys aren鈥檛 really like chickens: they haven鈥檛 been intensively bred for as long, so they retain more of their wild habits. They herd. They gobble. And turkey hens, unlike commercial egg-laying chickens, go broody.
鈥淏roody鈥� means that mothers become motherly, a real headache for breeders. It isn鈥檛 merely that the hen will hiss and peck when anyone tries to take her eggs. Her ovaries can regress and she鈥檒l stop laying altogether. Small-scale farmers nipped broodiness in the bud by putting problem hens in rooms filled with rocks, or out in the cold-anything to help them forget the nest. Try paying that kind of attention to a modern-day flock of 25 000.
Luckily, endocrinologist Mohamed El Halawani of the University of Minnesota in St Paul has come up with a novel tool: an anti-broodiness vaccine.
Endless tricks
His strategy is clever but logical. Mothering behaviour is promoted by the hormone prolactin, made in the pituitary gland when the turkey hen touches her eggs. Stop prolactin release, reasoned El Halawani, and mothering behaviour would be prevented too. He achieved this by injecting hens, a few weeks before laying, with another brain protein called vasoactive intestinal peptide, needed to trigger prolactin production. The birds make antibodies to this VIP, and these antibodies, when bound to the bird鈥檚 own VIP, inactivate it. Thus, no prolactin-and no broodiness. The hens just keep laying and laying and laying.
Efforts, meanwhile, are being made to ensure that every turkey tom who gives of his semen gets a fair crack at contributing to the next generation. The problem with pooling turkey ejaculate is that some toms, no matter how wondrous their traits, rarely fertilise an egg. Sperm from certain super-males wins out almost every time. Today, with the help of a turkey sperm motility test, breeders can tell just which bird鈥檚 sperm does what. Toms with super-fast sperm can be identified, and toms with wonderful traits but wimpy sperm can be given more exclusive treatment.
There鈥檚 no end to the tricks that scientists are trying. Breeding continues, for bulk and fecundity and also for such qualities as better feed conversion and disease resistance. Today鈥檚 birds are less likely to make you sick, because they can be sprayed with harmless bacteria that take up residence in their gut, inhibiting growth of Salmonella, and helping to prevent food poisoning. And there are edible films that can be sprayed on the carcass to kill such nasty bugs.
So Merry Christmas! Tuck in! As those in the business say, 鈥淢ay 1999 bring prosperity to you and the turkey industry.鈥�