Heterosis is the term for what we laymen usually call "hybrid vigor."
In brief, it's the generalized positive effect on growth, healthy, longevity, fertility, immunity and general thriftiness that one sees in an animal or plant whose parents were unrelated. It is the effect of the animal being heterozygous for most of the variable alleles in its genome; this is a general effect, and not the same as just "avoiding deleterious recessives" by the practice of inbreed and purge.
Despite the protestations of ornamental animal breeders seeking that perfect feather crest or flawless dime-sized spots, the biological reality of heterosis is not in question. Those who scoff at it --
I knew a mutt that died at age five of cancer! -- are in the company of those who disbelieve the biological reality of evolution via natural selection because
those wicked scientists are always arguing with each other, so obviously they don't really agree except when they are feeding us Good Christians into the maw of the savage Darwin.In fact, a robust manipulation of heterosis is the basis for almost all modern industrial agriculture.
Now, an ordinary animal breeder* can utilize the power of heterosis in both purebred populations and in crossbreeding. The least troublesome way (to registries and others who fret about purity) is to simply select breeding partners that are less related than the average within the "pure" registered population. Another way, sanctioned by many livestock registries, is to "grade up" mixed breed herds by utilizing purebred sires for a number of generations (3-7, depending on the breed/species), after which the youngest generation is considered "purebred." Yet another way is to simply crossbreed, selecting for production characteristics and ignoring fancy points.
When you are eating a steak, do you really care whether the steer it came from had a white face or was all black?
Virtually no dog registries have actual grading up programs**, and it is anathema to the kennel clubs that exist to perpetuate dog pageants. But the recent lemming-run popularity of "designer breeds" and the historic practice of crossing for performance (e.g. for lurchers or Alaska huskies for race teams) are both, in part, attempts to capitalize on heterosis.
On an industrial scale, the use of heterosis is much more sophisticated.
Highly inbred parent strains of animals are closely-guarded and coddled corporate assets. In poultry, male strains (all females born are culled) and female strains (converse) are crossed to produce the production animals. Production animals generally have truly impressive economies; they will not breed true, so the farmer must continue to pay the corporate overlords for grow-out stock. And because of the extreme selection practiced on the parent strains, the effects of heterosis are more targetted than that provided by Nature when two
normal unrelated animals reproduce.
Here are three hybrid birds, on their four-week birthday:
They were all hatched within a day of one another, and are all commercially-bred from highly selected parent strains. They are all representative of their respective genetics, brooded together, and all well-fed.
The chick at left is a golden comet pullet. The golden comet is a hybrid cross specialized for egg-laying. It is one of the varieties known as a "sex-link," which has the great virtue of segregating by color along gender lines. It is easy to tell the cockerels from the pullets among newly-hatched chicks -- females hatch out a reddish tint, males like yellow marshmallow peeps. They are a cross of silver-factored white rock hens and New Hampshire red roosters, and are produced mostly for the private small flock owner.
If I wanted to breed my own golden comets, I could lay my hands on the purebred parent stock of the appropriate breeds and start makin' em. Comets have normal lifespans, normal immune response, normal growth rates, high egg production, early maturity, and hardiness.
The chick in the center is one of the 100+ colored range broilers, or "Freedom Rangers," that I'm raising for meat this year. I know virtually nothing about the parent strains, which would certainly qualify as commercial, without perhaps crossing the threshold into "industrial." These birds are engineered for medium-fast growth (slaughter weight in 9-12 weeks), good feed-conversion rations, and the ability to thrive when raised on pasture -- outdoors, unheated, uncooled, and with natural green things as a major part of their diet.
Some people keep a few of these hybrids past the normal slaughter age, adding them to their regular backyard flocks of laying hens and hobby birds. And they do fine. They live normal lifespans, and the hens are decent layers of very large eggs.
I did not weigh the chicks, but the range broiler is easily twice the body mass of the same age golden comet. Today we had one of the range broilers die in a freak neck-snapping accident, and we heard the commotion and found him right after he died. I dressed him out to eat. Whole -- with blood, feathers, head, feet and corn-stuffed innards -- he weighed 2 pounds, 4 ounces at 4 1/2 weeks. Dressed, 1 pound 6 ounces, which is exactly the weight of a supermarket "cornish game hen." A small meal for two people, but a meal. One of the comets wouldn't even be a mouthful.
The chick on the right is a Cornish x white rock industrial broiler. Every chicken you buy in a grocery store is one of these. I got four of them at the feed store in order to compare them to the range broilers. These are generally called Cornish crosses, or just Cornish x.
The front view photo doesn't really fairly portray the difference in size between the ranger and the Cornish x. The ranger is very slightly forward of the Cornish, which skews the perspective, and he's also standing up straight, so he looks bigger. But here's a top view of the same three chicks:
All four of the Cornish x chicks are named, but they all have the same name: Eric Cartman. Ken thinks they have passed the "big-boned" stage long since, and are now Chickie-the-Hut.
The naked parts are not because he's got some horrible feather-loss disease or parasites or is being picked. None of the above. His feathers are just fine -- it's just that he's growing so fast, they can't come in fast enough to cover him. You'd think he'd want a little chicken sweater, but in fact, he's often too warm in a stall that is cool for the other chicks. The Cartmans feel like squishy oversized hot potatoes when I pick them up. At the county fairs in late summer, one can often see the regular chickens, including large and very fluffy ones, looking perfectly comfortable, while the Cornish x entered in the 4H "meat pen" competition are nearly expiring from the heat.
He's at least 2/3 larger than the range broiler chick, has possibly three times as much flesh in his breast, and
check out those feet. Notice that in the top view he is lying down. It was the very devil to get him to stand for the front view photo. Despite having feet and legs like Greek pillars, Cornish x have a hard time standing much, and do a lot of lying around.
A "Cornish game hen" is nothing other than this bird, slaughtered at this age.
This bird is designed to live 6-9 weeks. There is a decent probability that he could drop dead of congestive heart failure before reaching that milestone. Or he could "go off his feet" and be unable to lift his own weight, making it necessary to cull him. To slow down the growth a little, I follow the common practice of restricting feed from the Cartmans; at night, these four little piggies go into a dog crate, where they are kept away from the food for 12 hours. (It was the door of this crate that killed the range broiler today; I've fixed the issue.) If you don't restrict feed when raising Cornish x, you are much more likely to have a lot of birds go DRT before their time. The woman I bought my goats from had two Cornish chicks that she was restricting even more drastically, feeding only twice a day; they looked like real chickens to me. Their feathers nearly covered their whole bodies.
If someone keeps a Cornish x past about 11-12 weeks of age, the probability that it will go off its feet or go spurs up for no apparent reason in the next couple months approaches 100. If the bird doesn't actually die that soon, it will suffer quite a bit from the imbalance between its body mass and its skeleton and circulatory system. This is the messy fate of too many Easter chicks -- Cornish x are cheap to buy, and are cute, fat, and yellow as day-old peeps.
This chick is the offspring of a male from a closely-guarded, highly inbred proprietary sire strain, and a female from a different, closely-guarded, highly inbred proprietary dam strain. These strains are quite different from the Cornish and Plymouth Rock breed chickens that are available to regular farmers and hobbyists. The breeding stock do not gain weight the way the hybrid offspring do, but they still must be kept under a regimen of severe feed restriction if they are to be kept alive long enough to breed.
Farmers who produce chickens for roasting and broiling buy Cornish x chicks from enormous hatcheries. If a farmer raises a standard breed or a DIY hybrid instead of buying Cornish x, she'll be at a significant competitive disadvantage, as they will take much longer to grow to a smaller size eating more feed. There's no practical way to create one's own hybrids on a small scale. Their ubiquity is an unstoppable economic force; they are the Microsoft of feathers.
As grotesque as the Cartmans are, and as much as I loathe the industrial "proprietary" nature of their origins, I do see the point in a bird that goes from egg to freezer in only two months. The labor savings of such a bird are significant. And their feed-conversion ratio is nothing short of astounding, with industrially-raised birds turning 2.5 pounds of feed into a pound of flesh. I could certainly see raising a batch of them in the Fall, say, in some short nook of time before weather got bad and we just needed some more chicken in the freezer. But I'd feel guilty about them; not about slaughtering them, but about keeping them alive.
I'm fascinated with these three ways of exploiting heterosis. But the Cornish cross chickens show that it's not an unalloyed blessing for the animal. A "random" mutt -- meaning, a mutt whose parents practiced selection for themselves -- of any species is more likely to be vigorous, long-lived, fertile, disease-resistant, and cancer-free than is an inbred individual of the same species. But human selection for extremes, and human selection for traits that are burdensome or frankly cruel to the animal born with them is not limited to the deranged inbreed and purify practices of fancy breeders. In the hands of those who view animals as widgets, not beings, a fanatically controlled and organized hybridization program presents the same assaults on the welfare of the "genetic material."
*Plants are more complicated to view as a group, since the option of self-pollination in some species and the wide variability of different species
in re heterosis makes generalizations almost invariably invalid. But suffice it to say that there is no benefit to you, the home gardener or consumer, or to the planet, from planting or eating a hybrid tomato -- just plenty of benefit to the industrial seed company. Corn, different story. Fruits, whole 'nother ball of wax.
** The ESCR's "Step-In" program is close, but it is explicitly limited in scope to animals considered "purebred" English shepherds, but from unregistered bloodlines. In a former age, the found of one of the commercial registries that served ES, the Animal Research Foundation, had a program to mix in various other breeds -- including Beauceron and curs --to engineer a dog for his own purposes. The descendants of these mixes are inextricable in the English shepherd gene pool.
