Here is a little vignette for you; a taster of the BBSRC-funded chicken biomechanics project my team has underway in collaboration with Jonathan Codd’s team at Uni Manchester. I did not know about the following situation until a couple of years ago during my former PhD student (now postdoc) Heather Paxton‘s research on chicken mechanics.
Regard this chicken, slit open along the midline to show the viscera. The huge pectoralis muscles (breast meat) have been pushed aside; the right side’s are clumsily outlined (I blame caffeine?) in blue.
Then consider the heart, within the jagged, shamefully and ineptly drawn green circle. I’ll come back to that.
So this broiler chicken took 6 weeks to reach this size, of about 3 kilograms (6.6 lbs). Fifty years ago, before artificial selection was imposed on a MASSIVE scale (many billions of chickens per year worldwide, bred in a complex pyramid scheme of crossed strains), that same chicken strain would have taken 15 weeks to reach a normal slaughter mass of roughly 2 kg (4.4 lbs). The major selection, of course, has been for edible meat, especially that lovely breast muscle’s white meat.
If we look at red junglefowl, to a large degree the “wild type” ancestors of domestic chickens that are native to southeast Asia, the leg muscles take up about 7.7% body mass per leg vs. about 6.3% in the broiler. Just a small decrease, but probably an important one, and something our research focuses a lot on (walking ability, lameness, activity levels etc). But that’s a subject for a future post. In stark contrast, the breast muscles (back to the blue ellipse above) have gone from 7% to up to 11.6% body mass per wing; a huge change!
Now let’s return to another large muscle, the one within the green circle above; the heart. Not only must the heart, which has become relatively larger by perhaps 25%, pump blood to a body that has enlarged by >50%, but it also must perfuse the giant pectoral muscles, which have enlarged by >65%.
Herein lies the problem… You probably can predict what happens.
Several syndromes may develop, but the one I want to cover here is called deep pectoral myopathy (AKA “Oregon disease” or better yet “green muscle disease”, a very appropriate term as you’ll see below). Basically, the giant pectoralis muscles receive inadequate blood flow from the smallish heart, because the muscles are so big and under so much pressure, creating resistance to flow, and so the muscles begin dying from within. A picture tells the story:
While surely uncomfortable for the birds and hence a welfare problem, it is usually not found until the animals are slaughtered, and then of course the meat is destroyed rather than delivered for human consumption. Because of the welfare problems and loss of meat (i.e. financial loss), the poultry industry is trying to remedy this problem. W’e're working on aspects of this as well, as part of our study of how the locomotor and ventilatory systems of chickens develop and have evolved.
I am blogging this as a great example of how anatomy can go haywire and become imbalanced when evolutionary selection pressures are intense and highly specific (e.g. almost single-minded human selection for large breast muscle). It is also a conundrum that human society faces: while chicken meat seems more efficient and more ecologically sound than some other meats, and there is growing demand for meat as the human population grows, how do we balance welfare concerns with food security, economics and other factors? And how do we judge when artificial selection has gone too far? I do not present an answer because the answer is not easy, and because my team is still learning about how to answer it.