Gallus gallus domesticus

Chickens. The UN's Food and Agriculture Organization estimated that in 1990, the world's population of chickens was around 10.6 billion birds. For 2023, that number has grown to some 34.4 billion animals. And that figure only accounts for animals alive at any given moment. It's worth remembering that a typical "broiler" (a breed raised for meat) is slaughtered about 42 days after hatching. So, for example, while it was estimated in 2020 that there were 1.2 billion broiler chickens alive in the U.S. at any one time, over 9.2 billion were raised that year. Unlike broilers, egg-laying hens can live 6-8 years, but they only produce eggs for about 3-4 years. That's because females are born with all the eggs they'll ever have already stored in their (single) ovary. When those eggs run out, that's it. In 2020, about 374 million were alive in the U.S. at any one time. USDA data suggests the average American consumed 97.7 pounds of chicken meat and 279 eggs during 2022, with 285 projected for 2023. And this in the wake of the avian flu outbreak last December that sent the price for a dozen eggs in January soaring to $4.82. There's a lot we could talk about here: ethical and environmental implications of commercial production methods, shifting medical opinions on health issues associated with eating eggs, intriguing insights into chickens' evolution, behavior, and intelligence… But for today, let's just consider one question: Why do laying hens go to all the expense of producing eggs even when there's no rooster around to fertilize them? You'd think, from the chicken's perspective, that would seem a massive waste of time and energy, with no baby chicks as a reward for her effort. As usual, we'll need some background biology before getting to the answer. First, that white or brown-shelled egg sitting in a carton in the fridge—it's not an egg. Rather, it's the yolk inside the shell that's the actual egg, a female reproductive cell. As noted above, a baby female chick hatches out of its shell with all the ova (egg cells) it will ever have already in its ovary. But at this stage, they're all very tiny. Most hens start laying when they reach 18 to 22 weeks old. At that time usually one (but sometimes two) of the tiny egg cells will start to swell with fats, proteins, cholesterol, vitamins, and minerals in a process that takes about a half-hour. When the now-enlarged egg cell is released from the hen's ovary, it moves into the first part of the oviduct. If the egg is to be fertilized by a rooster's sperm, it has to happen here, before any coatings are layered over the yolk. The yolk takes about 3 hours to pass through the next part of the oviduct (the magnum) where the so-called "egg white" or albumen (containing water and more nutrients) is layered around it. In the last part of the oviduct (the isthmus) two thin egg membranes are laid over the albumen in a process taking an hour or two. You can see these when cracking into one end of a hard-boiled egg. One membrane adheres to the shell and the other to the egg white. The last stage, taking some 20-21 hours, occurs in the hen's uterus (just after the oviduct) where the calcium-rich egg shell is added on. In the final few minutes before laying her egg, a thin protective coating called the "bloom" is placed over the shell. Its job is to protect the egg from bacteria and keep its insides moist. The bloom is removed when eggs are washed in a typical commercial operation, which is why we need to refrigerate most store-bought eggs. If the eggs aren't washed, this is unnecessary. The whole process takes 24-26 hours to produce one egg, and a healthy hen bred for laying (like a Rhode Island Red, Plymouth Rock or White Leghorn) can lay 200-300 eggs/year. So, again, why go to all that trouble just to lay a lot of unfertilized eggs? Recall that the egg cell can only be fertilized shortly after being released from the ovary, before the albumen, egg membranes and shell are layered over it. But the female can't know in advance if there will be a rooster present to do the job, so its body just keeps producing eggs until her ovary empties out. The chicken's direct ancestor (still alive in many tropical and subtropical areas) is the Red junglefowl. In the wild, this reproductive system works well because roosters make sure a high percentage of a hen's eggs will be fertilized. Humans took advantage of this quirk in the junglefowl's reproductive system when we began domesticating them about 8,000 years ago and, over many generations, selected some to be super egg-layers and others to be fast-growing broilers. What we know as, Chickens.