Category Archives: Just-so

Why Do Hens Lay Unfertilized Eggs?

(a Just-So Story)

[ I wandered down this line of thought just after finishing Richard Dawkins' excellent book, "The Greatest Show on Earth". Be warned that I am not an expert; I merely pretend to be one on this blog. Poetic license has been taken; speculations are far from scientifically proven.]

I’m talking about supermarket variety eggs, which don’t hatch in spite of several childhood hours spent with a lightbulb incubator. Why do hens lay them, indeed? An egg requires large quantities of protein and valuable calcium. A fertile egg is worth the trouble – after all, the hen’s primary Purpose in life is to replicate via eggs. But if the hen has not mated, it should be easy enough to avoid this costly expense. The primary trigger for egg-laying appears to be long days – signifying spring – rather than successful mating or presence of sperm. Why did natural selection not penalize such a glaring and expensive mistake and evolve something sensible?

Sunny side up

No tiny chicken here… whew!

Could it be due to artificial, human selection? Perhaps some enterprising man from long ago found a freak jungle fowl which laid without getting laid and assiduously nurtured that bloodline. After all, we have done so with other freaks like seedless bananas, so why not an embryo-less chicken egg?

[ Aside: There do exist seeded bananas. And some people consider baluts (fertilized eggs) a delicacy. I urge you not to click on those links as they cannot be unseen. I still have nightmares of seeded bananas... ]

Man is off the hook for this one, however. Chicken farmers do exploit the egg-laying trigger by faking spring and turning on the lights early, so their hens lay in all seasons. They have bred for egg qualities, but they’re not responsible for the unfertilized egg laying habit itself. Other species, both near and far, exhibit the same phenomenon. Among humans, it takes the form of menstruation – expelling the unfertilized egg and a goodish bit of nutrients which would have gone to nourish the potential embryo. Yes, the unfertilized chicken egg is directly equivalent to menstrual effluvium in humans. (I’m sorry, did you have an omelette for breakfast?)

The evolutionary design process is very counter-intuitive, so let’s remind ourselves of some of its peculiarities.

Firstly, evolutionary design is blind and lazy to the point of utter stupidity, at least as viewed by a human engineer. For example, a standard oxygen-nitrogen mix is assumed for any gas entering the lungs. In above-ground mammals, the trigger for breathing is based not on depleted blood-oxygen levels, but on rising blood-CO2 levels, which sounds downright loony. Usually, falling blood-O2 and rising blood-CO2 are strongly correlated. But in some cases, like a roomful of nitrogen, both blood-O2 and blood-CO2 levels drop. So if you’re curled up with a book in a room and a sinister hand floods it with pure nitrogen, you won’t turn a hair; you will keep reading until you keel over and die.

Since roomfuls of nitrogen didn’t normally exist in nature, we got by without an explicit oxygen check, even though it would be so simple to add one. Evolution is test-driven development in a very strong sense: if there’s no selection-pressure test, there’s no code.

Secondly, DNA’s operation is unlike anything we’re used to. It’s like seed data for a multi-step, multi-layered process where the output of one stage is input for the next. Embryology – the process of development from single-celled zygote to full-blown animal – is best thought of as an exceedingly complicated origami, going through different folding stages, as the organism, the blooming flower of the DNA seed, is revealed.

All descendant species arise from gradual, successive modifications of parent species. Each step must be a viable, reproducing life form in its own right. Design-from-scratch is just not done. This means animal bodies are riddled with design artifacts from their ancestors and the path they took while evolving from them.

The change from an ancestor to a descendant species is achieved mostly by distorting embryological processes. Speeding up this, slowing down that relative to the other, so that the expanding, warping, folding origami of the growing embryo takes on different shapes in the descendant as compared to the ancestor. There’s the famous example of the laryngeal nerve which illustrates what this can lead to. In the fish design, this nerve was routed straight from brain to gill. In the (fish-descended) mammalian design the routing is preserved, though the surrounding geometry has got distorted to awkward levels due to the neck. So the nerve comes all the way down from the brain, loops around the aorta and goes back up, finally reaching the now-modified gill (larynx). It reaches farcical heights in the giraffe, as you might imagine.

Pharyngeal arches

Pharyngeal arches

This concept of evolutionary action taking the shape of distortion of embryology might help you understand why the growing human embryo initially looks somewhat fish-like, with pharyngeal arches and all (a.k.a. “Ontogeny recapitulates phylogeny”) before being squeezed like silly putty towards a human shape.

Even grossly different mechanisms like egg laying and giving live birth – which appear to be so different as to require very different origins – can be shown to be evolved via a clear series of gradual steps from one to the other. Eggs can be held in and develop more and more inside the mother, being laid later and later in the gestation cycle. Taking this extreme, you get eggs which don’t have shells, held inside the mother for the entire gestation cycle and “laid” at maturity. There’s actually a fascinating little skink which alternates between egg-laying and live birth depending on the survival tradeoffs for the mother.

Skink with embedded eggs

A yellow-bellied three-toed skink carrying embryos, visible as light orbs inside its body. (Photograph courtesy Rebecca A. Pyles)

To summarize:

  1. We have more similarities than differences with our ancestors and cousins as far back as the fish. After all, the important inventions – circulatory system with blood and heart, digestive tract, liver, kidneys, sense organs, brain, bone – had already been done, so land animals should really be considered minor modifications of fish.
  2. Inertia. Systems which worked well in the past will continue to be used, unless acted upon by an external selecting force.

Let’s apply these points to our problem.

We know from all the salmon-run videos we’ve seen that many fish practice external fertilization, using water as a mixing medium for sperm and eggs. Indeed, it seems natural that underwater external fertilization was used by the first sexually reproducing animals. In such cases, you can’t really have sperm presence in the reproductive tract trigger egg production, which must start well in advance of any actual mating. For salmon and most other species, place and time of year is an extremely important factor, since laying eggs at the wrong time or the wrong place would severely penalize or even completely ruin their chances of survival. The availability of sperm is almost never really an issue. If the female is ready to lay eggs, chances are extremely good that there is a male nearby willing to do his duty.

This technique could have easily been continued even with the advent of internal fertilization, change in body plan, moving out of water and so on, in the absence of any real selection pressure to the contrary.

Mammals too have an estrous cycle, where they prepare an endometrial layer and ovulate in advance of mating. This layer is either reabsorbed or – in the case of apes and humans – bloodily discharged, in case that fertilization does not take place. The mammalian reproductive system is perfectly capable of optimizing resources and avoid “firing blanks” by suspending the estrous cycle during off-breeding-season periods or while the mother is nursing.

So here we are, at the somewhat anticlimactic answer: natural selection never penalized unfertilized egg laying because it never came to its attention. In the wild, the non-presence of males in the presence of fertile females almost never happened, and the few natural “blank” eggs were simply too rare to bother optimizing for.

The implications for the human condition are uncomfortable, to say the least: it means we’re falling into the “blank” error case much more frequently than natural selection designed for.

Anyway, hope you enjoyed this little evolutionary thought experiment, and I hope I haven’t permanently ruined your appetite for egg products.