According to Wikipedia, some birds lay unfertilised eggs. This seems like a bad idea, survival-wise. After all, the egg can’t be fertilised once its shell has formed, and producing eggs that won’t hatch is a pretty big drain on resources. Wouldn’t this be strongly selected against?
Since this happens, there must be a reason. Why?
When wild birds lay unfertilized eggs, it’s accidental. Sometimes the male hasn’t been successful in transferring sperm; in other cases the female have undergone some other stimulus that causes them to lay despite not having mated. Of course this is selected against, since it is a waste of resources. However, since the process normally works fine, it is probably better to have it on “autopilot” to some extent rather than try to fine tune it so much so that all accidental laying of unfertilized eggs is eliminated (which is probably impossible anyway).
Domestic hens, on the other hand, are very prone to lay unfertilized eggs because they have been selected for it. Hens that did so were picked to produce the next generation by humans because they were good egg producers. And it doesn’t really matter to the hens that they produce a lot of eggs, since they receive all the food they can eat anyway. As long as humans pick such hens to breed from, and reject ones that don’t produce so many unfertilized eggs, it doesn’t matter that so many eggs are “wasted” from the hen’s point of view.
Hmm… good question.
WAG: Any birds in the wild that manage to lay unfertilized are those that didn’t manage to mate in the first place. Meaning, they have a pretty low (comparatively speaking) fitness to begin with. The additional disadvantage of wasting resources on unfertilized eggs could be insignificant compared to whatever other disadvantages the bird already has.
In birds that have successfully mated, whatever traits that lead to being able to produce unfertilized eggs will have no disadvantage. In fact, they might have some major advantage in this case, maybe since resources have been allocated to those eggs even before the mating has occurred.
More WAG: Perhaps this phenomenon is more common in birds that usually mate just once or twice (in the wild), meaning there is very little chance of future reproduction, and by the time they lay an unfertilized egg, they’ve already missed their chance.
Not necessarily. Even birds that are successfully mated may sometimes lay unfertilized eggs.
No, in either case there will be a disadvantage (and I fail to see how this could be an advantage).
I’m not sure if you are referring to mating, or copulating. Birds that breed as pairs usually copulate multiple times. In any case, female birds are able to fertilize several eggs from one copulation. I doubt there is much of a correlation here.
Reproduction is so important that the advantages outweigh the disadvantages. A similar thread here not to ling ago dealt with why women get monthly cycles, which is basically the same thing (as opposed to a system that only turns on after mating or something).
I was thinking that this phenomenon might be an example of a simple tradeoff. The genes that allow for unfertilized eggs (even after copulating, as you say) might also be genes that lead to fast-developing eggs, or something else that gives an advantage in most cases.
Yes, in normal circumstances “keeping the pump primed” is more critical than wasting a few resources if things don’t go exactly right.
Humans lay unfertilized eggs too. Other mammals too, I think.
Could you repeat the question, please? I don’t get it.
No they don’t. Obviously they don’t lay eggs in the sense the OP is using the term. If you are taking about ova, then humans certainly don’t “lay” them.
Platypodes and echidnas, I suppose, but no others.
It seems quite clear to me. Some birds lay unfertilized eggs. The OP wishes to know why, since this seems a waste of resources that should be selected against. A reasonable question.
But the wasted resources seems comparable in both cases. Especially since (I have heard that) humans are born with all the ova they’re ever going to have (I don’t know if that’s also true of birds or not). So why waste the unfertilized ones?
Unfertilized bird eggs are costly because it takes a lot of time, energy, and nutrients from the mother for the egg to grow. Compared to that, the amount of resources a human uses to grow and maintain ova is trivial - we’re talking about a single cell (plus some support cells), versus an egg that weighs a rather substantial percentage of a bird’s weight. And even though human females are born with a limited number of ova, that number isn’t really a limiting factor in terms of reproduction.
You should have been clearer about what you meant.
A large hen’s egg weighs about 2 ounces, or about 4% of the weight of a 3-pound hen. Human ova are about 0.2 mm in diameter. I’m not going to work out the weight, but obviously it is infinitesimal compared to that of a chicken egg. If we take the weight of a human female as 125 pounds, she would have to have an ovum weighing 5 pounds to be comparable to that of a chicken. Even if we take into account the loss of menstrual blood if the egg is not fertilized, the cost to human females of non-fertilization in any one month is negligible compared to the cost of one unfertilized egg to a chicken or other bird.
Thank you, Colibri and lazybratsche. I was thinking of “resources” in terms of limited opportunities to reproduce, and neglected the much larger size of the bird eggs. Thanks for reminding me.
So the whole period thing is a myth in (human) women? Someone should tell them, as they are wasting a lot of money on pads and tampons.
Oy. :rolleyes: I was specifically referring to the eggs… Yes, there is a menstrual cycle, and yes, it does use a lot of resources…
As I’ve pointed out, although some resources are expended the actual energetic cost is trivial compared to a bird’s egg.
The two phenonomena are quite different: in humans, a relatively small cost is incurred - probably amounting to a few ounces of tissue, including menstrual blood, or much less than 1% of body weight. The benefit is the possibility of a pregnancy, which in evolutionary terms is the main objective of existence.
The production of unfertilized eggs in birds is accidental. It is not something that happens routinely, except in cases where it has been artificially selected for in domestic animals. As such, it is better compared to a miscarriage or still birth, rather than ovulation and menstruation in humans.
I suspect that the production of unfertilized ova in all animals in the wild is pretty rare. Whenever the female is in heat, the egg gets fertilized.
Generally speaking, women average a fluid loss of about 60mL in a “normal” menstrual period, but as high as 240mL may be “normal” for some women. Of that, about 50% is blood, and the rest is…well…other stuff. Water, cells from the uterine lining, etc.
Even 240 mL (about a cup) of fluid once a month is nowhere near as “expensive” to produce as the chicken’s eggs for the month if viewed as a percentage of her body weight.
Mayo Clinic cite for 60-240 mL figure.
The American College of Obstetricians and Gynecologists cite for 50% blood statistic.
Yes, humans are pretty unusual in having aseasonal estrus. Most animals only ovulate at particular times of year, or in some cases only in response to mating itself.
Assuming a density approximately that of water, 60 ml = 2 ounces, 240 ml = 8 ounces. That is, between 0.1 and 0.4% of the body weight of a 125 lb woman. Given that most of this weight is actually water, the energy expenditure is pretty low…
Only if she succeeds in finding a mate.