The Wild and the Domestic: Chickens and Broodiness

Note: This program originally aired August 17, 2013.

Last week’s show, was admittedly, very fun, and I hope you enjoyed it (to listen click here). This week however I want to take a few moments and relay some interesting information I discovered, pertaining to chickens and broodiness, and the poultry community in general.

It turns out that if you do any research on the causes of broodiness (the tendency of a hen to want to sit on a nest and incubate eggs, instead of just visiting the nest to lay a daily egg), two distinct conversations emerge. The first includes the backyard chicken community, those raising a small flock of birds for eggs and pleasure. Their information is purely anecdotal and covers a wide range of observations and advice. The other conversation is from the scientific community, particularly the scientific community working for the poultry industry. The thrust of their research is in preventing broodiness, as it interrupts the hens’ laying, and laying eggs is what it is all about (for that sector of the industry). Every day a hen lives but doesn’t lay an egg is a waste of resources in the eyes of industry. These two perspectives make for interesting reading, as you can imagine.

I am curious about what triggers a hen to go broody, and in asking this question we have to keep in mind a few things. First, chickens are birds, and second they are domesticated and selectively bred. In saying that they are birds, we acknowledge that they retain some physiologic and behavioral traits of wild birds. Their domestication has reduced or altered some of those traits. In the wild birds mate and then lay fertilized eggs, usually a set number, a clutch, and then incubate them. This is generally tied to environmental conditions, particularly day length. Chickens are birds, so yes, they lay eggs. They were domesticated approximately 4000 years ago from a south Asian bird called Red Jungle Fowl. These wild ancestors look a lot like chickens, but have a much more typical reproductive pattern, laying fertilized eggs in clutches a few times a year. In the domestication process however, the relatively rare (in the wild) trait of laying unfertilized eggs has been emphasized over and over again, to the point where domestic chickens can lay an egg nearly every day for a year or more, with the rate of egg laying tapering with age. Hens can lay eggs for their entire lives and never even see a rooster.

If you ask an online backyard chicken keeping forum what causes a chicken to go broody, or better yet, how to make your chicken go broody, you will get a plethora of responses which range from dumb luck, to leaving eggs under the hen, to putting dummy eggs or even golf balls under the hen, to day length and season, to breed. The most common theme seems to be the accumulation of eggs in the nest, which appears to trigger something in the wild bird part of the chickens’ wee little brain, something that says “yes, you have your clutch, now sit on it”. If we think about the implications for chicken neurology, that means that the part of the chicken brain that decides to be broody is separate from the part of the chicken brain that is used for mating; interesting, and based on our evidence, totally anecdotal. If you look in the scientific literature for the cause of poultry broodiness a totally different plot develops. It turns out there isn’t much in the literature about the causes of poultry broodiness, only how to prevent it. And that is because most of the research on chickens is done for the benefit of the poultry industry, either by industry scientists or agricultural extension agencies. These studies describe in detail what happens when a chicken goes broody (brain chemistry changes in ways similar to mammals, the pituitary gland triggers the production of the hormone prolactin, which among other things, inhibits gonadotropin stimulated ovulation, i.e. egg laying), but don’t really examine why chickens exhibit this behavior. It is sort of like the question is mute, chickens are birds, that’s why they go broody, what we need to know is how to stop it. And that is what the studies are all about—tests of what they can inject in or feed to chickens to prevent the surge of prolactin that causes the cessation of egg laying, which is the bottom line for the egg industry.

So I never got my question answered. Science has never looked at the question of why a chicken goes broody or how to make a chicken go broody, because industry has no need for that, they have machines that incubate eggs instead. On this I will throw my lot in with the backyard chicken crew, observing, questioning, testing and doing informal science with the best of them. And not knowing the answer doesn’t really bother me.  I actually prefer that some things lay beyond our control, and that mysteries in nature abound. If there were no mysteries, our curiosity would have no food, no medium on which to grow and flourish, and that would be as soul killing as any cement floored mechanized egg producing factory farm. So the next time you crack an egg, pause, and ponder, what you know about that egg, and more importantly, what you don’t know.

References:

Advice from Mother Earth News: http://www.motherearthnews.com/homesteading-and-livestock/encourage-hens-to-hatch-eggs.aspx#axzz2actDPDU

Originally published in Backyard Poultry http://www.themodernhomestead.us/article/broody-hens-1.html

From the USDA (mainly focused on how to prevent broodiness in commercial flocks): http://www.ars.usda.gov/is/ar/archive/nov95/broodiness1195.htm?pf=1

 Way , way , way more than you ever wanted to know about prolactin: http://www.sp.uconn.edu/~ttc02001/MCB3210-5210/RA%20%28V%29/PRLR%20Signal%20Transduction%20Pathways.pdf

The original chickens, Red Jungle Fowl: http://www.gbwf.org/pheasants/junglefowl_red.html

A bit about chicken domestication:
http://www.sciencedaily.com/releases/2008/02/080229102059.htm

Of Mice and Oogonial Stem Cells

Note: This program first aired on Deecmber 29, 2012.

I was asked by a listener recently, if female chickens are born with all of the eggs they will lay over the course of their lives, like human females are. This made me wonder if all females have this characteristic, if that is a distinguishing feature of being female—that the cells that become the gametes form in the embryo, and are not replenished as the individual matures. Conventional wisdom states that we females have a set amount of eggs (more than likely more than we will release in the reproductively active period of our lives), we do not make more as we go along. This is in contrast to males, who are able to continually replenish their supply of gametes.

We need to pause for a moment and return to high school biology, and revisit the concepts of meiosis and mitosis. Our cells have two sets of DNA in them, one from our mother and one from our father. That DNA is in the form of chromosomes. Mitosis is simple cell division, we get two cells when we started with one. The chromosomes in the cell all line up, split in half, and each half then replicates itself in the new cell; each new cell ends up with two sets of chromosomes, just like the parent cell had. Meiosis is cell division that results in the formation of gametes. What makes gametes special is that they have only one set of DNA in them, a mash up of DNA from the mother and the father. In meiosis, the chromosome pairs line up, swap DNA and then get separated from each other into two separate cells. Then the chromosomes themselves split in half, and the halves end up in two more cells, yielding a total of four cells from the original one, each with a single set of genetic information. That is what makes gametes gametes, they have only one set of DNA and they need to join with another gamete cell to get a full set of genetic information, enough to make a new individual. The traditional thinking has been that , at least in mammals, meiosis occurs in the ovaries of the embryo. As a female’s reproductive organs are forming, meiosis is going on, and all the eggs the female is going to have are formed then.

Before we get to the chicken issue, I have to break it to you that the conventional wisdom here has been shown to be wrong. As early as 2004, researchers discovered that in mice, the females seemed to be able to maintain a supply of healthy living eggs, even when fed toxins that would kill their follicles. This meant that the mice could generate new eggs, a discovery that flies in the face of the conventional understanding that mammals are born with the only eggs they will ever have. Since then researchers have identified the stem cells in human ovaries, called oogonial stem cells, that do the same thing for us. These oogonial stem cells are able to undergo meiosis at any point and produce new oocytes or eggs. What isn’t clear is if these oogonial stem cells are just a back up system, or if they share responsibility for egg production with the original meiotic cells in the ovary.


So, on to chickens. It turns out this is a very difficult question to answer definitively. Most of the literature refers to the “all the eggs you’ll ever make” phenomena as occurring in either mammals, or “higher animals ” (which by the way are apparently all vertebrates except fish). There is, thus far, no evidence of oogonial stem cells in chickens, but that doesn’t mean they aren’t there, it just means no one has looked, which is a different thing altogether. Likewise, this conventional wisdom principle has likely been laid like a blanket on top of our knowledge of all “higher animals”, meaning, it is unlikely that scientists have studied the embryos of every species close enough to declare that all higher animal females are always born with all their eggs. To be fair, closely related things usually behave in closely related ways, so this blanket technique may not be as imprecise as I am making it sound. We do know that certain worms continually make new oocytes, so it is not unheard of in that natural world, but chickens are closer to the higher animals than to round worms.

The final word? I suspect that chickens, like humans, make most of their eggs during the development of the embryo, and may, like humans, potentially retain the ability to generate new eggs as needed. This question illustrates nicely, the limits of our knowledge, and way we construct what we know, or think we know about the natural world. Thanks Greg, and keep your questions coming.


References:

From Science Daily, a digest of the original study from Massachusetts General Hospital and the University of Edinburgh, demonstrating that female mice can grow new eggs: http://www.sciencedaily.com/releases/2012/07/120726180259.htm

National Geographic’s take on the same study: http://news.nationalgeographic.com/news/2012/02/120229-women-health-ovaries-eggs-reproduction-science/

Need a review of mitosis and meiosis? Check out this nice slide show from the good folks at PBS: http://www.pbs.org/wgbh/nova/miracle/divide.html

A bit here on the traditional take on female oocyte development: http://anthro.palomar.edu/biobasis/bio_2.htm

Whoa. Here’s a scientific article about what can regulate meiosis in chickens!! http://www.biomedcentral.com/1471-213X/8/85

The truth? Its complicated… http://rstb.royalsocietypublishing.org/content/277/955/201.short