And if you think you know why, don’t be too sure about it.
Most adult female birds have only a single ovary and oviduct, the left ones. That’s in contrast to other vertebrates that have two of them, including humans of course. The loss of the ovary was long-presumed to be a weight saving device that enabled more efficient flight and some sources still claim that as fact.
The direct dinosaur ancestors of birds had two ovaries but the earliest birds had only one. We have compelling evidence that both statements are true from recent discoveries of their fossilized soft tissues, including parts of their reproductive systems – further circumstantial evidence suggesting that the loss of the right ovary might be related to weight loss and its advantage for flight.
This is a Long-billed Curlew egg, produced by the left ovary of course, that was presumably scavenged by predators on Antelope Island. The development of the shelled egg like this one, as opposed to eggs without shells like those of amphibians, was possibly the most significant development of land vertebrate evolution. Because the shelled egg doesn’t dry out it allowed vertebrates to invade the land much more efficiently instead of being restricted to water for their reproduction. But I digress, though only a little – the evolution of the land egg is a subject I used to love to cover in my zoology classes.
But let’s see if all the evidence supports the theory that adult birds lose their right ovaries as a weight saving device.
Adult male birds fly but they have testes on both sides. And the single ovary is of comparable size to the testes – even during the breeding season when the testes increase considerably in mass. So why don’t male birds lose one of their testes? They have to fly too and a single testis could certainly get the job done.
An alternative theory presented by some suggests that the fragility of the eggshell might be the key as to why the ovary is lost. If the female has two adjacent oviducts and both have eggs passing through them as they mature would the eggs be “clanging against each other” during flight and other strenuous activities, possibly breaking or damaging one or more of them?
Perhaps the real reason the ovary is lost is influenced by both – loss of weight for flight and to prevent egg damage. Or maybe it’s something we haven’t even thought of.
On a related note here’s something else bird gurus wonder about. Why is it always the right ovary that is lost and never the left? After all, when female birds are hatched (birds are hatched, not born – at least not in the biological sense) they have both rudimentary ovaries but only the left one develops. Why is it never the right instead?
There’s so many things about birds that we simply don’t know or understand but I sure do think they’re fun to chew on.
Ron
I have an 18-year old male Rouen duck whose testes, late in his life, became so enlarged at the start of mating season as to press on his sciatic nerve, causing him extreme lameness. A Deslorelin implant each January shrinks his testes, thereby eliminating his lameness during mating season. Yes, he is a pet duck and not for eating.
Ron: Fascinating stuff I did not know, and I learned even more from some of the posts.
Did you look into brain size in male songbirds?
No, I’m afraid I didn’t, Richard. Too little time and too many things going on…
Other than my “smarty-pants” reply to Everett, I have no comment on this at the moment, since I’m truly biology-challenged, but perhaps later when I have time to digest all the other comments and your lesson, Professor! 😁
🙂
I was late seeing this today, but let me add a bit more interesting (at least yo me) information now, if I may. This is condensed from an Ornithology textbook I am currently writing, but I’ll spare you the more detailed biology of this.
A number of species in the Order Falconiformes have two functional ovaries as do a few other birds. When both ovaries function, eggs are usually produced alternately which helps keep weight down and prevents eggs “banging” into each other while in the oviduct. (Although, it only takes one day for an egg to travel down the oviduct.) For most birds, very early during embryonic development, germ cells begin to migrate to the site in the embryo where the ovary will eventually form. There are more of these cells that migrate to the left side of the embryo than the right side which ultimately leads to the development of an ovary only on the left side. Additionally, cells on the right side usually begin to degenerate before an ovary can be formed. If both ovaries and oviducts were functional and contained mature eggs, the females of many species would have great difficulty, or even be unable to fly until unburdened of mature eggs. This situation actually occurs in some species of steamer ducks that are very nearly flightless to begin with. Females of these ducks with mature eggs may not be able to fly on the day prior to laying an egg.
Everything about birds is endlessly fascinating to me. Thanks for sharing this interesting tidbit with others. It’s certainly not information you will find in a field guide.
Valuable and interesting information you provided that certainly fleshed out the subject for us, Dan. Thanks very much for providing it!
I tried to do further research on the subject (other than what I found online) by using Cornell’s new version of the Handbook of Bird Biology, which I own. I think it’s a fantastic resource with a huge amount of good information but there’s just so much there, spread throughout the 700 page text, that I find it extremely difficult to locate specific information even though I’m relatively sure it’s there. I don’t even find the exhaustive index to be very helpful in finding info. Frustrating…
Learning every day. Learning before I have even poured my first cup of tea this morning. With something to ponder on for several days to come.
Megathanks to you and the knowledgeable people who visit.
Thank you, EC. Not sure what we’d do without “the knowledgeable people who visit here”.
Tea. Now that’s a beverage I never took too…
I come from a coffee loving family. Food sensitivities removed it from me a couple of decades ago. I have to drink my tea weak too.
Wow! Biology lessens, spelling lessons, grammar lessens and so much entertainment all in one easy read. I learned a lot and laughed as much at the blog and the comments. I am so glad I have been introduced to FeatherPhotograpphy. It is always a great part of my day.
Loved your first sentence, Betty. Thank you.
I had no idea that female birds had only one ovary. The possible reasons are quite intriguing. Thank you for teaching me something new yet again.
The “why” of it all is quite intriguing, Susan. Thanks.
Ahhh!!!! Darwin, Genetics, DNA, the master of evolution, loved teaching this stuff. Used to have mock trials, using outlandish stuff freshman had debate. Oh memories, fantastic memories. When you come right down to it what caused the change/elimination in the specific DNA code to rid the right ovary. Did it happen in mass or over time. Whatever it was it had to happen to enough individuals so that whatever breeding there was would perpetuate. What an interesting topic, many thanks!
Dick, I actually used to show the old black and white version of Inherit The Wind to my biology students – the version with Spencer Tracy. Don’t think I could get away with it these days…
Does that mean you think as Tracy (the colonel) said “We are marching backwards” today (in some respects)?
We sure are, especially when it comes to climate change. But on other things too.
I agree completely, that is why Nov 6 is so important to our Republics history!
Facetiously I’ll offer that the left one is gone because the female is always right.
Fascinating bit that I did. It know. Thanks, professor.
Ha, that might work, Arwen, if it was the left one that doesn’t develop. But actually it’s the right one they’re missing. I made a similar mistake in my original text.
Well, it turns out that in several falconiformes the right ovary is alive and well and can create ova https://onlinelibrary.wiley.com/doi/pdf/10.1111/ahe.12121 and this is also true in kiwis and about 80+ other bird species. https://link.springer.com/article/10.1007/BF01640692
8. Despite textbook statements paired ovaries in birds have been recorded in individual birds of at least 86 species belonging to 16 different orders and paired oviducts in birds have now been recorded in individuals of at least 34 species belonging to 13 different orders.
9. Although it has long been known that paired ovaries occurred commonly but irregularly in some members of the carinate order Falconiformes, species of this group form only 52% of the total number of bird species in which paired ovaries have now been recorded. If normal sexing routine in birds included the examination of the right side of the body cavity in addition to the usual examination of the left side, additional species (and additional orders) with paired ovaries might well be found.
Wow, I didn’t know some of that, Martha. And apparently a lot of other normally reliable sources don’t either.
I’ll correct my post so it doesn’t say that kiwis are the only birds to have a right ovary.
But apparently it still applies to most birds.
I have a male bird who when he came to breeding age started sneezing and his voice became hoarse. I took him to the vet who said that when some male birds come into breeding season their testes enlarge so much they put pressure on other organs. He did an xray of my bird and the testes were huge and pushing on air sacks and lungs. So perhaps this is another weight issue with birds. When an organ is not needed it shrinks thus less weight for a flighted animal. And yes my bird sneezes and is hoarse when he bird comes into season for the past 8 years.
Wow, that’s quite a story, April. I guess there’s having cojones and then there’s HAVING COJONES!
Why????????????
Ron. Good thing I didn’t have you in biology. Thanks for the lesson, now that I’m too old to be graded. Diana
Ha, you’re never too old to be graded, Diana. 🙂
Here’s another bit of the mystery about eggs. I often feed commercially grown quail to my rehab and falconry birds, and I remove the intestines before giving them to the raptors. The difference is size between the non-breeding testes and the breeding testes is remarkable. But even more remarkable is that the eggs in the females are a golden cluster, like grapes (or really more like the flowerets on a flower head) surrounding the ovary and oviduct. They are shell-less of course and range in size from pinhead to grape. By the time the birds have been frozen and thawed, the ovary/upper oviduct complex seems to have absorbed or gone to pieces, leaving the cluster of yellow. There can even be a shelled egg further down, ready to be ejected. I often leave the shell-less eggs in the bird when I feed, on the assumption that those little gold balls have a lot of nutrients in them. Another pedantry correction: in the second to last paragraph you say the left ovary is lost, the right retained. Farther on in the sentence you get the left/right correct, so there’s another typo for you to fiddle with. Great post!
Dang, that’s a mistake that I was afraid I might make, Sallie. I actually meant to do another read-through to check for that very error but I had an appointment to meet a friend at Farmington and I was running a little late so I didn’t do it. Thanks for pointing it out. I fixed it.
Interesting info you provided on the eggs and ovaries. Thanks for that too.
Interesting bit to ponder. Add it to the other bits to contemplate about birds. Makes for interesting ‘sitting and waiting’ time out in the field! 🤔
Yes, our mind sure can wander while sitting and waiting for birds. Thanks, Kathy
Ron – very educational and informative, but a painful reminder why my 10th grade biology grade was a C- 🙂
Everett Sanborn, Prescott Arizona
PS: nothing but rain rain and more rain here – in terms of filling up our lakes it has been one of our best monsoon seasons in a long time
I didn’t do particularly well in high school biology either, Everett. I didn’t get the biology bug until college. Congrats on your copious rain – sure wish we had some of it.
Ha! I can do you one better (or half better), Everett — my 10th grade biology grade was a D! Maybe it’s a Sanborn thing, 🙄
Ok, here’s my 10th grade biology story, Chris. I was a poor student in HS – never studied or did homework, just coasted and was mostly thinking about girls. In the first two quarters of bio I got low C’s but for the last 2 quarters of the year the daughter of family friends became our student teacher. Her name was Kathy Owen, older sister to one of my good friends Chris.
So “for Kathy” I decided to work and study for a change for the third quarter. My final percentage that quarter was the highest B+ it could be without being an A-. I just assumed Kathy would give me the A- because I was so close and we were friends. She didn’t, I got a B+ and I was pissed off with righteous indignation. So for the 4th quarter I really slacked off and got a D.
But I always used that as a teaching point with my own students. I would tell them the entire, honest story and then pause for effect and then say, “Boy, I really showed her, didn’t I?”.
My students always got the point… And it definitely reduced the amount of arguing and complaining I got when I posted grades. They knew I wasn’t going to budge.
I’d heard that about one ovary developing related to flight somewhere along the line – interesting as to the possibilities for that…:) Of course, then if something happens to the remaining ovary………on and on it goes! Thx for the speculation on both sides…….:) Cold front (43 this morning) from Alberta and British Columbia came through with a vengence yesterday afternoon/last night bringing VERY heavy smoke. Heavy enough to dig out the little ionic air cleaner for hubby Joe (88 with heart issues) – saved his bacon more than once over the years – improves life for me these days also! Would love some of that cool air in the house for sure!
Thanks, Judy. We got some of that smoke yesterday too – it had been clear for several days before. I’m sick of it.
Fascinating, I had no idea. Thank you again Ron, you continue to be a great teacher!
Thank you, D. I try but I don’t always succeed.
Ron,
Hate to be a pedant, but given your precision with words:
Testis = singular
Testes = plural
Tom
I’m well aware of that, Thomas. I was in the process if fixing that and several other typos when your comment came in. Thanks for the heads up as that would be an easy one to miss.
I’m glad of that. You remind me so much of my my own HS biology teacher. He loved to catch us out on such matters and that lesson has stuck with me forever.
Keep up your great work: I look forward to your blog post every day….and congratulations on your recent milestone!
Tom
Whenever I’m reading something serious and “formal” (not including things like simple, informal conversations on places like social media and personal emails) careless typos drive me to distraction. So I hate to be guilty of it myself.