Joined at the Hip: These Twins are BIRDS!

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Conjoined twin Barn swallows, Hirundo rustica,
found July 17, 2008 in Searcy, Arkansas after they fell from their nest.

Image: Samuel Peebles (Daily Citizen, AP Photo).

Last week, a homeowner discovered a pair of young barn swallows that had fallen out of their nest after a sibling flew off to learn to forage from its parents. That's a common event, but this particular pair of young birds were remarkable: they were joined at the hip -- literally.

Conjoined twins -- sometimes known as "siamese twins" -- have been described in humans and other mammals as well as in reptiles, but not among birds that anyone recalls. This might be because conjoined twin birds die before anyone finds them.

"I can't even say it's one in a million -- it's probably more than that," said Karen Rowe, an ornithologist with the Arkansas Game & Fish Commission. "There's just very little to no records of such a thing."

Unfortunately, the twin birds did not survive. By the time the person who discovered them had notified wildlife officials, the two birds had stopped eating. One bird died shortly thereafter, on Friday morning, and a veterinarian later euthanized the surviving twin.

The birds first appeared to have only three legs, but further examination found a fourth leg tucked up underneath the skin connecting the pair. Additionally, X-rays showed that the birds were fully formed, alhtough it is not clear if they are identical or fraternal twins. Their bodies are being sent to the Smithsonian to gain a better understanding of how these birds developed.

According to Rowe, the twins had to develop from a double-yolked egg. But even if they had survived, it would have been difficult to teach the birds to fly, added Rowe, who is a master of understatement.

Barn swallows are aerial hunters that capture and consume insects in midflight. They build construct familiar mud nests that they plaster to walls and often to support structures in barns, hence their name.

Source (quotes).

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By the way, why is this so much rarer in birds than in reptiles? Do you think that cursorial conjoined twins would be more viable than flighted conjoined twins, living longer and thus providing more more opportunity for our observation? And, can reptiles also produce double-yolked eggs?

sara, the part of your question I can personally answer is, "can reptiles also produce double-yolked eggs?" Yes; certainly snakes can.

I grew up in Colorado, and on one occasion had the joy of helping to clear a massive rattlesnake infestation out of a neighbour's basement. Rattlesnakes don't lay their eggs, they carry their eggs in their body and then "give birth" when the eggs hatch -- but one snake we killed proved to be a female full of eggs (yes, we were butchering them; rattlesnakes make good eatin', and rattlesnake chili is a treat not to be missed!), and one of the eggs had a double yolk, and two tiny embryonic snakes developing.

If both birds and snakes can have this happen, I can think of no reason why it wouldn't occasionally happen in other reptiles.

By Luna_the_cat (not verified) on 21 Jul 2008 #permalink

I'd be very surprised if the pair were fraternal twins, since conjoined twins in both birds and mammals typically arise through incomplete splitting of the embryo at the primitive streak (gastrulation) stage. There are a number of classic experimental and descriptive studies of twinning in chick, quail, and duck embryos; I'm fairly certain these are summarized by Romanov in The Avian Embryo (I'm at work right now, but I'll check my copy when I get home).

There's a 2006 paper by Mazzullo et al., reporting a case of conjoined twins from a hybrid ostrich mating. The twins hatched, but died 24 hours later. They were the "cephalopagus" type, sharing a head and brain, with fused first cervical vertebrae, and then two spinal cords and bodies. I can send you the pdf, Grrl, if you're interested.

Many of the classic reports on twinning in avian embryos indicate (not surprisingly) that more abnormalities are found when the female birds and/or eggs are in poor condition, and when eggs are not rotated or maintained at the correct temperature and humidity. This is certainly consistent with my own anecdotal memory of the incidence of conjoined twins and other abnormalities among chicken and quail embryos, from my doctoral research.

I can't see how these two could have developed from separate embryos in a two-yolked egg and ended up joined simply by virtue of having been smooshed together.

It would also seem to me that if they were fraternal twins that did accidentally end up conjoined, they'd have been nonviable on account of mutual rejection.

But then, what do I know? I'm not a biologist. I don't even play one on TV. :-)

"I told her to go get the cat to just let it go after them"

Wotthehell was this woman thinking?

By themadlolscien… (not verified) on 22 Jul 2008 #permalink

I can't see how these two could have developed from separate embryos in a two-yolked egg and ended up joined simply by virtue of having been smooshed together.

I'm going to have to hunt up the exact reference for this, as I can't remember the precise details and I can't find it online -- however, there exists, in one of the medical school museums of congenital deformity, a truly horrific preserved fetus. In this case, fraternal twins developing together the womb somehow ended up positioned face-to-face, with their faces pressed together, from mid-pregnancy -- and the cell signalling which tells the developing face to join up along the central line ended up fusing the left half of one twin's face to the right half of the other twin's face, and vice versa, resulting in a giant, two-brained head with an extra-broad face consisting of half from each twin on each side of the skull. (The baby/ies did not survive birth.)

During embryological development, there is some fairly powerful chemical signalling going on to knit developing tissues together into a completed, joined up body. Also during this time, a great deal of immune response is still undeveloped. Presumably, if the birds were fraternal and from two different zygotes, either the two birds were genetically similar enough that there was no immune response anyway or the immune response was not yet developed enough to make tissue rejection an issue; and the cell signals to knit developing muscle and skin together was enough to override anything else.

By Luna_the_cat (not verified) on 23 Jul 2008 #permalink