The point at which the embryo stops separating varies. Just as Siamese twins can be joined at the head, breast, or hip, so too can snakes be joined at varying places on their bodies.
Although it's difficult to be certain, captive inbreeding may cause more two-headed births than in the wild.
"There are no statistics available, since the majority of two-headed snakes cannot survive long after birth in the wild," said Van Wallach, a researcher at Harvard University's Museum of Comparative Zoology.
"The few examples we have are fortuitously captured," he continued. "Many more individuals are surely born, but we never see them. My guess is that they are occurring with greater frequency in captivity than in nature. There is no way to test this, of course, because we can't sample the wild specimens we are unaware of."
"It's not an evolved trait, so each two-headed animal would be highly individual," said Burghardt. Where the split occurs along the body determines how much duplication of organs there is and the degree of competition between the two heads.
"If the two heads are very close together it's going to be much more difficult for them. With more separation, they can act a little more independently," he said.
Individuals, Not Freaks
Each head of the king snake at Arizona State University was supported by a separate neck, but they shared a single stomach. The two-headed black rat snake that lived for close to 20 years at Burghardt's lab had two complete throats and stomachs. Pictures of the ladder snake in Spain show two completely separated heads that join the body at about "neck" level.
That snake is destined for the lab of Enrique Font, a biologist at the University of Valencia. It's too early to tell which organs may be duplicated, he said.
"As I have not yet been able to examine it, I'm reluctant to speculate as to what could be done with it," he said. "To the best of my knowledge, the snake has not been sexed yet. If it is a male, I would be interested in finding out how the snake courts a female.
"In some snake species," he added, "the male rubs its chin against the female dorsum during courtship. As this particular snake has two headswhich may have different ideas about courting and matingand two chins, it would be nice to find out how the two heads manage the deal and also to find out what the female's response is."
Font is also interested in looking at howand whetherthe two heads cooperate in targeting and capturing prey, and what role two brains play in regulating hunger and mediating other behaviors.
"These animals shouldn't be looked at as freaks," said Burghardt. "They're organisms with motivations and individuality just like any other. They provide us with an opportunity to study cooperation and the processes of controlling the same body with two nervous systems. Studying them might provide some insight into the survival issues faced by Siamese twins."
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