Is A Two-Headed Snake One Snake Or Two Snakes?

Is A Two-Headed Snake One Snake Or Two Snakes?
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Alumni Distinguished Service Professor of Psychology and Ecology & Evolutionary Biology at the University of Tennessee, past editor of the Journal of Comparative Psychology

Two-headed snakes seem to be produced by a mishap early in embryological development. The classic report on them is Cunningham’s 1937 book, which documents the many ways in which the two-headed syndrome is expressed.

Do the conjoined snakes have different personalities?

I have had several two-headed snakes in my career; the most long-lived and most studied was IM, a two headed black rat snake that lived for almost 20 years. The two heads were in one body but seemed to have somewhat different personalities and would often compete in how they would avoid obstacles and over food, even though, we often said, the food was going into the same stomach and body. Experimental studies of hunger and satiety, reported in 1993, led to the surprising finding that while each head ate readily, they had different prey size preferences and seemed to reach satiety independently. This fascinating finding was resolved when we discovered through x-ray videos of IM swallowing mice that IM had two separated stomachs! This seems to be the first such case, and again shows that two-headed animals can be diverse, and assuming that only the head is duplicated is not warranted, though frequently assumed, as we did at first.

Paul Andreadis

Herpetologist at Andreadis University

Can only one “head” be hungry?

In the case of the two-headed snakes that seem to find their way into the popular media, often there are two heads that join near the neck, and then there is a single body posterior to the junction. It is unknown how much anatomy is genuinely shared. In other words, do both heads receive sensory information from all parts of the shared body? In most cases, we just don’t know. We eventually learned by x-rays that the two-headed rattlesnake had two digestive tracts inside the single body. So maybe it is no surprise that when one head was fed to satiation, the other head often was still hungry.

Is a two-headed actually two individual snakes?

But is a two-headed snake one individual, or two? Since our behavioural instincts, and our emotions, and our individually unique personalities, and our thoughts of higher cognitive function are all attributes of our brains, and since a two-headed snake has two brains (albeit genetically identical ones), it really is two individuals. Consider the difference in appetite between the two heads in the rat-snake. The left head was quite a ravenous eater, the right head a much more restrained feeder. Yet they were identical twins. We don’t have a good term for such an individual, or pair of individuals, so in our original paper on the subject, we referred to the snake as a single subject. But two brains, with differing appetite levels, attached to the same body? Quite a challenge for defining what an individual is!