This story is doing the rounds at the moment: a two-headed adder seen in Yorkshire. Here is the BBC's take on it: http://www.bbc.co.uk/nature/24289923
A live two-headed anything is indeed a very rare sight, and one worthy of great attention. Somehow the Beeb has decided it is an adult female though, when it is definitely a juvenile, and (I would say) of indeterminate sex. The age of the animal(s) is pertinent to the story because of whether such an animal might survive. Could it hunt, eat and grow successfully? Could it wend its way through tangled undergrowth normally, and in the late autumn find itself an underground void to hibernate for the winter?
The age of this adder is crucial to knowing the answers to these questions, but looking at the photograph being circulated, it does not look like an adult animal (30-70cm). It has the proportions, and the dorsal zigzag shape, of a juvenile animal (around 10-15cm at birth). Adders give birth to live young around September in the UK, so it could be one of this year's young. Or it could be a yearling.
In the adder, both sexes are ginger or brown when born. Adult females may retain the ginger colouring, or may take on various shades of brown, or a sandy or toffee colour. Their dorsal zigzag is ginger or brown, of a darker shade than the body colour. Males can be grey or bronzy-brown (which often causes confusion in sex determination), but their zigzag is always black. This is a very reliable rule. Melanistic examples of both sexes can be black or very dark brown however.
Juvenile males tend to become darker than females of the same brood within a year or two. They develop a black zigzag by the time they are adult within four years, whereas females stay ginger or brown throughout their lifetime (except when melanistic).
In this case, the Beeb has asserted incorrectly that the adder is an adult female. One would need to know its size, and see the tail of the animal for a better idea of sex (males have relatively longer tails, even as juveniles). But even then, sex determination might not be possible.
Without a scale, it is hard to say if this adder is a newborn, or one that has survived (and grown) for a year already. Given the presumable difficulties in surviving, it would be very interesting to know which is the case.
(By Chris Gleed-Owen)