Polymelia: Animals with Extra Legs.
There are many different conditions that can cause animals to be born with extra legs (a condition called polymelia). Some examples are described below.
The main causes of extra limbs is the partial development of a conjoined twin, of a genetic mutation (spontaneous or inherited) that affects fetal development. It can also occur due to the effect injuries, toxins and/or infections during crucial periods of fetal development.
Also known as: supernumerary limb(s).
There are quite a few examples of cattle that survive in good health with extra legs. The extra limbs often remain small and vestigial, allowing the bull or cow to function in a normal manner. The often seem to be attached to one side of the withers, where the animal can carry them fairly easily, and they remain relatively small in size.
However they can also be large or comprise of up to seven legs and other body parts. Or cases like this one from 1981 which are more disabling to the calf, as the extra limbs diverge at the "knee" rather than higher in the body, and so impede locomotion and normal body postures.
Polymelia in cattle is sometimes found to be associated with chromosomes having multiple breaks. In other cases is results from an incomplete conjoined twin.
For further examples see: Japan (1985), Korea (2001), Korea (2007), Brazil (2008), United States/Nebraska (2007), India (2010). These examples show polymelia happens around the world and to many different species and breeds of domestic cattle.
In 2008 a six-legged fawn was found. The fawn was separated from its mother and rescued when it was found under attack from dogs.
As with many of the examples in cattle, the extra limbs belonged to a partially developed conjoined twin. In general, this condition seems to be especially rare in deer.
This dolphin, caught off the coast of Japan, was documented to have an extra set of fins (2006). This abnormality was though to reflect that dolphins evolved from species that has hind legs. These fins are not really an abnormality, but may be cause by a mutation that activated a feature that was carried by the dolphins evolutionary ancestors, but is not normally expressed in modern dolphin. Fins of ths type are reported to occur occasionally and various species of dolphin and whale.
Deformities, including extra limbs, are being seen more often in frogs around the world. Different explanations have been offered for these deformities including the effects of pollution, hormones or climate change. The preferred explanation now is that deformities are produced when parasitic worms infect tadpoles.
In 2007 a four legged duck was hatched in the United Kingdom. However due to an accident "Stumpy" (right) ended up losing one extra leg. Another seemed to break off naturally. Thus Stumpy ultimately ended up living life as a conventional two-legged duck.
Even more unusually, this goose (2008) has a third wing.
In 2002 the shown to the right sheep was born in the Netherlands with a fifth leg. Polymelia is a defect considered not uncommon in sheep. Unlike the example pictures, with sheep the extra limbs are often attached to the hindquarters.
Extra limbs have also been known to occur in horses.
Extra Fingers and Toes
Many species are sometimes born with extra fully or partially formed digits.
Human polydachtyly in the fingers is generally caused by a mutation that effects fetal development and may be associated with other abnormalities, however these may be very mild. Anne Bolelyn is one of the more famous examples, haiving six fingers on her right hand.
Polydachtyly is also found on cats. Hemingway was known to own a cat with extra toes and its descendant still live in the Hemingway Museum. Hemingway acquired his cats from a sailor, many sailors believed these cats to be good luck.
Extra limbs, when they occur, are general not functional or useful. In only a few cases, such as the human child Jie-Jie, is the limb sensitive to touch and able to move in a relatively normal manner. Other human examples include: Josephene Myrtle Corbin (1868-????) and Maxine Mina (1896-????)
The two most common causes of polymelia seem to be:
- Fragile chromosome resulting in chromosomal breaks, or
- an incompleted conjoined twin, and
- (in the case of amphibians) parasitic worm infection.
A snake cannot really be said to have duplicate limbs, as it isn't supposed to have any in the first place. However in 2009 this snake was found with a very uncharacteristic appendage, a leg!
As with the dolphin this probably reflects an accidental activation of ancient snake DNA, from a time when they still had legs. Some modern species have small limb buds, ancient snakes had small hind legs, and even older ancestors would have had a full set of four.
In 2008 a shark was caught that appeared to have two hind legs.
See also this hub on humans with tails.
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