Why are mules and hennas sterile?
Because they are hybrid animals. In other words, their parents were separate
species. Mules are the result of mating between horses and donkeys. Usually
different species can't mate, but if they are successful, their offspring
usually can't reproduce themselves. There are exceptions to every rule
especially in science and there are instances where hybrids can reproduce,
such as matings between wolves and closely related dogs.
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Update: June 2012