Publications that do not allow for the pronoun "who" to be used in describing an animal include the Times of London, Reuters, the Houston Chronicle, the American Psychological Society's manual, the Chicago Manual of Style, Parrot's Grammar for English Language Teachers, Cambridge Advanced Learners' Dictionary and Encarta, among others.
The researchers also examined the British National Corpus World Edition, a 100-million-word collection of spoken and written materials, to see how frequently the word was actually used to describe animals in the real world. The researchers found 738 sentences in which the usage is found, a frequency of .76 per 100,00 words. Although not a frequent phenomenon, the usage is certainly popular, they wrote.
"I was pleasantly surprised to see so many uses of ‘who' with animals and so many reference works and style guides saying that it is okay," said Jacobs.
Interestingly, there were certain factors that made speakers and writers more likely to use "who" rather than "which" -- one of those was the type of animal involved. They ranked the animals in order of how often they occurred in the research: