When you let out a big yawn during a boring lecture, you succumb to a reflex universal among animals. Researchers from the State University of New York looked at the videos of 29 mammals yawning to calculate the average length of their yawns. The animals had their brain weight already documented in an earlier research.
Researchers realised that brain weight and the number of neurons in the outer layer of the brain, called the cortex, can reliably predict yawn length. Humans, with about 12000 million cortical neurons, had the longest average yawn, lasting a little more than 6 seconds. The yawns of tiny-brained mice, in contrast, were less than 1.5 seconds in duration.
Gorillas, horses, walruses and African elephants may be huge in size but they have shorter yawns than humans because their brain size is smaller compared with ours. The findings suggest that the length of yawn does not correlate to the size of the body but to the size of the brain.