In 2013, Georgia Institute of Technology mechanical engineer David Hu and his colleagues discovered a “law of urination”: All mammals weighing more than 1 kilogram empty their full bladders in about 21 seconds (standard deviation 13 seconds).
Last year Hu followed that up with a law of defecation: Despite a rectum length varying from 4 to 40 centimeters, mammals from cats to elephants defecate within a nearly constant duration of 12 ± 7 seconds. A layer of mucus helps feces slide through the large intestine; larger animals have more feces but also thicker layers of mucus, which aids their ejection.
From the journal Soft Matter, whose cover artist deserves some kind of award.
(David L. Hu et al., “Hydrodynamics of Defecation,” Soft Matter 13:29 [August 2017], 4960-4970.) (Thanks, Colin.)