An elephant takes the same amount of time to empty its huge bladder as a cat - despite holding 3,600 times more urine. That's a whole lot of pressure.

The controlling factor of urination duration is the length of the urethra, which gets longer at a predictable ratio as an animal gets bigger. All animals have urethras of the same aspect ratio: a length-to-width ratio of 18. This is rare among animals. Usually, body parts change in relative size, such as the eyes and brain.

As the urethra gets longer, the effects of gravity increase and there's more pressure pushing out the urine.

Source: http://www.sciencealert.com/most-mammals-take-21-seconds-to-pee-regardless-of-their-size ...

Some mammals, such as camels, polar bears, and seals, have full nictitating membranes. That is the transparent or translucent third eyelid that can be drawn across the eye for protection and to moisten it while maintaining visibility.

In humans, the plica semilunaris (also known as the semilunar fold) and its associated muscles are thought to be homologous to the nictitating membranes seen in other animals. In most primate species, a plica semilunaris is present, but a fully developed nictitating membranes still exists in lemurs and lorisiform primates.

Unlike the upper and lower eyelids, the nictitating membrane moves horizontally across the eyeball (shown below). In some diving animals, such as beavers and manatees, it moves across the e ...