A mutation introduced webbed digits on the front legs of a prehistoric mammal. The animal became more reproductively successful because it could glide greater distances when it launched itself from tree tops.

Hair elongates at the same rate all year round.

Birds begin migrating when their pineal gland detects that days are as long as nights.

Spiders have 8 long slender legs relative to their body in order to evenly distribute their weight over a wide area so that a delicate web can bare the load without being damaged.

Small fish swim in large schools.

Red blood cells give up their mitochondria (organelles that carry out cellular respiration), in order to better carry oxygen without using it up.

Thirst and urination coordinate to keep the total volume of blood fairly constant in an organism.