The Mexican tetra comes in two forms: one lives in rivers, the other in pitch-dark caves. In cave embryos, eyes start to form, then degenerate and become covered by skin. In darkness sight is useless and costly: in young fish, the eye and visual brain can consume up to about 15% of resting energy.
In 2022, a young tagged bar-tailed godwit flew from Alaska to Tasmania in about 11 days of nonstop flight, covering 13,560 km without eating, drinking or landing. Before departure, these birds pile on huge fat reserves and shrink their digestive organs, which are useless during the journey.
To escape rivals, a dung beetle rolls its ball of dung in a straight line. At night, it holds its course using the glowing band of the Milky Way. In a planetarium, researchers showed that beetles rolled straight under a starry sky but wandered aimlessly when the galaxy was switched off. It was the first animal known to navigate this way.