Which animal has the longest pregnancy?

An African elephant carries a calf for about 22 months. The calf is born weighing something over a hundred kilograms and can walk within hours, and much of that long gestation goes into brain development - an elephant needs to arrive already able to keep up with a moving herd and to start learning from it immediately.

Among vertebrates generally the record belongs somewhere stranger. A frilled shark is thought to gestate for around three and a half years, and an alpine salamander can take up to three depending on altitude. There is also a deep-sea octopus that was observed brooding one clutch of eggs on the same rock for four and a half years without eating, which is not pregnancy but is arguably worse.

Elephant herds run on this timescale. Females are pregnant or nursing for most of their adult lives, calves are looked after by the whole group, and the accumulated memory of the oldest female - where water is in a drought, which routes are safe - is what the herd survives on. Killing the eldest animal removes information the others cannot reconstruct.