Which animal has the longest gestation among land mammals?

The African elephant, at about 22 months. The calf arrives weighing over a hundred kilograms and walking within hours, and much of that long gestation goes into the brain - an elephant needs to arrive able to keep up with a moving herd and to start learning from it immediately.

The reproductive arithmetic that follows is what makes elephants so vulnerable. A female is pregnant or nursing for most of her adult life, produces perhaps four to six calves in fifty or sixty years, and cannot compensate for losses quickly. A population reduced by poaching does not bounce back in a decade; it takes generations.

What the calf is born into matters as much as the gestation. Herds are led by the oldest female, and experiments playing recorded calls of strangers found that groups with older matriarchs responded appropriately while those with younger ones did not - knowing which voices are dangerous takes decades to learn, as does knowing where water is in a drought. Killing the oldest animal, which poachers do because she has the largest tusks, removes information the herd cannot reconstruct.