In the insect and fish worlds, where the female forms are generally larger and stronger than the male, the female is generally more pugnacious and predatory than the male.

Among birds-of-prey, where also the female form is larger and stronger than the male, the psychic differences seem very small.

Among eagles and other allied forms, which are strictly monogamous, the affection of the female for the male is so great that she is said never to mate again if the male dies, and both watch over and care for the young with extreme solicitude.

The ostrich male form, though perhaps larger than the female, shares with her the labour of hatching the eggs, relieving the hen of her duty at a fixed hour daily: and his care for the young when hatched is as tender as hers.

Among song-birds, in which the male and female forms are so alike as sometimes to be indistinguishable, and which are also monogamous, the male and female forms not only exhibit the same passionate affection for each other (in the case of the South African cock-o-veet, they have one answering love-song between them; the male sounding two or three notes and the female completing it with two or three more), but they build the nest together and rear the young with an equal devotion.