Precociality and altriciality
In the biology of birds and mammals, altricial species are those in which the young are underdeveloped at the time of birth, but with the aid of their parents mature after birth. Precocial species are those in which the young are relatively mature and mobile from the moment of birth or hatching. They are normally nidifugous, meaning that they leave the nest shortly after birth or hatching. These categories form a continuum, without distinct gaps between them.
Precociality[edit]
Superprecociality[edit]
Extremely precocial species are called "superprecocial". Examples are the megapode birds, which have full-flight feathers at hatching and which, in some species, can fly on the same day they hatch from their eggs.[4] Enantiornithes[5] and pterosaurs were also capable of flight soon after hatching.
Another example is the blue wildebeest, the calves of which can stand within an average of six minutes from birth and walk within thirty minutes;[6][7] they can outrun a hyena within a day.[8][9][10] Such behavior gives them an advantage over other herbivore species and they are 100 times more abundant in the Serengeti ecosystem than hartebeests, their closest taxonomic relative. Hartebeest calves are not as precocial as wildebeest calves and take up to thirty minutes or more before they stand, and as long as forty-five minutes before they can follow their mothers for short distances. They are unable to keep up with their mothers until they are more than a week old.[10]
Black mambas are highly precocial; as hatchlings, they are fully independent, and are capable of hunting prey the size of a small rat.[11]
Phylogeny[edit]
Precociality is thought to be ancestral in birds. Thus, altricial birds tend to be found in the most derived groups. There is some evidence for precociality in protobirds[12] and troodontids.[13] Enantiornithes at least were superprecocial in a way similar to that of megapodes, being able to fly soon after birth.[5] It has been speculated that superprecociality prevented enantiornithines from acquiring specialized toe anatomy seen in modern altricial birds.[14]
Terminology[edit]
In birds, the terms Aves altrices and Aves precoces were introduced by Carl Jakob Sundevall (1836), and the terms nidifugous and nidicolous by Lorenz Oken in 1816. The two classifications were considered identical in early times, but the meanings are slightly different, in that "altricial" and "precocial" refer to developmental stages, while "nidifugous" and "nidicolous" refer to leaving or staying at the nest.[19]
The two strategies result in different brain sizes of the newborns compared to adults. Precocial animals' brains are fully developed at birth relative to their body size, limiting their knowledge largely to their instincts but providing them immediate access to their bodies. However, as adults, their brains do not develop any further, and they memorize little from birth to death. Altricial animals' brains are comparatively undeveloped at birth, thus their need for care and protection, but their brains immediately begin developing at birth and do so independently, adapting to individual circumstances, and hence as adults, altricial animals attain more versatile brains than their precocial counterparts, capable of strategic thought based on experience. Thus altricial species, ultimately, possess greater potential for skill and adaptability, which is notably a factor in human developmental success.[2]