Altricial

Altricial means "requiring nourishment" and refers to a pattern of growth and development in organisms which are incapable of moving around on their own soon after hatching or being born. The word is derived from the Latin root "alere" meaning "to nurse, to rear, or to nourish" and refers to the need for young to be fed and taken care of for a long duration.Ehrlich, P., D. Dobkin, D. Wheye. 1988. The birder's handbook: A field guide to the natural history of North American birds. NY: Simon & Schuster Inc.]

In bird and mammal biology, altricial species are those whose newly hatched or born young are relatively immobile, have closed eyes, lack hair or down, and must be cared for by the adults. Altricial young are born helpless and require care for a comparatively long time. Among birds, these include, for example, herons, hawks, woodpeckers, owls and most passerines. Rodents and marsupials are altricial, as are cats, dogs and humans.

On the opposite end of the spectrum are precocial animals in which the young have open eyes, have hair or down, large brains and are immediately mobile and somewhat able to defend themselves against predators. For example, with birds that nest on the ground, such as ducks or turkeys, the young are ready to leave the nest in one or two days. Among mammals most ungulates are precocial, being able to fend for themselves almost immediately after birth.

Different animals employ different precocial and altricial strategies with no clear distinction between the two states and having a range of intermediate states. The ability of female to obtain nutrition and contribute to the pre-natal and post-natal development of their young appears to be associated. Precocial females are able to provide protein-rich eggs and such their young hatch in the fledgling stage - able to protect themselves from predators (ducks or turkeys) and the females have less involvement post-natal. Altricial females are less able to contribute nutrients in the pre-natal stage; their eggs are smaller and their young still in need of much attention and protection from predators. This may be related to r/K selection; however, this association fails with a number of cases.Starck, J. M. & R. E. Ricklefs (1998) Avian Growth and Development: Evolution within the Altricial-Precocial Spectrum. Oxford University Press. ISBN 0-19510608] In the case of mammals it has been suggested that large adult body sizes favor production of large, precocious young, which develop with a long gestation period. Large young may be associated with long lifespan, extended reproductive period and reduced litter sizes. It has been suggested that altricial strategies in mammals may be favoured if there is a selective advantage to mothers that are capable of resorbing embryos in early stages of development. [Eisenberg, J.F. 1981. The Mammalian Radiations. An Analysis of Trends in Evolution, Adaptation, and Behavior. Athlone Press, London.]

In birds, the terms "Aves altrices" and "Aves precoces" was introduced by Sundeval (1836) and the terms nidifugous and nidicolous by Oken (1816). The two classifications were considered identical in early times, but the meanings are slightly different in that altricial-precocial refer to developmental stage while nidifugous and nidicolous refer to their leaving or staying at the nest.

The two strategies result in different brain sizes of the newborns compared to adults. Precocial animals' brains are large at birth relative to their body size, hence their ability to fend for themselves. However, as adults, their brains are not much bigger or more able. Altricial animals' brains are relatively small at birth, thus their need for care and protection, but their brains continue to grow. As adults, altricial animals end up with comparatively larger brains than their precocial counterparts. Thus the altricial species have a wider skill set at maturity.

References

External links

* [http://www.stanford.edu/group/stanfordbirds/text/essays/Precocial_and_Altricial.html The altricial-precocial spectrum in birds]
* [http://www.biologie.uni-muenchen.de/ou/morpho/downloads/agd1.pdf Avian Growth and Development. Evolution within the altricial precocial spectrum. J. M. Starck and R. E. Ricklefs (eds). Oxford University Press, New York, 1998 Chapter 11]