Altricial-precocial spectra in animal kingdom

Abstract

Traditionally, the terms altricial and precocial are used to described the state at birth of birds and mammals. We suggest an explanation for why birds evolved from precocial to altricial and mammals from altricial to precocial. However, the concept is not confined to these groups alone and is an important pattern underlying all animal metabolism. Being able to quantify it, helps position species in multi-dimensional diversity trait space. We here argue that a quantification of the state of development of an individual at birth, in the context of its life cycle, can best be done by comparison with the state at puberty. Two natural quantifiers for the altricial-precocial spectrum exist in the context of the Dynamic Energy Budget (DEB) theory that are applicable to the whole animal kingdom: maturity and maturity density at birth divided by that at puberty. These quantities have been estimated, in combination with other parameters, for some 875 species belonging to all large phyla. We study how taxa are ranked according to both the maturity and maturity density ratios. We conclude that only the maturity ratio qualifies as quantifier for the altricial-precocial spectrum. We were able to retrieve known patterns in altriciality for birds and mammals, while the concept is now applicable to all animal taxa. This new quantifier for animal altriciality can be linked quantitatively to other properties. These linkages are studied by examining how maturity parameters interact with other DEB parameters and implied properties. With the exception of mammals, cartilaginous fish and insects, bigger-bodied species show a clear tendency for to be more frequently altricial. We discuss possible explanations. Age at birth as fraction of age at puberty was found to be proportional to the maturity ratio to the power 1/3; this is consistent with DEB's co-variation rules, but does not follow from them. Maturity at puberty follows a Weibull distribution with great accuracy while that at birth does not, which is consistent with the idea that Weibull distributions can be expected when many factors contribute. We found that species from all taxa with a very high allocation fraction to soma, tend to have low maturities at birth and puberty. We discuss the possible implications of these patterns.