Early diversification of the avian brain:body relationship

Abstract

Scaling of avian brain:body mass throughout the diversi®cation of the class was investigated by analysis of a large collection of adult brain and body masses. Linear regression model analysis of whole-class brain:body scaling resulted in scaling exponents ranging from 0.574 to 0.609, values which exclude several prior empirical and theoretical estimates. Taxonomic level-speci®c analysis of brain:body scaling was performed by major-axis regression of trait variances partitioned among levels of taxonomic distinction. Brain:body scaling exponents varied markedly among avian orders, but were not easily related to ecological differences among taxa. Avian brain:body scaling exhibited a partial taxon-level effect, in that scaling exponents vary with the taxonomic level of investigation. However, scaling exponents were greatest at the family level, a pattern not consistent with prior ontogenetic or genetic covariance models of trait diversi®cation. Instead, it is suggested that initial diversi®cation among birds was largely through body size diversi®cation, while later diversi®cation of families within orders contained a relatively greater degree of brain size diversi®cation. Avian developmental mode, known to in¯uence avian brain size at hatching, was associated with relatively little variance in adult brain mass. Avian brain:body diversi®cation has occurred relatively uniformly in precocial taxa, while diversi®cation within altricial taxa is marked by a relatively high degree of brain mass diversi®cation among families within orders.

Big brains, enhanced cognition, and response of birds to novel environments

“…It is commonly found in comparative analyses that a pattern is stronger at higher taxonomic levels (55) because of a greater signal-to-noise ratio in comparisons between less closely related taxa, making it easier to detect real trends. In our case, much of the variation in brain size resides at the family level (59). This high variation is likely to have further facilitated the detection of clear patterns, emphasizing the utility of adopting a hierarchical approach (66) in future analyses of brain evolution.…”

“…Most of the variation in relative brain size resides at higher taxonomic levels (37,58), which can been interpreted as extensive diversification in brain size early in the avian radiation (59). Thus, it is relevant to consider how variation in relative brain size relates to variation in establishment success at higher taxonomic levels, such as among families.…”

Small organ size contributes to the slow pace of life in tropical birds

“…Feather mass was greater in temperate species, a likely requirement for their larger thermoregulatory demand. Despite its high energy turnover and diversification in size among bird taxa (Nealen and Ricklefs, 2001), brain size was similar in tropical and temperate birds.…”

A Comparative Analysis of Social Play in Birds

“…For the brain size measures, we log-transformed our tabled values and calculated residuals from Nealen and Ricklefs' (2001) major axis regres-sion of avian brain and body masses. To correct the age of first reproduction for body size effects, we combined the values in Table 2 with the broader survey provided in Appendix 2 of Gaillard et al (1989) and conducted a major axis regression on the log-transformed data (Sokal and Rohlf, 1981;Seim and Saether, 1983).…”

“…Brain size and body size for this set of 66 species were log-transformed, and residual brain sizes were calculated from Nealen and Ricklefs' (2001) allometric regression (Figure 1b). For analysis, the species residuals were averaged within genera, as discussed above, and genera that were included in the social play data were removed.…”