A critique of comparative studies of brain size

Abstract

Newcastle upon Tyne NE2 4HH, UKIn recent years, there have been over 50 comparative analyses carried out in which social or ecological variables have been used to explain variation in whole brain size, or a part thereof, in a range of vertebrate species. Here, we review this body of work, pointing out that there are a number of substantial problems with some of the assumptions that underpin the hypotheses (e.g. what brain size means), with the data collection and with the ways in which the data are combined in the analyses. These problems are particularly apparent in those analyses in which attempts are made to correlate complex behaviour with parts of the brain that carry out multiple functions. We conclude that now is the time to substantiate these results with data from experimental manipulations.

Socially induced brain development in a facultatively eusocial sweat beeMegalopta genalis(Halictidae)

“…Byrne & Bates 2007a,b;Healy & Rowe 2007;Dunbar & Schultz 2007). Applying this hypothesis to insect societies in general may be problematic because insect sociality spans a wider spectrum of social organization than for vertebrates.…”

“…Direct tests of the basic prediction of the social brain hypothesis-that the specific cognitive challenges imposed by sociality should be reflected in brain organization-have faced several difficulties, which can be overcome by intraspecific studies with insects. One problem involves quantifying social complexity, or measuring behavioural repertoires, across different taxa ( Facultatively social bee brain volume A. R. Smith et al 2161 (Healy & Rowe 2007). The relatively specific functions of different lobes in the insect brain facilitate tests of whether brain morphology reflects presumed cognitive challenges (e.g.…”

“…Edelman & Changeux 2001;Ricklefs 2004;Goodson et al 2005;Healy & Rowe 2007). Neural tissue is energetically expensive, and thus specific brain regions should enlarge only when needed to meet functional demands (Niven & Laughlin 2008).…”

“…Pérez-Barbería et al 2007). It is often problematic, however, to compare social behaviour across species because of the lack of uniform behavioural metrics or biologically meaningful definitions of social complexity (Byrne & Bates 2007b;Healy & Rowe 2007). It also may be problematic to apply the hypothesis to the many social insects, such as ants and honeybees, in which individual decision-making is relatively simplified and social complexity arises via self-organization, rather than via increasingly sophisticated individual behaviour (see Gronenberg & Riveros 2009 and references therein).…”

Endocranial volumes of primate species: scaling analyses using a comprehensive and reliable data set

“…One important limitation of previous research into primate brain evolution has been sample quality (reviewed in Healy and Rowe, 2007). Most authors have relied either on brain mass data derived from very small samples (e.g., Stephan, 1966, 1969;Stephan et al, 1970;Stephan et al, 1981) or on endocranial volume data from compilations without specification of sample size or sex (e.g., Clutton-Brock and Harvey, 1980).…”