<p>Evolutionary increases in mammalian brain size relative to body size
are energetically costly but are also thought to confer selective
advantages by permitting the evolution of cognitively complex behaviors.
However, many suggested associations between brain size and specific
behaviors - particularly related to social complexity - are possibly
confounded by the reproductive diversity of placental mammals, whose
brain size evolution is the most frequently studied. Based on a
phylogenetic generalized least squares analysis of a data set on the
reproductively homogenous clade of marsupials, we provide the first
quantitative comparison of two hypotheses based on energetic constraints
(maternal investment and seasonality) with two hypotheses that posit
behavioral selection on relative brain size (social complexity and
environmental interactions). We show that the two behavioral hypotheses
have far less support than the constraint hypotheses. The only
unambiguous associates of brain size are the constraint variables of
litter size and seasonality. We also found no association between brain
size and specific behavioral complexity categories within kangaroos,
dasyurids, and possums. The largest-brained marsupials after
phylogenetic correction are from low-seasonality New Guinea, supporting
the notion that low seasonality represents greater nutrition safety for
brain maintenance. Alternatively, low seasonality might improve the
maternal support of offspring brain growth. The lack of behavioral brain
size associates, found here and elsewhere, supports the general
‘cognitive buffer hypothesis' as the best explanatory framework of
mammalian brain size evolution. However, it is possible that brain size
alone simply does not provide sufficient resolution on the question of
how brain morphology and cognitive capacities coevolve.</p