Bigger brains are costly. Thus, this must be compensated by increased opportunities to reproduce, such as longer lifespans. The routes to these changes are different for precocial and altricial species. Also, cooperative breeding can mitigate the costs, and so the effects may be absent.
“In all mammals, regardless of developmental mode at birth,
we predicted that neonates should become larger to compensate for the increased
impact of starvation on survival in large brained animals. The results
confirmed this prediction in both monotokous precocial species and polytokous
altricial species, and remained significant when phylogenetic nonindependence
was controlled for by contrast analysis. Thus,
large brained species must invest more in each offspring they produce."
...
In altricial lineages, brain mass was positively correlated
with lifespan and negatively correlated with annual fertility rates and litter
size.
Furthermore, we found that carnivore families in
which mothers often experience help from either the male or other group members
in rearing the offspring are characterised by a complete absence or even a
reversal of brain-mass-life history correlations (Table 2). In raw
species-level analyses, cooperatively breeding large brained species even
managed to produce larger litters in a shorter time period than smaller brained
species, whereas in IC analyses, only lactation length was significantly
negatively correlated to brain mass. Carnivores without helpers showed the
pattern expected for altricial, polytokous mammals.
...
The analyses of a large sample of mammalian species showed
that relatively large brained mammals reduce their fertility rate by either
reducing the number of offspring per litter in multiple litters or by
distributing the reproductive effort over a longer time period if they have
single births. In either case, the reduced fertility is compensated by a longer
lifespan, although not generally enough to maintain net reproductive rates at
the same level (Isler and van Schaik, 2009). However, any form of cooperative breeding, producing energy inputs into the
female or the dependent infants by the male or other group members, allows
some large brained carnivores to even increase their reproductive rate, whereas
maximum lifespan is not prolonged. Thus, the main predictions of the
Expensive Brain hypothesis are fully supported by the results of the
comparative analyses."
This basically says that cooperative breeding allows for
bigger brains without further increases in either reproductive rate, neonate
mass, maximum lifespan, litter size, or a decrease in gestation length,
lactation length, or time to sexual maturity. Cooperative breeding for the win!
