Data from: The evolution of a placenta is not linked to increased brain size in poeciliid fishes

Maternal investment is hypothesized to have a direct influence on the size
of energetically costly organs, including the brain. In placental
organisms, offspring are supplied with nutrients during prenatal
development, potentially modulating brain size. Previous research has
predominantly focused on mammalian species that exhibit both pre- and
postnatal provisioning, in which effects on brain size have been observed
during both developmental stages. Here, using eight poeciliid fish
species, we test if those species with placental structures (i.e.,
matrotrophy) invest more resources into offspring brain development than
species without placental structures (i.e., lecithotrophy). The prediction
is that matrotrophy may entail higher nutrient provisioning rates to the
developing embryo, resulting in larger offspring brain sizes, compared to
species with a lecithotrophic strategy. To test this prediction, we took
non-invasive brain size measurements during the first four weeks of life,
comparing these to somatic growth measurements. Contrary to our
expectations, we did not find any differences in brain size between the
two maternal strategies in poeciliids. Furthermore, we did not find any
differences in how relative brain size changed over ontogenetic
development, between placental and non-placental species. In contrast to
the marsupial/placental transition, the fish species investigated here
only exhibit pre-natal provisioning, which may reduce the potential for
maternal investment into brain size. Consequently, our results suggest
that coevolution between placental structures and juvenile brain size is
not a general pattern in vertebrates.