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 8 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 4 weeks of life, comparing these to somatic growth measurements. Contrary to our expectations, we did not find any differences in brain size between the 2 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 prenatal 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.