Habitat complexity influences neuron number in six species of Puerto Rican Anolis

Abstract

Elucidating the selective forces shaping the diversity of vertebrate brains continues to be a major area of inquiry, particularly as it relates to cognition. Historically brain evolution was interpreted through the lens of relative brain size; however, recent evidence has challenged this approach. Investigating neuroanatomy at a finer scale, such as neuron number, can provide new insights into the forces shaping brain evolution in the context of information processing capacity. Ecological factors, such as the complexity of a species' habitat, place demands on cognition that could shape neuroanatomy. In this study, we investigate the relationship between neuron number and habitat complexity in three brain regions across six closely related anole species from Puerto Rico. After controlling for brain mass, we found that the number of neurons increased with habitat complexity across species in the telencephalon and 'rest of the brain,' but not in the cerebellum. Our results demonstrate that habitat complexity has shaped neuroanatomy in the Puerto Rican anole radiation and provide further evidence of the role of habitat complexity in vertebrate brain evolution.

Keywords: Anolis; habitat complexity; isotropic fractionator‌; neuroanatomy; neuron number.

Figure 1.

Relationship between relative neuron density and relevant habitat complexity. Each point represents an estimate for a single individual, with colour representing species and shape distinguishing different ecomorphological classes. Relevant neuron density is the residual for each individual when the number of neurons is regressed on brain region mass, yielding a normalized measure of neuron density in neurons per milligram. Habitat complexity decreases with increasing values of RHC. Significant relationships are plotted with a solid line.