Abstract
Adaptive characterizations of primates have usually included a reduction in olfactory sensitivity. However, this inference of derivation and directionality assumes an ancestral state of olfaction, usually by comparison to a group of extant non-primate mammals. Thus, the accuracy of the inference depends on the assumed ancestral state. Here I present a phylogenetic model of continuous trait evolution that reconstructs olfactory bulb volumes for ancestral nodes of primates and mammal outgroups. Parent-daughter comparisons suggest that, relative to the ancestral euarchontan, the crown-primate node is plesiomorphic and that derived reduction in olfactory sensitivity is an attribute of the haplorhine lineage. The model also suggests a derived increase in olfactory sensitivity at the strepsirrhine node. This oppositional diversification of the strepsirrhine and haplorhine lineages from an intermediate and non-derived ancestor is inconsistent with a characterization of graded reduction through primate evolution.
Highlights
In mammals, olfactory receptor neurons situated in the olfactory epithelium of the nasal cavity send axons through cribriform foramina of the ethmoid bone and enter the olfactory fossa
An ongoing challenge for comparative olfaction research is to assess the appropriateness of scaling olfactory bulbs (OB) volume by expressing it as a ratio to some other measurement
body size (BS) and total brain (TB) volume are highly correlated in mammal groups and both of these traits are commonly used as scaling factors in comparative research
Summary
Olfactory receptor neurons situated in the olfactory epithelium of the nasal cavity send axons through cribriform foramina of the ethmoid bone and enter the olfactory fossa These axons synapse with mitral cells located in the main olfactory bulbs (OB) of the telencephalon. Olfactory sensitivity (the detection threshold of a concentration of odorant molecules) is related to the number of receptors in the olfactory epithelium and the physical size of the OB is correlated with this receptor neuron population size [1,2,3]. On this basis, interspecific studies of mammalian olfaction have typically used OB.
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