Meeting Abstract
Selective brain cooling (SBC) is a mechanism by which artiodactyl mammals stabilize brain temperatures below rising body temperatures. By lowering hypothalamic temperature, SBC significantly reduces evaporative water loss (≤6 liters/day). SBC is thus a hypothesized adaptation for surviving warming and drying climates. This physiology is driven by counter-current heat exchange across a high surface area cerebral arterial meshwork called the carotid rete (CR). The CR functionally and anatomically replaces the internal carotid artery (ICA), delivering cooled blood to the brain. Absence of the ICA relegates different branches of the embryonic aortic arches to supply the CR and brain. Compensatory branches vary on a suborder-specific basis: Suinamorpha by aortic arches 2 and 3; Camelidamorpha by arch 3; and Ruminantiamorpha by arch 1. These divergent cranial arterial development patterns suggest that there may be different mechanisms for achieving SBC within artiodactyls, and that the CR may be homoplastic. Here, I use basicranial osteological correlates to survey aortic arch contributions to the CR across Artiodactyla. I then map these results onto a phylogeny of artiodactyls and use ancestral character estimation to infer the evolutionary history of CR development. This analysis infers independent evolution of the CR and SBC for each suborder. Features that arise via such homoplastic parallelism are typically considered to be the result of developmental constraints, rather than the result of adaptive responses to selective pressures. This is a surprising conclusion given the hypothesis that the CR and SBC are adaptations to hot, dry environments. Future studies should incorporate specimens from the fossil record to better parse between parallel and convergent evolutionary mechanisms that may underlie cerebral arterial patterns in artiodactyls.
