Meeting Abstract
Arterial thermoregulatory mechanisms (e.g., carotid retia in artiodactyls and felids) have been recognized as drivers of selective brain cooling, which is important for some endotherms that live in predominately hot climates. Among birds, thermal physiology and osmoregulation (e.g., water balance, renal function) have been studied in the largest extant bird, the common ostrich, a giant flightless species native to African deserts. Adults have been shown to utilize ophthalmic retia—arterial structures convergently similar to mammalian carotid retia—to aid brain and eye cooling. How this feature forms and grows during ontogeny, however, is not well understood. Rete development is of interest because ostriches grow rapidly, attaining most of their body mass in the first 1.5 years of life. Thermal and osmoregulatory needs may be in flux during this period due to the individual or combined effects of body-mass, metabolic, and arterial surface-area scaling. To elucidate ophthalmic rete ontogeny, we employed vascular injection and µCT scanning on a developmental series of ostriches to visualize and digitally measure retial, endocranial, and eye volumes using Avizo 9. Retial volumes appear to scale with negative allometry, such that young ostriches hatch with relatively well-developed retia that change little during the accelerated growth of their early ontogeny. We compare this finding with rete ontogenies in other endotherms capable of arterial cooling to more broadly address the configuration of such structures in the context of thermal and osmoregulatory issues faced by large-bodied vertebrates due to climate change.
