Blood flow to long bones indicates activity metabolism in mammals, reptiles and dinosaurs

Abstract

The cross-sectional area of a nutrient foramen of a long bone is related to blood flow requirements of the internal bone cells that are essential for dynamic bone remodelling. Foramen area increases with body size in parallel among living mammals and non-varanid reptiles, but is significantly larger in mammals. An index of blood flow rate through the foramina is about 10 times higher in mammals than in reptiles, and even higher if differences in blood pressure are considered. The scaling of foramen size correlates well with maximum whole-body metabolic rate during exercise in mammals and reptiles, but less well with resting metabolic rate. This relates to the role of blood flow associated with bone remodelling during and following activity. Mammals and varanid lizards have much higher aerobic metabolic rates and exercise-induced bone remodelling than non-varanid reptiles. Foramen areas of 10 species of dinosaur from five taxonomic groups are generally larger than from mammals, indicating a routinely highly active and aerobic lifestyle. The simple measurement holds possibilities offers the possibility of assessing other groups of extinct and living vertebrates in relation to body size, behaviour and habitat.Keywords: metabolic rate; blood flow; bone remodelling; nutrient foramen; allometry INTRODUCTIONThe size of blood vessels is dynamically variable, responding to the blood flow requirements through them. Vessels throughout the body react specifically to greater blood pressures by thickening and strengthening the walls, and to greater shear stress (related to the velocity of the blood) by increasing circumference [1,2]. Organs that have higher metabolic rates require higher flow rates and therefore have larger blood vessels that service them. To test the hypothesis that differences in blood perfusion rates reflect differences in metabolic capacity between species, this study examines the correlations between femoral nutrient foramen size, bone volume, body mass and resting and maximum metabolic rates in living mammals and reptiles. As bones are the only tissues remaining from non-avian dinosaurs, the relationships between nutrient foramina size and metabolic capacity developed for mammals and reptiles can indicate the levels of activity and metabolic status of dinosaurs.Long bones of all amniotes, comprising the reptiles, mammals and birds, receive blood from three sources: (i) nutrient vessels, (ii) metaphyseal and epiphyseal vessels that combine after the cartilaginous growth plate is closed and (iii) periosteal vessels [3]. The nutrient system contributes around 50 -70% of the blood supply

Foramina in plesiosaur cervical centra indicate a specialized vascular system

“…First, the foramina could each have housed a normal nutrient blood vessel pair, consisting of an artery which enters the bone, delivering the oxygenated blood to the interior of the bone, and a vein draining the bone interior. This is the classical situation seen in long bones with a single large nutrient canal (e.g., Seymour et al, 2012;Nakajima et al, 2014) through which a terminal artery, known as arteria nutricia, enters and then ends inside the bone. Since osteogenesis of the vertebral centrum follows the same rules as that of a long bone, this is a plausible hypothesis for the ventral foramina, as recognized by Storrs (1991).…”

Dinosaur Energetics: Setting the Bounds on Feasible Physiologies and Ecologies

“…Recently Seymour et al (2012) have examined the vascularisation of long bones in dinosaurs, comparing this with mammals and extant reptiles. The scaling of foramen area with bone length for femurs is distinctly different in mammals and non-varanid reptiles, being greater in mammals.…”

A strong response to selection on mass-independent maximal metabolic rate without a correlated response in basal metabolic rate

“…It is challenging to explain how vertebrate endotherms evolved from their ectothermic ancestors because this evolutionary change occurred over 100 million years ago (Ruben, 1995) and fossils can rarely be used to determine metabolic status reliably (Seymour et al, 2012;Grady et al, 2014). Nonetheless, two general classes of models attempt to explain how endothermy may have arisen.…”