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Were dinosaurs lethargic, cold-blooded creepers or lively, warm-blooded creatures? This controversy has plagued paleobiologists for decades and prompted Roger Seymour at the University of Adelaide and collaborators in Australia, Canada and Germany to employ a novel measurement to investigate the matter: one in our very bones! The nutrient foramen of a bone is an opening through which blood vessels enter, supplying blood to the bone cells inside. These active cells require the oxygen and nutrients in blood to dissolve older bone and to deposit new bone during growth and maintenance (remodelling) throughout life. Seymour wondered whether the size of these entryways might indicate the amount of blood required to sustain the bone. Highly active animals might require more bone maintenance (because of a higher incidence of tiny fractures in bone caused by locomotion) and consequently a greater blood supply.

The team measured the nutrient foramen, volume and mass of the femur (the large leg bone crucial for support and locomotion) in 59 mammalian species and 40 reptilian species, ranging in size from mice to elephants and geckos to crocodiles. They then set about assessing how these data relate to body mass and metabolic rate values obtained from previous experiments. The researchers discovered that larger animals have larger bones and larger bone holes, but the holes in mammals are much larger than those of most reptiles of the same body mass.

The team also found that the scaling of foramen size is closely related to maximum whole-body metabolic rate during exercise in both mammals and reptiles, though less closely related to basal metabolic rate. This suggests that blood supply through the holes relates to bone maintenance requirements following activity, and that blood flow to bone is about 10 times higher in mammals than in reptiles. This fits with the fact that mammals have much higher metabolic rates during exercise than reptiles, requiring more oxygen to fuel their muscles and other tissues, including the cells responsible for bone remodelling. In addition to a larger nutrient foramen, mammals also benefit from higher blood pressures and blood that can carry more oxygen, all of which contribute to a much larger oxygen supply for mammalian tissues. Interestingly, varanid reptiles, including active hunters like the Komodo dragon and monitor lizards, had a foramen size similar to that of mammals. Varanids are capable of increasing their metabolic rates far above those of other reptiles, filling a niche akin to mammalian predators. Varanids also remodel their long bones, but other living reptiles apparently do not.

Next, the researchers took their analysis one step further to fossils from 10 species of dinosaur. They found that dinosaurs have even larger bone holes than mammals (even after correcting the results for body mass differences). This suggests that dinosaurs were highly active and lively, perhaps even more so than mammals! Make no bones about it, this implies that the notion of dinosaurs as sluggish and sloth-like may soon be as extinct as the charismatic creatures themselves. Novel applications of these parameters could also provide insight into activity levels of other living and extinct vertebrates.

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