Endothermic vs. Ectothermic

Deinonychus was a highly agile and active organism which would indicate that it would need a high metabolic rate to support this (McGowan, 1991). High metabolic rates are indicative of endothermy as well as a high ratio of bone to body fluid exchange which is similar to that of large mammals. This bone to body fluid ratio would indicate whether an organism has a Haversian System, which is a network of fine penetrating vessels for supplying the bone tissue with nutrient rich blood. If the organism did have a Haversian System then it would be capable of rapid growth and have a high metabolic rate. Microscopic examination of Deinonychus's bones indicates that it did have a Haversian System (Wilford, 1985).

Haversian bone is also present in some turtles and crocodilians, which are ectothermic organisms. It is also absent in small mammals and birds, thus it alone can not prove whehter an organism if endothermic but with other evidence it can support the notion (Wilford, 1985).

Deinonychus had an erect posture which means that a certain degree of blood pressure is needed to sustain it. An advanced four chamber heart is needed to maintain the blood pressure, as evident in modern endotherms. However, it is unknown whether Deinonychus had such a heart as it is unlikely htat such an organ would ever be fossilized (Wilford, 1985). However it does seem logical that, as it had a very active lifestyle, it would require such a heart to maintain it. This in turn would indicate that it might have been an endotherm.

Bakker has stated that the ratio of predation to prey indicates endothermy when there is more prey per predator ie/ if the predators make up between 1-6% of the total population then the predators were endothermic. However, if the predators make up between 35-60% of the total population then they would have been ectothermic. The reason for these numbers is that predators who are endothermic must consume more prey than predators who are ectothermic. Deinonychus's fossil record indicates that it made up between 1-3% of the total population, indicating that it was an endotherm (Wilford, 1985).

However, there are problems with this and they are that the fossil record is highly incomplete. Only a fraction of those organisms that lived are fossilized, thus those numbers which Bakker has accumulated are highly subjective.

Many Palaeontologists including Ostrom have thought that if any dinosaurs were to be endothermic, then it would be the small carnivorous ones such as Deinonychus (Wilford, 1985).