Thermal niche differences among Brazilian caimans are unrelated to blood oxygen affinity

As ectothermic organisms, most crocodilians adjust their body temperature by behavioral thermoregulation. However, the dwarf caiman (Paleosuchus palpebrosus) is unusual in this respect by being a thermoconformer that tolerates lower body temperatures than its larger sympatric crocodilian species. This suggests that P. palpebrosus is physiologically adapted to lower temperatures, which would allow it to occupy environments inhospitable to sympatric and larger species, thus avoiding competition for niche partitioning. Temperatures affect blood oxygen affinity and, consequently, oxygen delivery to the tissues. Since O2-hemoglobin affinity and its temperature-dependence may vary amongst species, we hypothesized that P. palpebrosus has a lower blood oxygen affinity and a lower temperature sensitivity than the sympatric species. Using a diffusion chamber, we measured whole blood oxygen affinity from the dwarf caiman, P. palpebrosus, and two of the co-occurring caiman species in Brazil: the broad-snouted caiman, Caiman latirostris, and the spectacled caiman, Caiman crocodilus. Measurements were made at 20 and 30°C and blood was buffered for pH = 7.4 at both temperatures. The P50 values at 20°C (2.98±0.39 mmHg for P. palpebrosus, 3.04±0.99 mmHg for C. latirostris, and 3.13±0.64 mmHg for C. crocodilus) were lower than P50 at 30°C (11.44±2.02 mmHg for P. palpebrosus, 8.71±0.93 mmHg for C. latirostris, and 9.19±1.55 mmHg for C. crocodilus), but did not vary among species in either temperature. Consequently, enthalpy values (-23.5±3.7 kcal/mol for P. palpebrosus, -19.0±6.5 kcal/mol for C. latirostris, and -19.0±5.9 kcal/mol for C. crocodilus) were also similar for all the species tested, indicating a similar change in O2 blood affinity with temperature. The present results do not support the hypothesis that particular aspects of the blood oxygen binding properties provide certain physiological adaptations underlying the unique tolerance to low temperatures in the dwarf caimans compared to the larger sympatric crocodilian species.