Meeting Abstract
Together with ratites, the tinamous are the most primitive extant birds (clade Palaeognathae). Unlike ratites though, tinamous can fly. The tinamous have the smallest heart size among all existing birds, which is expected to limit aerobic scope. Our aim was to characterize heart morphometry and to measure metabolic and cardiovascular function in two species of the genus Nothoprocta, the Ornate Tinamou OT, a highland species and the Chilean Tinamou CT, a lowland species. Relative heart size was 0.24% for OT and 0.28% for CT, significantly smaller than high and lowland chickens (0.54% and 0.42% respectively). Resting metabolic rate is 31% lower in OT than in highland chickens. When exhausted, OT and CT had elevated glucose and lactate levels suggesting a severe oxygen debt, and OT also showed limited capacity to maintain body temperature with a significant and persistent drop in body temperature (over 2h) after the exhaustive bout. Heart rate running on a treadmill at 3 km h-1 was 5% lower in OT, indicating that OT cannot compensate for the reduction in heart size with a faster heart rate. In anesthetized birds, cardiac output and stroke volume of OT was 40% of the cardiac output of chickens in identical conditions. Blood pressure was significantly higher in OT. Altogether, we provide evidence that heart size is a phylogenetically conserved trait among tinamous and that OT cannot compensate aerobically for its small heart. Instead, it relies on anaerobic metabolism incurring in a large oxygen debt while exhausted.
