The small heart of the Ornate Tinamou is compatible with endothermy and flight but compromises aerobic metabolism and thermoregulation during recovery from exhaustive activity

Abstract

Tinamous are an ancient family of neotropical flying birds. Scant data from a few species show that they have a small heart so our aim was to characterize relative heart size of two species of the genus Nothoprocta and assess the physiological limitations associated with a small heart size. Relative heart size (0.24% for the Ornate Tinamou OT, a highland species and 0.28% for the Chilean Tinamou CT, a lowland species) was significantly smaller than high and lowland chickens (0.54% and 0.42% respectively), without evidence of right ventricular hypertrophy. Resting aerobic metabolism was 31% lower in OT than in highland chickens. When subjected to exhaustive activity, OT had elevated glucose and lactate levels suggesting a severe oxygen debt when exhausted. This was further shown as a significant drop in body temperature after an exhaustive bout. Finally heart rate while running on a treadmill at 3 km h−1 was 5% lower in OT, indicating that tinamous cannot compensate for the reduction in heart size with a faster heart rate. Altogether, we provide evidence that heart size is a phylogenetically conserved trait among tinamous and that the Ornate Tinamou cannot compensate aerobically for its small heart. Instead, it relies on anaerobic metabolism incurring in a large oxygen debt while exhausted. Supported by FORMAS Centre of Excellence in Animal Welfare Science and career grant from Linköpings universitet to JA.