Alligators are cold-blooded reptiles that retain their body heat by seeking out warm environments. But body temperature changes can be a dangerous prospect for the heart. A healthy heart maintains a steady beat by propagating electrical signals throughout the tissue, but low temperatures interfere in this process by changing the electrical capacity of the muscle, possibly causing lethal irregular heartbeats. To learn what keeps alligator hearts safely ticking at low temperatures, Conner Herndon and his colleagues at Georgia Tech in Atlanta, Georgia, USA, set out to investigate why alligator hearts appear to handle temperature fluctuations much better than those of other species, such as mammals, that must maintain a constant body temperature.
Herndon and the team collected heart tissue from several American alligators in order to test how well electrical signals pass through the muscle. The team then trained high-frequency cameras on the hearts and stimulated them with electrodes at a range of heartbeat frequencies to calculate the speed of the electrical signal and how far the signal from one heartbeat travelled across the tissue at temperatures ranging from 23°C to 38°C. In addition, the team repeated the experiments on the hearts of rabbits – an animal that must maintain a constant body temperature to survive – to find out how well both hearts coped at low and high temperatures.
At low body temperatures (23°C), the alligator hearts maintained function much better than the rabbit's: the electrical propagation speed only dropped by 20% in the alligator heart while the rabbit experienced a 67% drop. The distance that the signal travelled in the rabbit's heart shortened to an alarming distance reaching only 3 cm – equivalent to the length of its heart. Shorter stimulus lengths put the animal at risk of heart failure because the muscle tissue farthest away is at risk of being stimulated by stray electrical signals emanating from other sites in the heart, causing an irregular heartbeat. In contrast, alligators maintained a longer propagation distance of 6 cm – double the 3 cm length of their hearts – ensuring full stimulation of the entire muscle tissue and well-coordinated heartbeats at low body temperatures.
The team also found that the alligators’ low heart rate helped protect it against the effects of cooler body temperatures. The alligators’ maximal heart rate only dropped from 70 beats min−1 at 38°C to 60 beats min−1 at 23°C, while the rabbits’ heart rates dropped from 400 to 176 beats min−1. Therefore, the alligator's longer and steadier heartbeat in the face of cold temperatures ensured better coordination of the electrical activity preventing irregular heartbeats.
In short, the alligator's heart protects itself from low body temperatures by keeping robust electrical conduction so that it is unaffected by falls in temperature that would prove fatal for other species. The results underscore the trade-off between species that are restricted to lower heart rates but can function over a range of temperatures, such as the alligator, and species that can sustain a high heart rate but are vulnerable to low temperatures. The results also provide new insights into physiological adaptations of reptile hearts that allow them to maintain fluctuating body temperatures: a slow, steady tick for a cool heart.