In the episode Islands of Planet Earth II, majestic marine iguanas steal the show with their ability to thrive on a barren, volcanically active island. These marine iguanas live on Fernandina, one of the Galapagos Islands, and can swim up to 30 meters by holding their breath for up to half an hour [1]. But why? The lack of food on land forces marine iguanas to look for food in the sea. They dive in the surrounding sea to eat algae that grows on the ocean floor. This selective pressure to find food has led to several adaptations that allow marine iguanas to stay submerged under water and gather food. Notably, their heart rates drop from 40 beats per minute (bpm) to 10 bpm while diving underwater and then immediately increases back to 40 bpm once they surface [1]. Their decreased heart rates allow the iguanas to conserve oxygen during their long dives.
Marine iguanas aren’t the only species to develop bradycardia, a slow heart rate, when diving under water. The tufted duck also exhibits bradycardia when diving and selectively constricts blood vessels to increase flow to vital organs [2]. Much like the marine iguanas, tufted ducks must dive to find food. In leatherback sea turtles, heart rates when diving are as low as 1.05 bpm [3]. Not all deep divers use bradycardia to regulate oxygen consumption when submerged underwater. Sea snakes redirect blood to reduce the amount of oxygen lost through the skin [4]. Selectively shunting their blood allows them to conserve oxygen and go on long dives much like the marine iguana. This example shows that natural selection has led to several independent adaptations that allow organisms to spend more time under water.
by Eunice Lee, Kelsey Berman and Birk Evavold
Planet Earth II, Episode 1, Islands, starting at approximately 18:56
References
[1] Bartholomew GA & RC Lasiewski. 1965. Heating and cooling rates, heart rate and simulated diving in the Galapagos Marine Iguana. Comparative Biochemistry and Physiology 16: 573-582.
[2] Woakes AJ & PJ Butler. 1983. Swimming and diving in tufted ducks, Aythya fuligula, with particular reference to heart rate and gas exchange. Journal of Experimental Biology 107: 311-329.
[3] Southwood AL., Andrews RD, Lutcavage ME, Paladino FV, West NH, George RH & DR Jones. 1999. Heart rates and diving behavior of leatherback sea turtles in the eastern Pacific Ocean Journal of Experimental Biology 202: 1115-1125.
[4] Seymour RS & MED Webster. 1975. Gas transport and blood acid‐base balance in diving sea snakes. Journal of Experimental Zoology Part A: Ecological Genetics and Physiology 191: 169-181.
The Evolution of Planet Earth 