The Galápagos Islands have a rich history in the field of evolutionary biology. In addition to being the islands where Darwin developed his theory of natural selection, it is also home to thousands of species of animals. One of these species is the marine iguana, or Amblyrhynchus cristatus, a land-dwelling reptile that can, fascinatingly, forage for algae in the sea.
The mechanisms that allow marine iguanas to forage for food underwater are based on body size and heating and cooling rates of the iguanas. Marine iguanas feed in two ways: intertidally or subtidally. Intertidal feeding has the advantage of taking less time and thus allows the iguana to feed more frequently, while subtidal feeding takes longer and requires the iguana to dive in the water. Research shows that large marine iguanas favor subtidal feeding despite the efficiency of intertidal feeding, especially when in competition with smaller iguanas, which favor intertidal feeding [3]. Additionally, because marine iguanas spend time both in water and out, they must be able to withstand a high range of temperatures. Ideally, they should be able to heat rapidly while basking in the sun and cool slowly in the ocean while foraging. Research shows that marine iguanas have heating rates much higher than their cooling rates, which would support this idea [1]. It also shows that larger male marine iguanas are favored by females and that larger females also produce larger eggs and larger clutches, which are better for the survival of the hatchlings [4].
One would assume that the ability to swim would require certain specializations, like increased stamina, especially compared to other species of lizards. However, research has shown that marine iguanas do not have an advantage, like higher stamina or higher tolerance to submergence, when compared to other species of lizards or iguanas; their ability to forage for food underwater seems to be a behavioral adaptation meant to allow the iguanas to take advantage of alternate food sources [2]. The marine iguana is truly a fascinating animal!
by Judy Oh
Planet Earth II, Season 2, Islands, starting at approximately 19:00
References
- Bartholomew GA & RC Lasiewski. 2003. Heating and cooling rates, heart rate and simulated diving in the Galapagos marine iguana. Comparative Biochemistry and Physiology 16: 573-582.
- Dawson WR, Bartholomew GA & AF Bennett. 1977. A reappraisal of the aquatic specializations of the Galapagos marine iguana (Amblyrhynchus cristatus). Evolution 31: 891-897.
- Kristztina G, Trillmich K & F Trillmich. 1986. Foraging strategies of the marine iguana, Amblyrhynchus cristatus. Behavioral Ecology and Sociobiology 18: 259-266.
- Wikelski M. 2005. Evolution of body size in Galapagos marine iguanas. Proceedings of the Royal Society B 272: 1985-1993.
The Evolution of Planet Earth 