‘Scuba-diving’ lizards
Anoles are a diverse group of over 200 species of colourful lizards in the genus Anolis. They are found in South America and Central America, including some Caribbean islands.1 Most species are excellent tree climbers, suited to life in the rainforests, but some are also found in deserts, urban areas, and even deep inside caves.2 Just a few species spend time in water, and scientists have made a fascinating discovery about these critters.
Over 6,000 species of lizards are known but none are fully aquatic, and just a few dozen are semi-aquatic (from 11 different families).3 Biologists at the University of Toronto have uncovered how certain species of Anolis can stay submerged for up to 18 minutes. One of the reasons anoles can stay underwater for so long is their lower metabolic rate, compared with mammals and birds. In addition, the big surprise is that, whether diving for food or hiding from predators, they can ‘rebreathe’ some of their own air.4 This stunning discovery is the first of its kind in a vertebrate animal. It is a handy trick, but how exactly do they pull it off?
At the swimming pool, if you try slowly breathing out through your nose, the bubbles don’t stay attached to your nose, but race to the surface of the pool. However, when these anole lizards exhale underwater, the air bubble that exits their nostrils stays attached, ready to be inhaled again. Think of them as lizard scuba divers! It is worth watching the short video posted on the blog of the paper’s lead author, Christopher Boccia.5 As the bubble grows, then disappears again, the lizard is extracting more oxygen from the air.
Your own skin lacks the properties to keep such a layer of air attached. And since the scaly skin surface of anoles is much more hydrophobic (‘water-hating’) than human skin, you might think it would be even harder for them to hold onto an air bubble. Paradoxically, the opposite is true.
Marvellous micro-spines
This water-repellent tendency of the Anolis lizard’s body surface results from numerous tiny spines (about 1.2 µm high) and even tinier spinules (about 0.5 µm). How small is that? Well, 1,000 microns (µm) = 1 mm. In fact, there are 11.2 million of such spines per square millimetre!6 The same microstructures do exist on the skin of terrestrial anole lizards but they are shorter (about 1.0 and 0.4 µm high respectively).7 While lowering the wettability of the lizard’s surface (making it hydrophobic), the spines and spinules actually capture and retain a thin layer of air next to the skin.6 Having a non-wettable skin brings benefits: it is self-cleaning, it reduces drag while swimming, and it helps conserve heat. But it also turns out to enable this underwater rebreathing ability.
Technically, this sort of breathing in ‘scuba diving anoles’ is called ‘plastron respiration’—plastron being the name for the thin film of trapped air. Biologists have long known about plastron breathing in various arthropods, but this is a first in reptiles, a discovery made independently by two groups of scientists.8 Most anoles don’t go scuba-diving like this, so is it an example of adaptive evolution by natural selection?
Evolution or design?
The team who reported this discovery believes that this scuba diving ability likely evolved several times independently, in different semi-aquatic species of Anolis lizards—maybe to avoid predation.4 Other experts think that changes in the architecture and position of the spines and spinules must first have preceded (or at least coincided with) the rebreathing behaviour of these species of semi-aquatic Anolis lizards.6 The latter may well be true.
Imagine that, in a population of semi-aquatic anoles, an individual was hatched with a mutation that caused its skin to have slightly taller spines and spinules—thus reduced wettability. One scenario is that the lizard would discover that, upon exhaling underwater, a bubble remained detached—a bit like an ‘aqualung’, which it could then rebreathe. Alternatively, since a mutation can cause several, seemingly unrelated changes in a creature (pleiotropy), maybe the modified gene altered the skin texture and the behaviour at the same time—if true, it would point to these lizards being designed to adapt in this way. If water-loving lizards with this beneficial gene variant (allele)9 bred together, it would increase the chances of the information for more hydrophobic skin being passed to successive generations. However, for the beneficial allele to spread through the whole population, the normal allele must have died out. Therefore, it must have given the ‘lucky mutants’ a significant advantage—maybe helping them escape from predators, or else enabling them to catch more prey underwater. If so, it would make sense that the genetic information enabling ‘scuba diving’ would take over such a population of anoles. Their progeny, with this altered skin and behaviour, would then be able to scuba-dive themselves.
However, this is not evolution in the sense of amebae to anoles, rather it is evidence that God designed creatures to vary—but within the limits of the created kinds.10 And certainly, pointing to changes in the size and density of a lizard’s skin texture (thus hydrophobicity) is a far cry from explaining how such a complex skin surface originated in the first place.
References and notes
- Losos, J.B. and Schneider, C.J., Anolis lizards, Current Biology 19(8):PR316–318, 28 Apr 2009. Just a single species is native to the southeastern US. Return to text.
- Scarpetta, S. and 8 others, Morphology and ecology of the Mexican cave anole Anolis alvarezdeltoroi, Mesoamerican Herpetology, 2:260–270, Sep 2015. Return to text.
- Bauer, A.M. & Jackman, T., Global diversity of lizards in freshwater (Reptilia: Lacertilia), Hydrobiologia 595:581–586, Jan 2008. Return to text.
- Boccia, C.K. and 14 others, Repeated evolution of underwater rebreathing in diving Anolis lizards, Current Biology 31(13):P2947–2954.E4, 2021. Return to text.
- Boccia, C., Repeated evolution of underwater rebreathing in diving Anolis lizards, anoleannals.org, 12 May 2021. Return to text.
- Baecken, S. and 5 others, Convergent evolution of skin surface microarchitecture and increased skin hydrophobicity in semi-aquatic anole lizards, J. Exp. Biol. 224(19):jeb242939, 2021. Return to text.
- See Table 1 of ref. 6. Return to text.
- The first, published in a blog post: Swierk, L., Underwater breathing by a tropical lizard, anoleannals.org, 20 Dec 2018. The second, see ref. 4; the lead author describes finding rebreathing in four Anolis species (ref. 5). Return to text.
- E.g. different alleles of the same gene code for different fur colours: Lightner, J., Colourful creature coats, Creation 28(4):33–34, Sep 2006; creation.com/coats. Return to text.
- To learn about this in more depth, see this 3-part article series: Carter, R., Species were designed to change: creation.com/species-designed-change-1, 1 Jul 2021; creation.com/species-designed-change-2, 22 Jul 2021; creation.com/species-designed-change-3, 12 Aug 2021. Return to text.
Readers’ comments
Comments are automatically closed 14 days after publication.