The effect of the last ice age is visible in American crocodile DNA

Sea levels — not varying temperatures or changing salinity — drove the genetic isolation of American crocodile populations on either side of Panama

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Those of you interested in crocodiles may have followed an ongoing debate in the scientific literature focused on identifying the main driver of the evolution of these magnificent beasts: Is climate a major factor or is sea level change?

One of the groups leading this debate are two researchers at McGill University. Geneticist Jose Avila-Cervantes, a postdoctoral fellow whose expertise lies in using a variety of next-generation molecular tools to study the evolution and ecology of Neotropical crocodiles, and his collaborator and supervisor, macroevolutionary biologist Hans Larsson, professor of vertebrate paleontology and evolutionary development and Canada Research Chair in Vertebrate Paleontology, just published a paper to update us on their thoughts on crocodilian evolution after they made a mistake in an earlier paper. This new paper also provides an interesting glimpse into the scientific process in action and how new data or new analysis (or fresh eyes) can affect how we look at existing puzzles.

Dr Avila-Cervantes and Professor Larsson used the opportunity to prepare and develop a more detailed analysis of how changing temperatures and rainfall had little effect on gene flow between the two populations of American crocodile, Crocodylus acutus, which have lived in isolation on opposite coasts of Panama for the past three million years. In contrast, they found that changes in sea level during the last ice age had dramatic effects on these stocks.

“The American crocodile can withstand large variations in temperature and rainfall,” explained Dr. Avila-Cervantes. “But around 20,000 years ago – when much of the world’s water was frozen and formed the vast ice sheets of the Last Glacial Maximum – sea levels dropped by more than 100 metres. This created a geographic barrier that separated the gene flow of crocodiles in Panama.”

Despite being very good swimmers, crocodiles are unable to travel long distances on land – nor can they climb 100m high cliffs and hills. Thus, the crocodile populations of the Caribbean and the Pacific were physically isolated from each other potentially for millions of years (Figure 1), and as a result each population carries its own unique genetic mutations.

Dr. Avila-Cervantes and Professor Larsson compared the climate tolerance of living American crocodiles with paleoclimate estimates for the last 3 million years for the region, a time span that includes the most extreme climate variations during the last ice age. They discovered that crocodiles could survive these climate variations.

“Finding that these animals would have easily tolerated the climatic fluctuations of the Ice Age speaks to their resilience over geological time,” co-author Professor Larsson said in a statement.

But the effects of sea level on crocodile genomes came as a surprise.

– This is one of the first times that ice age effects have been found in a tropical species, Professor Larsson pointed out in a statement. “It is exciting to discover the effects of the last ice age that still resonate in the genomes of Pacific and Caribbean American crocodiles today.”

“Only humans in recent decades of hunting and land development seem to really affect crocodiles,” Professor Larsson said.

Currently, one of the best-preserved crocodile populations in Panama lives in the middle of the Panama Canal on the Barro Colorado Island Nature Monument, as Dr Avila-Cervantes learned in previous work (ref).

“Conserving the population around this island may be our best chance to preserve the unique genetic signatures of Panamanian American crocodiles.”


Jose Avila-Cervantes and Hans CE Larsson (2023). Ice age effects on genetic divergence of the American crocodile (Crocodylus acutus) in Panama: reconstructing boundaries of gene flow and environmental ranges: a reply to O’Dea et al., Development 77(1):329–334 | doi:10.1093/evolut/qpac006

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