Effects Of The Last Ice Age Glaciation Is Visible In American Crocodile DNA

Sea levels — not varying temperatures nor changing salinities — drove the genetic isolation of American crocodile populations on each side of Panama

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Those of you who are interested in crocodiles may have been following an ongoing discussion in the scientific literature focusing on identifying the main driver of evolution of these magnificent beasts: Is climate a major factor or are changes in sea levels?

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

Dr Avila-Cervantes and Professor Larsson used this opportunity to prepare and develop a more detailed analysis of how changing temperatures and rainfall had little impact on gene flow between the two populations of American crocodile, Crocodylus acutus, living in isolation on opposite coasts of Panama during the past three million years. In contrast, they did find that changes to sea levels during the latest Ice Age had dramatic effects on these populations.

“The American crocodile tolerates huge variations in temperature and rainfall”, explained Dr Avila-Cervantes. “But about 20,000 years ago — when much of the world’s water was frozen, forming the vast ice sheets of the last glacial maximum — sea levels dropped by more than 100 metres. This created a geographical barrier that separated the gene flow of crocodiles in Panama.”

Despite being very good swimmers, crocodiles are incapable of travelling long distances on land — nor can they climb towering 100 meter cliffs and hills. Thus, the Caribbean and Pacific crocodile populations were physically isolated from each other potentially for millions of years (Figure 1), and as a result, each population carries their own unique genetic mutations.

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

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

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

“This is one of the first times Ice Age effects have been found in a tropical species”, Professor Larsson pointed out in a statement. “It’s exciting to discover effects of the last Ice Age glaciation 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).

“Preserving 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 C E Larsson (2023). Ice Age effects on genetic divergence of the American crocodile (Crocodylus acutus) in Panama: reconstructing limits of gene flow and environmental ranges: a reply to O’Dea et al., Evolution 77(1):329–334 | doi:10.1093/evolut/qpac006

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