I wanted to share a very cool natural history event that I recently learned about, which provides background to the fauna of the Americas. I found out about this because I was wondering why the US and Canada only have one marsupial species, the Virginia opossum, but this question led to so many answers (and even more questions!), I decided to share an overview of the whole event.
Overview of GABI
The Great American Biotic Interchange, or GABI, occurred during the late Cenozoic (this particular event occurring about 2.7 million years ago). This event describes the mixing of North and South American fauna that occurred when the Isthmus of Panama formed (as a result of underwater volcanoes that formed along a tectonic plate boundary), connecting the North and South American continents. Mammals, reptiles, amphibians, arthropods, flightless birds, and even freshwater fish were now able to migrate between North and South America. This also, however, closed the Central American Seaway, halting gene flow between the Caribbean Sea and the Pacific Ocean.
Before the interchange, South America was very isolated, and therefore had a high level of faunal diversification since they faced less competition. South America had all kinds of weird stuff, much of which is now extinct, while North America has a similar makeup of species that we are familiar with today (mostly boreoeutheria, which are placental mammals. A definitive characteristic of this group is that males of most boreoeutheria have a scrotum. Some male marsupials also have scrotums, which evolved independently, but are located anteriorly to the penis.)
Before GABI, most of South America’s terrestrial predators were almost exclusively birds, reptiles, and non-placental mammals. After, this niche was dominated by placental carnivores. This was initially thought to be due to competition, but it’s not that simple. Many of South America’s predators were already not doing so well before this (for reasons not agreed upon), so there were lots of empty niches for the migrant North American placental mammalian predators to fill. This can be seen as rats, mice, canines, and deer underwent a great degree of adaptive radiation (rapid diversification) upon expanding to South America.
Success or Failure
There are a few big factors that determined success or failure for these migrants. Migrating animals would have had to endure moist, tropical conditions, as they crossed from one landmass to the next. North American animals that were able to cope with these conditions faced similar conditions in South America. South American animals that could cope with tropical conditions, however, likely weren't able to deal with the more temperate climate farther north. Further, North America provided a larger and more competitive environment, as it was connected to Eurasia (via Beringia!), and therefore Africa, by extension. South America had only been connected to the smaller and less hospitable continents of Antarctica and Australia, so they were less competitive overall.
Although many South American native animals were not as successful in colonizing North America, one group, the xenarthrans (i.e. armadillos, anteaters, sloths), did much better than the rest. This group likely did well because many members have body armor or large claws, and therefore did not have to flee to survive. Their slow metabolism also meant they could live off of less abundant or nutritious food.
Now You See Them
Below, I have selected some recognizable species from North and South America that have origins in the other continent.
Notable North American fauna with South American origins: parrots, tanagers, hummingbirds, vampire bat, Virginia opossum, armadillos, porcupines, mountain lion (originally from North America, but was wiped out. The surviving population in South America reintroduced them to North America).
In Central America: poison dart frogs, toucans, 2 toed sloth, 3 toed sloth, anteaters
Notable South American fauna with North American origins: lungless salamanders, snapping turtles, coral snakes, rattlesnake, some pit vipers, New World sparrows, condors, pocket gophers, tree squirrels, cottontail rabbits, tapirs, wild horses, peccaries, deer, otters and other mustelids, skunks, wolves, foxes, mountain lions, panthers, guanacos/vicuñas (and their domestic descendants, llamas and alpacas!)
Reflections
I think it’s awesome that one simple question led me to learn about this event, and so many other mini side topics that I’d never learned about before. It makes me think about native and invasive species, and how much geography, geology, and plate tectonics affect the distribution of life on Earth. It is fascinating to observe the similarities between originally North American species and South American species with North American origins, showing the traces of past migrations and land connections. For example, it is strange to think that llamas, which are so iconic in my perception of South America, were introduced to South America via North America. And, that their camelid ancestors passed from the eastern hemisphere through North America, but these traces have been erased due to the fact that no extant ancestors remain in North America. Learning the context of these animals and landmasses gave me a new appreciation for them, and a new lens through which to view them. It’s a reminder of how interconnected and dynamic our planet is, and how much there still is to discover about the history of life on Earth.