Love avocados? Thank the giant ground sloths!

Love avocados? Thank the giant ground sloths!

Did you know that we can thank giant ground sloths for the avocados we have today? Giant ground sloths were one of the few ancient herbivores large enough to swallow avocados whole, thus serving as an important seed disperser for these delicious fruits that we know and love today!

Illustration of the giant ground sloths by Robert Bruce Horsfall/Source: Wikimedia Commons

 

Many plants, especially in tropical ecosystems, have evolved to rely upon animals to spread their seeds. Only extra-large herbivores such as the giant ground sloths had the ability to swallow avocado seeds whole, meaning that they could carry them around in their digestive tracts and eventually defecate them far away from the parent tree!

Evidence of these ancient symbiotic (mutualistic) relationships can still be seen today. For example, honey locust trees (Gleditsia triacanthos) have large sweet-smelling seed pods that were eaten by megafauna. They also have big, intimidating spikes on their trunks which likely served as an important defense against these giant herbivores. Now a popular city tree due to their ability to withstand poor conditions, modern versions of honey locust trees have been bred without spikes although their supersized seed pods still litter our bustling city streets.

Seed pods and thorns of the honey locust tree (Gleditsia triacanthos). Although seemingly powerless against modern tree dwellers these spikes likely served as vital protection against the large herbivores of the Pleistocene./Images: Freepik.com

How big were the giant ground sloths?

These ancient ancestors of modern-day sloths truly lived up their name. Like bears and anteaters, they had the ability to stand on their hind legs, making them the largest bipedal mammals to have existed. Over 100 species of giant ground sloths lived throughout North, Central and South America, ranging in size from the formidable Megatherium americanum which towered 3.5 metres tall (12 feet), and weighed up to 4 tons, to the considerably smaller 90-kg (200-lb) Cuban Megalocnus. 

 

Skeleton of the giant ground sloth (Megatherium americanum)/Source: Wikimedia commons

 

The giant ground sloths of North America disappeared around 11,000 years ago and their South American cousins followed suit around 10,200 years ago. However, amazing fossil evidence from 2007 revealed that 200-lb Megalocnus were still lumbering around the islands of Cuba as little as 4,200 years ago!

 

Skeleton of the considerably smaller Cuban ground sloth (Megalocnus rodens)./Source: Wikimedia Commons

Why did the giant ground sloths disappear?

Like many of the memorable megafauna of the last ice age, the exact cause of their extinction remains somewhat murky. Megatherium fossils have been found with cut marks on them, suggesting that ancient humans did consume these giant beasts. However, humans and the giant ground sloths of Cuba coexisted for a period of about 1,000 years. It is likely that a combination of over-hunting and/or climate change led to their demise.

 

Artist’s interpretation of the giant ground sloth (Megatherium) of the Pleistocene and two Glyptodon./Source: Wikimedia commons

 

Although the last of the giant ground sloths disappeared around 4,200 years ago, modern sloths carry on their legacy. Scientists have hypothesized that modern-day two and three-fingered sloths evolved from two distinct lineages of extinct giant ground sloths. In other words, although not closely related, two and three-fingered sloths both evolved independently to live in trees, in a process called convergent evolution. However, recent genetic evidence suggests that the species of sloths alive today may have evolved from an ancestor that was comfortable on the ground as well as in the trees.

 

The future of avocados

Without their ancient seed dispersers, the future of avocados is now in our hands. Unfortunately, avocado trees are quite sensitive to changes in temperature and water. A single avocado requires 60 gallons of water to grow. Fluctuations in rainfall and temperature due to climate change are causing shortages of these lucrative crops. Moreover, the expansion of avocado farms in Mexico has led to the destruction of pine and fur forests thus affecting monarch butterflies. The high demand for avocados has even caused gang warfare to break out in areas such as the Michoacán state of Mexico, where almost 80% of the US’s avocados come from.

Also known as “alligator pears” avocados became popular due to the targeted marketing efforts of Californian farmers in the 1990s./Image: Freepik.com

A sustainable future for avocados, means growing them in places where water is plentiful using agricultural methods (such as agroforestry) that make space for biodiversity.

 

-Katra Laidlaw