Megafauna are the largest animals on Earth, both living and extinct. The term literally means “large animals,” and biologists most commonly define it using a body mass threshold of about 45 kilograms (100 pounds), though some researchers set the bar higher at 100 kilograms for herbivores and 15 kilograms for carnivores. The word comes up most often in discussions of the giant creatures that roamed every continent during the last ice age, but megafauna are not just a thing of the past. Elephants, whales, hippos, and grizzly bears all qualify.
How Scientists Define Megafauna
There is no single, universally agreed-upon weight cutoff. The most widely cited threshold comes from paleontologist Paul Martin, who defined megafauna as animals over 100 pounds (roughly 45 kg). This definition originally applied to mammals, but researchers have since extended it to marine and freshwater environments. For freshwater species, several authors use a slightly lower threshold of 30 kg, since truly large animals are rarer in rivers and lakes.
Some ecologists take a more nuanced approach, using different cutoffs depending on the animal’s role. Herbivores need to exceed 100 kg to count, while carnivores qualify at just 15 kg, because a 15-kilogram predator already sits at the top of most food chains and shapes its ecosystem in outsized ways. Animals above 1,000 kg, like elephants and rhinos, are sometimes called “apex megafauna” to distinguish them from the merely large.
Famous Extinct Megafauna
When most people hear “megafauna,” they picture the giants of the Pleistocene epoch, the geological period that ended roughly 11,700 years ago. Woolly mammoths stood up to 3.5 meters tall and ranged across Europe, northern Asia, and North America. Giant ground sloths (Megatherium) in South America weighed as much as 4,000 kg, making them one of the largest land animals that ever lived. Saber-toothed cats, giant armadillo-like creatures called glyptodonts, and short-faced bears rounded out a world that looked dramatically different from today.
Every inhabited continent lost most of its largest species during the late Pleistocene and early Holocene. North America lost mammoths, mastodons, camels, and horses. Australia lost giant wombat relatives and enormous monitor lizards. South America lost ground sloths and car-sized armadillos. Africa and parts of Asia kept more of their megafauna, which is why elephants, rhinos, and large cats still survive there, but even those populations are diminished compared to their historical ranges.
What Killed the Giants
Two hypotheses dominate the debate: climate change and human hunting. Both have evidence behind them, but one has significantly more support than the other.
The climate hypothesis points to the intense warming and cooling cycles of the late Pleistocene, including rapid temperature swings and regional drying events. The problem with this explanation is that similar or even more severe climate shifts happened repeatedly throughout the preceding millions of years without triggering the same kind of size-selective extinction. Earlier climate upheavals killed off species, but they didn’t specifically target the biggest animals.
The human hypothesis argues that the spread of behaviorally modern humans across the globe lines up remarkably well with when megafauna disappeared on each continent. Australia’s giants vanished shortly after humans arrived there. The same pattern holds for the Americas, Madagascar, and New Zealand. A 2025 synthesis published in Cambridge Prisms: Extinction concluded that the linkage between human expansion and megafauna loss is “well supported by many types of evidence,” while climate as a major cause had “weak or no support.” Archaeological sites across southern South America show direct evidence of humans butchering extinct megafauna, with human cut marks found on megafauna bones at 13 out of 18 sites where those species were present.
The most likely picture is that humans were the primary driver, with climate stress occasionally making populations more vulnerable. But the extinctions were fundamentally unlike anything that happened before humans entered the scene.
Megafauna in the Ocean
Marine megafauna include some of the largest organisms that have ever existed. Blue whales reach 33 meters in length. Whale sharks grow to nearly 19 meters. Sperm whales can be 24 meters long, though 95% of measured individuals are under 15 meters. Southern elephant seals tip the scales at 5,000 kg, and even the ocean sunfish, a bizarre disc-shaped fish, can weigh 2,300 kg.
But marine megafauna aren’t limited to vertebrates. The colossal squid weighs nearly 500 kg. Giant Pacific octopuses can spread 9.8 meters across. Even invertebrates like the giant clam (with over 330 kg of soft tissue) and the lion’s mane jellyfish (with tentacles stretching 36 meters) qualify. The ocean supports larger body sizes partly because water buoyancy reduces the structural demands of supporting enormous mass.
Why Megafauna Matter to Ecosystems
Large animals reshape their environments in ways that smaller species simply cannot replicate. Their ecological roles fall into a few major categories, each with cascading effects on the landscape.
Nutrient Transport
Megafauna act as biological pipelines, moving nutrients across vast distances. Because they eat huge quantities of food, retain it in their guts for long periods, and travel far each day, they effectively redistribute nutrients from one part of a landscape to another through their waste. Two quantitative studies found that large animals increase the rate of nutrient movement across landscapes by at least tenfold compared to what physical processes alone would achieve. In cold, dry, or nutrient-poor environments, megafauna guts function as warm, moist incubation chambers that break down tough plant material far faster than it would decompose on the ground.
Nutrients that would otherwise stay locked in woody stems and leaves for years or decades get liberated through digestion, defecation, and the physical damage large animals inflict on vegetation. When megafauna trample and break trees, they move that stored energy into the decomposition cycle much faster.
Seed Dispersal
Many large herbivores eat fruit and disperse seeds over long distances. Some plant species in the Americas produce fruits that seem too large for any living animal to eat and spread. Ecologists call this the “megafauna fruit syndrome,” the idea that these oversized fruits evolved to be eaten by now-extinct giants like ground sloths and gomphotheres. Without those dispersers, the plants’ geographic range and genetic diversity have likely contracted.
This isn’t just a historical curiosity. In the modern Amazon, overhunting of the largest remaining seed dispersers, such as tapirs and large primates, is projected to cause a long-term decline in the high-biomass tree species those animals once spread. Fewer of these large, dense-wooded trees means less carbon stored in the forest, even if the forest looks structurally intact from above.
Living Megafauna and Rewilding
The megafauna that survived, primarily in Africa and Asia, are under intense pressure from habitat loss, poaching, and human-wildlife conflict. Conservation efforts increasingly focus on rewilding: returning large animals to landscapes where they’ve been lost, or introducing ecological stand-ins where the original species are gone.
Some rewilding projects are well established. European bison have been reintroduced across parts of the Carpathian Mountains. Przewalski’s horse was brought back to Mongolia after going extinct in the wild. Wolves were famously returned to Yellowstone National Park, triggering measurable changes in vegetation and river systems. Various species of rhinoceros have been reintroduced to areas in Africa and Asia where they were wiped out, and brown bears from Slovenia have been used to rebuild the dwindling population in the French Pyrenees.
More experimental projects push boundaries further. African cheetahs from Namibia have been translocated to India to fill the ecological role of the Asiatic cheetah, which was hunted to extinction there in the last century. In Colombia, a small population of hippos descended from animals that escaped Pablo Escobar’s private zoo has established itself in the wild, raising complicated questions about whether out-of-range megafauna can fill ecological roles left empty for thousands of years. In Australia, a population of banteng, a large Southeast Asian wild cattle, became established largely by accident but now represents a real-world case of a threatened megafauna species thriving outside its native range.
The logic behind these projects is straightforward: if megafauna shaped ecosystems for millions of years, then landscapes without them are missing a key engine of ecological function. Restoring large animals, or reasonable substitutes, could help rebuild nutrient cycles, control vegetation, and maintain the open, mosaic habitats that many other species depend on.