When people think of prehistoric wildlife, the mind’s eye tends to conjure images of very large animals. Indeed, the fossil record is studded with giants from all corners of the animal kingdom. Many of them eclipse their modern relatives in terms of size and some of them of so monstrously huge as to boggle the mind. Scientists of come up with a term for beasties of unusual size, megafauna.
What does “megafauna” mean?
To put in simply, the word “megafauna” translates as “large animal.” However, exactly what makes an animal megafauna is determined by the animal’s weight. There are several different thresholds for determining the size of megafauna. The smallest of these is 100 pounds. A great many animals are over 100 pounds, including those generally not thought of as being particularly big like pigs and humans. It can also apply to animals that are at least one ton in weight. Some break it down to demarkations of 500 and 1000 pounds, although these are not as frequently used. Megafauna can be further divided by diet into megaherbivores, megacarnivores, and even megaomnivores. While the term is often used to describe prehistoric animals, there are extant megafaunal species. Paleontologists may use the term to describe animals that are comparatively large compared to living relatives even if they do not meet the mass requirements. Many dinosaurs certainly are large enough to qualify as megafauna, but the term is usually reserved for Cenozoic animals. It is most frequently used when discussing mammals and flightless birds, only rarely being applied to reptiles, fish, or invertebrates.
What are the advantages and disadvantages of being big?
In the animal world, size matters. Being big can open up a lot of doors ecologically speaking. Large herbivores can reach vegetation unavailable to smaller competitors, and larger predators can take down larger prey items. Having a large body mass can serve as a form of protection, and not just from potential predators. Bigger animals can tolerate higher levels of stress, have an easier time maintaining their body temperatures in colder climates, and often can live a long time. However, there are costs with being big. Larger animals require more resources, in terms of both food and living space. Heavier herbivores have a harder time fleeing predators and heavier carnivores have to rely more on ambush-style hunting tactics. They also tend to have low reproductive rates, giving birth to a small number of young that mature slowly over the course of their long lifespans. Perhaps the biggest cost haas to do with gravity. The larger an animal gets, the more gravity acts upon its body. A big animal is much more likely to suffer catastrophic injury during a fall than a smaller one.
What habitats support megafauna?
Megafauna can be found in a variety of habitats around the world, but some are more likely to have megafauna than others. As a general rule, forests are not a good place to find really large animals. All the dense vegetation makes it harder for bulky bodies to force their way through, and the fact that food resources are widely scattered makes it difficult to find sufficient nutrition. Open environments tend to favor large body size. One of the best places to see modern megafauna are the vast savannahs of Africa. Animals native to polar habitats also tend to be larger to conserve heat. In a condition known as island gigantism, small animals that arrive on islands are released from predation and/or competitive pressures often become larger than their mainland ancestors. Sometimes large body size is achieved to fill in an ecological niche that would otherwise be filled by species unable to get to the island. This is particularly true for birds and rodents. Marine organisms tend to show a trend toward gigantism. Being freed from the confines of gravity, they can achieve far greater sizes than their terrestrial counterparts. Because reptiles are cold-blooded, larger species are more common in hotter climates and were more common when the earth was warmer in the past. For insects and other arthropods, size is limited by their armor and their respiration. They breathe via snorkel-like tubes called trachea that absorb oxygen directly form the air and transport it to where it needs to go. The bigger the arthropod, the bigger the trachea needed. Unfortunately, their legs and muscles to not increase to scale along with the trachea. If modern arthropods were to be any larger, they would be unable to move under their own power and would suffocate. During the Carboniferous Period about 350 million years ago the atmospheric oxygen content was at 35%, which allowed arthropods to grow to enormous sizes without necessarily increasing the size of their trachea as much. Examples of king-size Carboniferous creepy crawlies include griffinflies, which resembled dragonflies with yard-wide wingspans, and Arthopleura, a millipede that could grow to nearly the length of a car. Because rainforests have such thick plant cover, they contain more atmospheric oxygen than other habits which is why the tropics is the best place to find giant arthropods.
What types of megafauna are most common?
Megaherbivores are the most common variety of megafauna. Plant material is harder to harder to process, both mechanically and chemically, than animal tissue and as a result many herbivores have long, complex intestinal tracts and capacious, sometimes multi-chambered stomachs. In order to haul around all of those guts, animals need to be big. As a general rule, those herbivores that utilize handgun fermentation rather than rumination tend towards greater body size. Today there are very few terrestrial animals exceeding one ton, let alone half a ton, and the majority of them are ungulates. The largest extant land animal is the African bush elephant at around five tons, although the prehistoric rhino Paraceratherium and straight-tusked elephant Paleoloxodon both compete for the title of largest land mammal of all time at up to 20 tons. Megacarnivores are less common than megaherbivores. Many are considered to be apex predators. Their maximum body size is considerable smaller than that of the herbivores, topping off at about a ton. Carnivores heavier than 45 pounds have to start taking prey items at least their own size in order to balance out the energy expended hunting versus the calories gained. Once a carnivores hits the 2000-pound mark, the energy expended hunting is simply too great. This is why there are no full-time insectivores larger than the giant anteater. The only modern terrestrial carnivore to come close to that size is the polar bear, although they tend to average closer to 100o pounds. Several prehistoric predators, including a number of sabertooth cat species, achieved this weight. The largest carnivoran of all time is the southern elephant seal, which can reach weights of four or five tons thanks to its primarily aquatic lifestyle. Despite the fact they are biologically carnivorans, bears are one of the few omnivores to be classified as megafauna. The need for slightly more guts probably explains why they grow larger than their more carnivorous relatives, like dogs and mustelids. Some breeds of domestic pig can weigh as much as a bear, although they cannot get as large in the wild.
Are megafauna endangered?
Sadly, a greatly many megafaunal species are endangered. Thanks to their slow maturity rates, they are particularly vulnerable to being exploited by humans. They faced such varied threats as poaching, deforestation, and pollution. The end of the Pleistocene was marked by the extinction of groups of megafauna from across the world and many of those extinctions have been linked to human hunting. Megafaunal are often keystone species, helping to shape the environments in which they live. Elephants physically alter the landscape by knocking down trees, and the as bison migrate across the plains their hooves churn up and aerate the soil. Large animals are vital to transporting essential minerals and nutrients throughout their ecosystems. The depletion of ruminating megafauna decreases the amount of methane released into the atmosphere and in turn changes the global climate.