Part 1: Where we all began
By Theresa Peluso
Being mammals ourselves, we perhaps identify more closely with other mammals than with any other class of animal. Unlike with the other animal classes discussed in this series on The Web of Life,
I’ve found it necessary to split this article into two parts, because there is so much to share about mammals.
According to the most current data, there are approximately 5,450 species of mammals.
The single largest order is the Rodentia, which comprises most non-flying mammals. About 1,500 of existing mammal species are rodents. (Although Rodentia includes muskrats, porcupines, voles and groundhogs, it does NOT include rabbits, hares, shrews, moles and hedgehogs.) The next largest orders (based on genetic data, and not on anatomic similarities), are bats and Soricomorpha (mainly shrews and moles). These are followed by primates, which in turn are followed by the carnivora (Feloidea (e.g., members of the cat family, hyenas, mongooses)), Canidae (e.g., members of the dog family, bears, skunks, seals, raccoons), and Pinnipedia (walruses, seals, sea lions).
The total number of wild mammals in the world is estimated to be about 130 billion. Researchers have determined – despite the (understandable) difficulty of getting reliable estimates – that the most numerous of these (by population) are squirrels, bats, mice and rats, at 7 billion.
The total number of domestic mammals is estimated to be over 6 billion, with cows (1.5 billion) at the top of that list, followed by sheep (1.1 billion), pigs (1 billion), dogs (900 million), goats (860 million), rabbits (709 million), cats (600 million), tame water buffalos (175 million) and cats (60 million). (See https://www.worldatlas.com/articles/most-populous-mammals-on-earth.html .)
However, in terms of biomass distribution on earth (in gigatons of carbon), the picture changes. The biomass of wild mammals is .007 Gt C, and for livestock (dominated by cattle and pigs) it’s 0.1 Gt C, which is more than 14 times greater than the biomass of wild mammals. As a result of intense whaling and exploitation of other marine mammals, marine mammal global biomass has decreased from approximately 0.02 Gt C to roughly 0.004 Gt C – that means it is one-fifth of its original global biomass. By the way, the biomass of humans is 0.06 Gt C, which is nearly 9 times greater than the biomass of wild mammals. (Information from https://www.pnas.org/content/115/25/6506 .)
The smallest mammal in the world is the bumblebee bat (Craseonycteris thonglongyai), weighing in at just barely 2 grams and measuring approximately 3 cm in length, about the size of a large bumblebee. Contrast that with the Blue Whale, the largest known animal to have ever existed, with a length of 30 metres and a mass of 180 tonnes or more. It’s mind-boggling that both these mammals evolved from the same ancestors, and share all the mammalian characteristics. (In the case of some whales, a few of these characteristics are present in the fetus, but not in the newborn calf.) Unlike reptiles, from which our earliest ancestors diverged, the jawbones of the first mammals evolved to allow the skulls of later mammals to expand sideways and backwards, giving us bigger brains. The development of teeth enabled mammals to extract more nutrients from the food they ingested. Other characteristics that all mammals share are endothermic systems (bodies capable of generating heat internally), hair and fur on the body; mammary glands; a diaphragm; four-chambered hearts; three small ear bones; and glands that secrete fat, sweat, and scent. Mammals also care for their offspring, which gives the young greater chances of survival.
Mammals are found on all continents, including the harshest environments on this planet – from the most inhospitable deserts to the ice-covered North Pole, from the deepest oceans to the air.
The first mammals evolved from a population of mammal-like reptiles called therapsids about 200 million years ago, at the end of the Triassic period (the first of the three periods of the Mesozoic Era). (This first appearance of mammals was about 50 million years before the appearance of the first birds.) These original mammals coexisted with dinosaurs throughout the Mesozoic Era, which ended about 65 million years ago, when the mass extinction event occurred. All the various groups and sub-groups of therapsids went extinct at various times during this era, except for those that evolved into the mammals that we know today. A split in the early mammal population about 180 million years ago resulted in the monotremes, a group of mammals, like the duck-billed platypus, that lay eggs rather than giving birth to live young. Another split from the placental mammals, about 140 million years ago, resulted in a branch of marsupials, such as the kangaroo. Between 105 and 85 million years ago, the placental mammals split into their four major groups: a) hoofed mammals, whales, bats and dogs, b) primates, rodents and others, c) anteaters, armadillos and others, and d) elephants, aardvarks and others. Further sub-splits happened along the way. Some of these splits were the result of the original land mass (or super-continent), Pangea, breaking up into the different continents we know today). Another major influence on the evolution of mammals was the drastic climatic changes during these thousands of millennia, and the changes in topography as the tectonic plates continued to shift and oceans froze and melted.
Some biologists think that if every mammal species that ever evolved was alive today, the grand total would be about 5 billion – which would mean that – not including recent mammal extinctions – approximately 99 percent of all these species are not around anymore.
One of these mini-extinctions, which affected mostly the largest mammals, happened near the end of the Pleistocene era (this era lasted roughly from 2,590,000 BCE to 11,700 BCE). The cause of this extinction is difficult to identify because there is little evidence and because factors such as environmental conditions and human activity levels varied so much from one geographical area to another. Many scientists support the theory that drastic changes in climate caused these extinctions, changes that would have made it difficult for very large mammals to adapt. Yet, animals such as mammoths died out in some parts of the world, but survived in others, and some extinctions happened when there were no significant fluctuations in climate. Another theory is that humans, over the last 50,000 years, hunted these animals to the point of extinction. However, the fact that some megafauna died out but not others, plus the fact that human populations at that time were very low and their weapons very primitive (although they could very well have used fire as a means to trap and kill animals), raises doubts about this theory. Yet another theory is that contagious diseases, transmitted by humans or their animals, may have been the cause of these animals’ mass demise. But there are numerous holes in this theory as well.
Whatever the cause, during prehistoric times thousands of species died, such as the meat-eating kangaroo, the sabre-tooth tiger, the four-metre long armadillo with a medieval weapon (morning star) for a tail, the two-tonne wombat, the walking whale, the marsupial lion and the bear-dog. (See https://www.tor.com/2014/02/07/ten-fossil-mammals-as-awesome-as-any-dinosaur/ and http://www.bbc.com/earth/story/20160208-the-lost-giants-that-prowled-the-australian-wilderness for more fascinating examples of these extinct mammals.)
The mammals of the Mesozoic Era were very small – few were larger than mice – which helped them to escape the notice of the dinosaurs that dominated the earth at that time. They survived by feeding on plants, insects and small lizards, hunting at night, and living high in the trees or in burrows in the ground. In fact, their small size (and the fact that they had fur coats and a warm-blooded metabolism) was what enabled them to survive the mass extinction event, believed to have been caused by a massive comet or asteroid colliding with the earth, plunging it into a “nuclear winter” and destroying most of the life-sustaining vegetation.
In the space of a few million years after this incredible impact, the fossil record shows an explosion in the diversity of mammals – once the dinosaurs were no longer a threat to survival. However, it wasn’t until 6 million years ago that humans diverged from their closest relatives, the chimpanzees and the bonobos. But that’s a topic for yet another article.
What is the importance of mammals in the web of life?
Mammals perform a role in our ecosystems both as prey and as predator. Herbivores such as rabbits and deer serve an obvious role as consumers of plant material and as prey to numerous omnivores and carnivores. Omnivores such as shrews and rats eat both plant and animal matter, and serve as both predator and prey. Carnivores such as racoons, dolphins and coyotes are necessary to control the populations of other mammals. Unlike human hunters, predators tend to target the old, weak and diseased animals, and thereby maintain the strength of the prey population. The availability of prey, in turn, controls the size of predator populations, which decline during periods of scarcity.
Some mammals play an important part in seed dispersal. Bats in some tropical and desert ecosystems are considered a keystone species in this regard. In temperate climates such as ours, bats perform an essential role in controlling pests. Scientists estimate that in the United States, bats perform ecosystem services of $3.7 billion per year in reduced pesticide use. Primates are also good seed dispersers, and are important for forest regeneration, as well as for retaining balance in the ecosystem. Other good seed dispersers are marsupials and rodents.
Some large mammals, like elephants, play a huge role in maintaining soil health. As elephants eat small trees, defecate, urinate, and trample and churn the soil, they keep certain pest vegetation from overrunning the grasslands, while dispersing seeds for new trees to grow. The same is true of bison and moose in North America. Beaver too, despite their relatively small size, also affect our landscape in ways that are beneficial to other organisms, and improve biodiversity by cutting down big trees and making dams that flood the landscape, thereby creating meadows and marshy vegetation. When their populations get too big though, they become prey for other mammals like wolves.
Re-introducing large-bodied mammals to an ecosystem where they had previously been made extinct can generate huge improvements. For example, re-introducing the gray wolf in Yellowstone Park in the early 1990s helped to whittle down the elk population, which had been depleting the vegetation in the park. As a result, today there is a healthy population of willows, aspens and cottonwoods, beaver, fish and songbirds. As a bonus, the carcasses left by the wolves also nourish other animals such as ravens, eagles, magpies, coyotes and bears.
Marine mammals play similar roles in helping to conserve the health and diversity of their ecosystems. Some, such as walruses, seals and killer whales, act as top-level predators, deep in the ocean or near the surface. By keeping certain species such as sea urchins under control, mammals; for example, sea otters, enable kelp bed forests to thrive.
Mind you, there are also negative aspects to some mammals. In some countries like Bermuda, certain rats like the Black Rat and the Norway Rat, which are both native to Europe, have caused the decline of many native and endemic species by eating the food on which native species depend, catching and eating native birds and lizards, and stealing eggs from birds’ nests and sea turtle nests. They are considered the world’s worst invasive species.
In addition, rats and mice are hosts for many diseases that can affect humans and other mammals, such as Lyme disease, plague, hantavirus pulmonary syndrome and Rocky Mountain spotted tick fever. Other mammals that humans consider pests are deer, groundhogs, rabbits, and in some regions, wild pigs, the reason being that they get into our gardens, houses and garbage, and damage farm crops and pastures. The solution in many cases could be to have a healthy population of native predators like wolves, coyotes, fishers, foxes, and cougars to reduce the numbers of these undesirable mammals. Also, keeping expansion of our own human habitations under control, and providing natural, connected, corridors for these animals would enable them to thrive without encroaching on our properties.
On the upside, these mammal pests provide food for other animals. Rats (surprisingly) are considered good to eat by people, and are part of the diet of some cultures. Being scavengers, rats eat the garbage that people leave around. They are also able to smell diseases like tuberculosis in people, and serve as test subjects for the various medicines and other products that people use.
It’s truly remarkable that, over the course of 200 million years, so many species of mammals have survived so many climatic and geographic upheavals. And yet now, during a period where these upheavals are – for now – mostly minor, their survival is at risk. In fact, we humans have the ignominy of having a new era named after us: the Anthropocene era – because our activity during the last few millennia has triggered an episode of rapid extinction, on a scale that has occurred only five times in the last 500 million years. We are now in the process of causing the sixth mass extinction, and by the year 2,100, it is expected that up to half of the world’s current species will be extinct because of our actions. While it is open to debate whether humans exterminated the giant mammals that existed during prehistoric times, there is no question that our recent actions are threatening the survival of mammals of all kinds, everywhere. But this will be the subject of Part 2, along with positive steps we humans can take to try to reverse this alarming trend.
Coming next – Part 2: The Current State of Wild Mammals and How to Change It