40% of mammal species are rodents. With 56 million years to make their mark, rodents are found in vast numbers everywhere but Antarctica.
Like humans, rodents breed at an appalling rate. Likewise, every rodent has its own personality. In the face of stress, some mice have passive personalities, and so appear stoic, while the proactive put up resistance.
With one exception among 2,200 species in 27 families, rodents must gnaw, as their 2 front teeth grow all the time. As a rodent gnaws, its incisors scrape together, keeping them sharp. Rodent molars, the chewing teeth, are set far back into the jaw, affording the strongest possible bite.
The gnawing exception is a forest-dwelling shrew-rat, with oddly shaped incisors and no molars. The shrew-rat’s mouth adapted to its favorite food: soft-bodied earthworms that need to be nabbed but require little chewing.
The family of squirrels extends to 285 species in 58 genera. They evolved 40 MYA.
Squirrels have quite the body: strong and impressively elastic. A squirrel can rotate its ankles 180° to keep a grip while climbing, no matter which way it is facing. Squirrels can leap 10 times the length of their body: more than twice what a human can manage at the fastest sprint.
Part of a squirrel’s strength lies in its elaborately veined tail, which also acts as a thermal regulator. In the summer, the tail wicks off excess heat. In winter, the tail shunts warm blood to its body core.
While rats have quite limited vision, squirrels are sharp-eyed. Their peripheral vision is as sharp as focal eyesight. While color vision is limited (dichromatic), their pale-yellow lenses cut down on glare.
Squirrels have prodigious memories. For years they can remember the many thousands of locations where they have stashed food. Squirrels also have long memories of their experiences, including whether problem-solving attempts they tried worked or not.
Squirrels can recall techniques which they have not used for a long time. ~ English zoologist Théo Robert
Squirrels are gregarious. Males leave their natal group when grown, finding a mate in another community. A female remains with her family for life. All the females in a society are related, which affords sharing a tribal vocabulary.
For protection against predation ground squirrels forage socially. If one spots a predator, she will call out. A ground predator raises a trill alarm, while a flying hawk solicits a whistle. Young squirrels learn the differences in calls.
A trill call is only given by females in the presence of other squirrels in the family. At a trill, others stand up and look around to spot the menace.
A whistle causes all squirrels in earshot to hustle for cover. That makes a whistle somewhat selfish, as the scramble likely leaves any predator confused to the source. By contrast, a trill is a bit risky, identifying caller location, and so an altruistic call. But a female is calling to family, and nothing is more important than family.
Squirrels meeting give each other a kiss, which is actually a sniff, as identification. A squirrel’s mouth has a scent gland.
Squirrels can tell how closely related they are by smell. Some family are more important than others.
Squirrels learn from their peers, particularly how to steal food, and to recognize food thieves. They cache surplus food and will do so deceptively if they think they are being watched.
Squirrels in rattlesnake territory mask their scent by picking up pieces of shed snakeskin, chewing it, and then licking their fur. They’ll also use scent from other sources: soil or other surfaces of snake repose that are scent laden. Juveniles and adult females practice this protection more than adult males, who are less vulnerable to predation by snakes by dint of their superior speed and maneuverability.
Squirrels can assess how dangerous a snake is by the sound of its rattle. Squirrels understand that rattlesnakes can see in infrared. A squirrel heats up its tails and then waves it as a warning to a nearby snake, who suddenly senses the squirrel as a threat. Other squirrels nearby join in. The tactic works.
On the odd chance that a snake does not slither away, squirrels further intimidate a snake with aggression. Most common is kicking sand at a snake, forcing its retreat.
Squirrels foretell food production by surveying local trees that produce tasty seeds. Red squirrels will breed extra pups in anticipation of a bumper crop of spruce cones months before the cones even materialize. Spruce cone seeds are a major food source for these squirrels, which makes the squirrels a spruce-tree enemy.
As an adaptive stratagem, spruce trees irregularly produce different cone quantities: years of few cones, followed by a bumper crop. This technique, mast seeding, is practiced by several tree species, to optimize the prospect of thriving in the face of animal predation of their seeds.
Spruce trees bud each year. Some buds differentiate to tree branches, while others go to cones. Squirrels nibble the buds and somehow detect that a masting event of bumper cone production is forthcoming the next summer. In the evolutionary race, red squirrels have outmaneuvered their meal ticket.
They are diabolically clever animals. ~ English biologists A.H. Barrett-Hamilton & M.A.C. Hinton
Rats are extraordinary creatures: cunning, fearless, smart, and curious. Their capacity for viciousness is quite manly. If cornered, a rat will fight to the death.
Rats are nearsighted. Vision is quite poor beyond a meter. Living in close quarters does not reward excellent eyesight, except up close, where rats’ visual processing is at least as sophisticated as humans.
Rats otherwise have keen senses, particularly an astonishing sense of smell, excellent hearing and sense of taste, and an exquisite sense of touch, accentuated by extremely sensitive whiskers and body hair. Rats use their facial whiskers much as humans use their hands and fingers when finding their way in the dark.
Motion can be sensed 15 meters distant. By smell, rats can tell whether a predator is within that range, or another rat, and whether male or female.
Rats are immaculate. They spend up to 30% of their time grooming. Rats are good swimmers, and happily bathe when clean water is available.
Rats are strong and agile; capable climbers that can jump 10 times their height, and squeeze through tight spots.
Rats are omnivorous, with a high metabolism: consuming 1/3rd their body weight per day. Seeds, grains, seafood, and meat are favorite foods. Rats will not eat white flour, as it lacks adequate nutrition.
Rats prefer to dine in secrecy. They readily hoard food.
Rats are stealthy, wily hunters. They are fine fishers and gather shellfish when seaside. Small animals, domestic fowl, and wild ground birds are ready prey. Stolen bird eggs are an especial treat.
Hungry rats are bold. They will attack the newborn or sick of larger animals, whether a piglet or human infant, even an adult human that appears helpless.
A rat can survive 2 weeks without food but must have up to 40 milliliters of water a day. Rats typically build their nest close to supplies of water and food.
Rats have many predators, so it is crucial that they navigate their environment efficiently and flexibly. Rats are better at mazes than people, and superior at tasks requiring precise timing. Rats excel at remembering various inventories. They exceed human capacity in intuitively counting objects.
Rats make decisions based upon past experiences. They suffer regret from poor choices. Novel situations require more analysis before a decision is made.
A rat has its own personality and temperament. Some are hard workers, other are slackers. Some are good-natured and cheerful, others somber.
But all rats like being tickled, especially tummy tickled. It is a social joy. Rats tickle one another and chirp with glee. Tickling makes for a happy home. In contrast, being petted is not pleasing.
Rats certainly have a sense of pleasure. It is likely they have a sense of humor, though stolid researchers haven’t figured out a way to test for that.
Rats behave like people in numerous ways. They make facial expressions: grimacing with pain, sighing in relief. A rat can tell another rat’s emotional state by its facial expression. Rats dream of the day’s events and who knows what else.
Rats develop a sense of trust, or mistrust, in others. Rats are generally empathic, and selectively altruistic. Rats enjoy sex and know good sex from bad.
Rats are prolific, breeding year-round. Females gestate for 22 days and may mate again 2 days after giving birth. A female can produce 16 litters per year, 6–12 pups per litter (kittens is the European term for rat younglings).
Rats are very altricial. Pups are born completely vulnerable: blind, deaf, and helpless. A pup is weaned in 4–5 weeks. Rats reach sexual maturity by 2–3 months, if they live that long.
Mortality is high. Pups are sensitive to temperature swings or other environmental upsets. Predators are aplenty. Disturbed mothers commonly kill and eat their young.
Life as a rat is tough, regardless of age. Most live but 6 months to a year, though some survive to 5 years.
Rats congregate in large packs. Females share parental care, while all adults protect pups from danger.
Old World rats lived in Central and South Asia before furtively traveling the world with human explorers. Black and brown rats are the best-known species. As stowaways on sailing ships, by the 19th century, the adventurous rat had colonized the world.
Black rats originated in South Asia, descending from a tree dweller. They prefer high places.
Brown rats came from northern China and Mongolia. Once found in the forests, reliable food supplies lured them to farms and villages. Brown rats prefer to be underground, near water. They are avid burrowers.
Brown and black rats are mortal enemies. They will not crossbreed. They may live in the same house, but only so long as they remain apart: black rats upstairs, brown rats in the basement. War breaks out if they meet in between. The larger, more ferocious brown rat is likely to prevail.
Rats are wrongly dreaded for carrying diseases. A marked fear has been immigrants may carry illness from afar. But rats are hygienic, and fiercely territorial. Newcomers are rebuffed by residents.
It’s unlikely that a lot of diseases are going to be entering cities on rats walking into the local rat population. ~ American zoologist Jason Munshi-South
Badly misnamed, “naked mole rats” live in the grasslands of East Africa. Their entire existence is subterranean.
The naked mole rat is more aptly called a sand puppy by the locals.
They are neither moles, nor rats; but they are naked. And they do look a bit like puppies if you are half blind, which they are.
Sand puppies have no fur. Fur would be a serious disability for a lifestyle living in the dirt.
The sand puppy has several adaptations that give the good life to be had from rooting around underground. The desert sand puppy is an excellent example of hand-in-glove adaptation to a habitat.
Sand puppies have no pain receptors in their skin, which is otherwise unknown for rodents.
Sand puppies have an almost ectothermic metabolism. Because they cannot regulate their body heat, sand puppies need a habitat with a highly stable temperature.
Sand puppy lungs are tiny, and their respiration rate extremely low: helpful for living inside tunnels, where air supply is limited. To compensate, sand puppy blood has a high affinity for oxygen, thereby making the most of stale air.
A sand puppy also has a modest metabolism. Low metabolism and breathing efficiency mean that aging by oxidative processes is minimized. One consequence is that cancer is unknown in these animals.
Sand puppies sleep a lot. Thanks to their lifestyle, sand puppies are especially long-lived for a rodent: only 3 years for hard-scrabbling workers, but up to 3 decades for breeders.
The sand puppy and the Damara mole rat are the only known eusocial mammals. A sand puppy colony has a single breeding queen, along with 1 to 3 breeding males among the 20 to 300 individuals in a colony; membership of 75 is average. Reproductivity in other females in the colony is suppressed by a pheromone secreted by the queen.
Food is hard to find in the parched terrain where sand puppies live. A solitary puppy would not survive. Eusociality solved several problems that ensured species survival.
Besides the queen, who sole task is procreation, sand puppies perform different roles at different ages, appropriate to their aptitude. (It was once thought that sand puppies have a strict caste system, like eusocial insects. Instead, Austrian zoologist Markus Zöttl explains, “mole rat social organization has more in common with the societies of other cooperative mammals, such as meerkats and wild dogs, than with those of social insects.”) The young attend to the queen.
Diligent and robust rats act as soldiers, standing watch against predators. Odor distinguishes friend from foe.
A solitary sand puppy is a snack to a desert snake, but a snake invading a sand puppy tunnel will meet a fierce defense.
Sand puppies nearby attack en masse. If one manages to sink its teeth into the snake, it will the drag the snake deeper into the tunnel, where the snake is gnawed to death.
The smallest adult sand puppies are laborers: digging for food and maintaining the burrows. Workers chew their way through hard soil in hopes of a robust root to feed the colony back home.
Eating dirt is no fun, even for a sand puppy – so a worker will tuck a block behind its teeth before gnawing its way to a tremendous tuber. The block acts as a barrier to dirt or other debris that could choke an industrious puppy.
For a barrier block a sand puppy picks a suitably sized wood sliver or bulb husk shaving. A puppy will pause to reposition his block as needed. A block is replaced when it becomes worn.
The eusocial sand puppy is an extreme example of the tendency which rodents that live in exposed habitats have: to form dense colonies underground, in burrows. Other examples include the Arctic ground squirrel, marmots, viscachas, and various voles. A curious culmination of this trend is the prairie dog, of which there are 5 species. The one with the broadest distribution, and the most studied, is the black-tail, which lives in the Black Hills of South Dakota.
Prairie dogs see in dichromatic color. They have good visual acuity: a necessity for spotting predators, and for visual signals at a distance with other prairie dogs.
Prairie dog hearing is in the same audible range as humans. Their sense of smell is exceptional. Prairie dog sociality is grounded in intimate olfactory reckoning.
A local colony is termed a town, which may span a hundred or more hectares in a tunnel complex. Black-tail prairie dog towns may have as many as 1,000 residents.
Towns are physically separated into wards, which define territories. A ward averages 0.05–1.01 hectares. Ward boundaries may be defined by a ridge, rocks, stream, shrubbery, or trees.
Each ward is home to a coterie: a family group comprising up to 30 or more members who all share a burrow. A typical coterie has 1 adult male, 2–3 adult females, and a few pups. But coterie composition varies. Family groups with more adult females may have more than 1 adult male. Prairie dogs have a flexible social system.
Adult males in a coterie tend to get along only when they are related. Unrelated breeding males in a coterie tend to be indifferent or antagonistic to each other.
A single male may have a harem of 2 or 3 groups of females, in which case the different female groups are not often friendly to each other.
Males maintain the burrow and defend territory. Mothers care for their offspring. All pups are sired by the resident adult male.
A male leaves his natal group when sexually mature: going forth to find another coterie to defend and breed in. Females tend to stay in their natal group for life, and so form the foundation of stability in a coterie. The same territory is passed on to the next generation.
Larger coteries tend to break up, with some individuals emigrating to form a new coterie. An adult male ventures into an adjacent unoccupied space and starts a burrow. He may be soon followed by 1 or a few adult females.
The resident male of each territory defends his territory. Interactions at ward borders may happen 20 times a day, each lasting a few minutes. Two males will stare at each other, chatter, and may make bluff charges.
In any such close encounter, males invariably raise their tails and expose their perianal scent glands, then sniff each other out. These glands reveal an animal’s health; an honest communication that typically ends the encounter before it turns into much of a fracas.
If a fight breaks out, which only happens infrequently, prairie dogs bite, kick, and ram each other. If a female spots an intruder that is her size or smaller, she may take him on herself. Otherwise, she calls the resident male.
Coterie members all recognize one another. They kiss when they meet: touching lips with open mouths and teeth exposed to get each other’s scent. They may then simply brush past each other. But family members with cordial relations will take time to groom each other when they meet. One lies down while the other nibbles its fur.
Occasionally, a kiss ends with the two laying side by side for a while, then heading off together for a bite to eat. Prairie dogs are herbivores.
Allogrooming is a frequent pastime. Pups are especially fond of it: often pursuing adults to present themselves for grooming.
Prairie dogs employ a rich set of communication protocols: olfactory, visual, and auditory. Their vocalizations clearly demonstrate that prairie dogs have a language all their own, including cultural transmission passed down through generations.
A predator in the vicinity raises a special high-pitched barking. The barking spreads from burrow to burrow. Such calls tell the nature of the predator. A hawk or eagle elicits a different yip than a snake or coyote.
The behaviors that various predators incite also differ. A prairie dog in the flight path of hawk dives into his hole. Those outside the flight path stand and watch the action.
Coyotes inspire watching. Prairie dogs will attentively stand outside the burrow entrance. Those inside the burrow will come out to stand and watch too.
Prairie dogs have other calls for events that do not involve predators. A male gives a slow, intermittent bark to defend its territory. A female guarding her babies in a burrow against intrusion has a distinctive muffled bark.
Being chased after losing a fight, a prairie dog typically emits a churring sound. This signal of submission lessens the hostility of the pursuer.
The most dramatic black-tail prairie dog display is the jump-yip territory call: an adult male stands, stretched out, its head back, and forefeet in the air, while making a call. A jump-yip from one can inspire others nearby to do the same.
A Keystone Species
Prairie dogs are a keystone species, with considerable impact on ecosystem diversity and abundance. Their burrows improve soil nutrient cycling and moisture retention, which benefits plants and earthy organisms. Like beavers, prairie dogs are ecosystem engineers.
Prairie dogs cultivate their homestead; not by digging, but by eliminating unwanted species. They cut back tall grasses around their towns. This removes cover for predators. It also allows shorter plant species to grow, which are otherwise inhibited by the tall, shady grasses.
Many quick-growing flowering species sprout around prairie dog towns, providing seeds that these animals are fond of. If an unwanted plant starts to grow, its shoot is often ripped off at the base. The plant is left to wither under the Sun’s steady gaze.
Prairie dogs know the plants they prefer. Many are a rich source of water, which is vital for survival, as prairie dogs rarely drink water.
As landscapers, prairie dogs construct mounds around each tunnel entrance using excavation soil. These mounds are maintained, as they provide a valuable vantage point to watch for predators, or for raids by prairie dogs from other towns. The mounds also help prevent flooding when the heavy seasonal rains come.
It is often thought that herbivores negatively affect the plant community. Prairie dogs illustrate that this is not necessarily the case. Their grazing alters plant composition in the community, while increasing nitrogen uptake in local vegetation, thereby creating a gyre that advances plant species richness.
The positive ecological effect of prairie dogs cascades through the larger food web in their habitat. Bison and pronghorn preferentially graze in prairie dog towns.
When these hoofed animals first appear, the little dogs are wary. Once the prairie dogs determine that these beasts are benign, they often began feeding close by, considering these larger animals good cover.
Bison find burrow entrances attractive dust bowls to wallow in. The bison churn the mound soil. If damage is done, a crew of prairie dogs come out to repair.
Sometimes bison favor certain spots, repeatedly rooting around. In such instances of serious bison business, the entrance is abandoned. Prairie dogs clearly understand the situation.
Mice and rabbits are known to seek refuge in a prairie dog burrow. They are usually tolerated.
Insects are more abundant where prairie dogs live. So too insectivorous birds and rodents, as well as those that prey upon them and prairie dogs.
More than 160 vertebrate species have been identified as positively affected by prairie dogs: a small fraction of all that benefit from their presence.