White-Footed Mouse

Common Name: White-footed Mouse, Wood Mouse – A descriptive common name that is not very helpful; several other small woodland mice have white feet and are virtually indistinguishable from the one that bears the name.

Scientific Name: Peromyscus leucopus – The generic name is a combination of musculus, the Latin word for mouse, and pero, a boot of untanned hide. The specific name is Greek for white, leukos, and foot, pous. Taken together, a booted mouse with white feet.

Potpourri: The mice and rats of the family Muridae have a strained relationship with the most prolific members of the family Hominidae, us. Mice are beloved as caricatures; Mickey Mouse is the mainstay Disney character, Jerry, the prankster mouse is foil for the cat Tom in Hanna-Barbera cartoons, and Mighty Mouse is the anthropomorphic super-rodent of Terrytoons. Mice are feared as sinister home invading pantry marauders and as bearers of diseases that range from rabies to hantavirus. Rats, which are essentially big mice, are detested as despicable vermin, the term rat-infested a synonym for dystopia. That has not stopped them; murids are the largest and most successful family of mammals, comprising over half of all species in the class. There are about 280 genera and over 1,300 species of rats and mice worldwide. In North America there are 78 species divided into a taxonomy of three subfamilies: The New World rats and mice are Sigmodontinae, the introduced Old World species like the Norway Rat and the ubiquitous House Mouse are Murinae, and the closely related voles and lemmings are Arvicolinae. [1]

The White-footed mouse is one of seventeen species in the genus Peromyscus that are virtually identical in appearance so that only an experienced zoologist (or another mouse) can tell them apart. The predominant features are protruding eyes, large ears, a body-length tail and a coat of short, dense fur dorsally brownish with white underparts and feet. Collectively, they are sometimes referred to as deer mice as they are similarly hued in the apatetic ecru shades of open woods and both have large, trumpet shaped ears as animals ever wary of predation. Or, more poetically, “the white-footed mouse sports a chestnut coat, a white vest, reddish brown trousers, and white stockings. His eyes and ears are uncommonly large, causing his head to resemble a deer’s in miniature.” [2] Most of the look-alike species are limited to a relatively small locality according to habitat preference, as indicated by common names like Canyon Mouse, Cactus Mouse, and California Mouse. Only the White-footed Mouse mostly toward the east and middle west and the Deer Mouse in northerly and westerly regions have continent-size populations. If you see a mouse in the woods, it is almost certainly one or the other. [3] There is a simple explanation for the proliferation of rats and mice into every nook and cranny not otherwise occupied; it is in their DNA.

Mammal predecessors appear in the fossil record approximately 200 million years ago at the same time as the dinosaurs who allowed only the inconspicuous to hide away in secluded dens to scavenge for plants and an occasional insect. In this the primordial cauldron savaged by terrible lizards and bodacious brontosaurs, evolution slowly but inexorably produced the class Mammalia, all having warm blood, body hair, and the ability to produce and excrete milk to feed the young that were born live. The egg-laying monotreme mammal branch currently represented only by the egg laying duck-billed platypus and the echidnas split away roughly 170 million years ago followed by the pouched marsupial mammals about twenty million years later. Placental mammals ultimately prevailed, eventually comprising 19 distinct orders from rodents to carnivores. Their ascendance to global dominance began 66 million years ago when a meteor struck the earth in the vicinity of Mexico’s Yucatan Peninsula near the town of Chicxulub for which its impact crater is now named; the world was plunged into darkness. Concurrently, a massive lava flow on the Deccan Plateau of India filled the air with sulfur. The dinosaurs, along with three quarters of all terrestrial plant and animal life perished. The age of mammals began with a big bang that defined the beginning of the Cenozoic Era and the end of the Mesozoic. [4]

The fossil record at Corral Bluffs in Colorado provides an osseous account of what happened next – everything larger than a rat perished. Ferns and a few flowering plants provided the sustenance for the long struggle that was to ensue, testing the resilience of nature’s DNA prescription. Within 100,000 years, the mammals had doubled in size and number, feeding on the fruits of the palm trees that succeeded the ferns. Absent the depredations of dinosaurs, the mammalian population burgeoned; within a million years, 100 pound Eoconodon’s were feeding on legumes as the forest canopy recovered. [5] The mammalian diaspora was aided and abetted by the breakup of Pangaea into Laurasia and Gondwanaland as precedent to the six continents (Eurasia is one) as they are now configured. Gene sequencing techniques have established that there are four superorders of mammals that each radiated as clades from four individual common ancestors. The sloths and armadillos of South America are in the superorder Xenarthra, the aardvarks and elephants of Africa are in Afrotheria, and the cats, pigs, and horses of Eurasia and North America are in Laurasiatheria; all are named for their land mass of origination. The mammals of the fourth superorder are called Euarchontoglires that radiated from the northern continents of Laurasia to become the primates, the rodents, and the rabbits. The near human adaptability and dexterity of rodents is no longer a mystery; they are among our closest mammalian relatives. [6] Perhaps they deserve some reconsideration.

There are many reasons why rodents, especially white-footed and deer mice, should be appreciated as benign rather than maligned as pests; they are demonstrably intelligent, affectionate, and perhaps even compassionate. In the laboratory, white-footed mice master complex mechanical lever and shutter devices with split second timing to obtain different rewards according to sequence and selection. They are capable of navigating mazes with hundreds of blind turns with unflinching determination. When solitary mice are placed together, they learn from each other and develop what can only be construed as a mutually beneficial relationship, maybe friendship. When mice are placed in control of their own lighting using levers, they regulate it to match their preferential conditions of low light for activity periods and very low light during inactivity. As testing of mouse intelligence has not been comprehensive, it is probable that the cognitive demonstrations to date only touch on their abilities. [7] Even though female mice have multiple litters in the course of a single year, they are valiantly protective of their young. A white-footed mouse inadvertently flushed from her nest by a woodsman’s ax was seen to run “out upon a limb as far as she could, and jumped to the ground, a distance of full four feet, the young still adhering to her.” [8] These attributes are as commendable as they are successful; mice are prolific. Accordingly, they are prime menu entrees for many snakes, carnivores and birds, especially owls.

Mice are equally if not more promiscuous than oft cited rapidly breeding rabbits. The average white-footed mouse is sexually mature at the age of six weeks and can have up to four litters of about a half dozen pups every year for its nominal three year life span. The geometric population implications are profound – one pregnant mouse could produce a million offspring by the end of its third year, becoming a great-grandmother many times over. However, in the harsh reality of being near the bottom of the animal food chain, few live that long; the wild mouse population is replaced every year due to attrition with average population densities that range from four to twelve mice per acre. [9] High population turnover rates incubate rapid evolutionary change in response to environmental stress and mice are no exception to this rule. The Chicago Zoological Society conducted a genetic analysis comparing 56 of the mice in their specimen collection dating back to the 1850’s to 52 captured during the study in local parks. The results were dramatic; only one of the “modern” mice had the DNA sequence of those predating 1950 leading to their conclusion “that the ‘molecular clock’ may sometimes, and sporadically, tick blindingly fast.” [10] While this was attributed to the aggregate environmental effects of human population growth, targeted predation can result in even more notable changes. Owls can catch and eat five mice a night, a thousand in a year; about one fifth of all field mice end up as owl pellets comprised primarily of indigestible mouse bones and fur. The unusual white mice endemic to Floridian coastal areas contrast starkly with the dark mice further inland; their origin had been a long-standing mystery. To resolve the conundrum, a graduate degree thesis study was devised using six thirty-foot long caged owl enclosures with either white beach sand or darker forest loam as substrate. The experiment consisted of simultaneously releasing one white mouse and one dark mouse for the owl to selectively skewer. The results were unequivocal; owls preyed on contrasting mice so that white mice on sand were more likely to run free. Subsequent DNA analysis confirmed that beach mice had gene mutations in a region that controls for the white fur color. [11]

Survival of the fittest removes the physically slow, mentally deficient, or color contrasted from the mouse gene pool, leaving only the best of breed to propagate. Mice are therefore very fast, very clever, and have apatetic fur color. They are also quite good at finding food and shelter; they have no need for clothing beyond the dense fur that mammalian heritage provides. The key attribute necessary for food foraging in the forest underbrush is smell. Approximately 40 percent of the rat or mouse brain is devoted to smell compared to about 3 percent for humans. [12] This is the impetus that draws the house mouse (Mus musculus) to the cupboard and the Norway rat (Rattus norvegicus) to the dumpster. Both are originally from central Eurasia and were unwittingly transported on the ships of colonizing Europeans. The Spanish, French, and then English brought the house mouse to the south, north and east probably multiple times over a century of migration. The Norway rat is thought to have first been introduced in boxes of grain brought by the German Hessians during the American Revolution. Both are scourges of human settlements as food competitors; a mice population estimated at 82,000 per acre in California’s Central Valley devastated the food supply in 1927. The etymology of the species name musculus could be the Sanskrit musha, meaning thief; equally likely is the Latin movere, to move. Both attributes are necessary in equal measure as the warm blooded mouse has high energy demands to energize its only means of defense – blinding speed to the nearest shelter. [13]

A man’s home may be his idiomatic castle but a white-footed mouse’s home is its palladium, the refuge of last resort. But rather than expend precious energy reserves to build one, white-footed mice will not infrequently appropriate an abandoned bird’s nest or animal burrow. They have been sighted ensconced in flying squirrel nests, in a variety of bird’s nests (usually within ten feet of the ground), and even in the dome of a muskrats hut in the center of a pond. “Unlike the common house mouse, Mus (sic) leucopus has not been degraded and contaminated by living with the lords of creation; on the contrary, he avoids the habitation of man, preferring the sweet nuts, seeds, and berries of the woods to the refuse of the kitchen.” [14] The energy demands of warm-bloodedness are put to the test in winter; this is especially true for the non-hibernating white-footed mouse as its diminutive size and lean musculature provide little room for energy dense body fat storage. Conserving energy is crucial – nests are snuggly fitted out with insulation; multiple mice inclusive of non kin nest-mates huddle together for warmth; and torpor – a dawn to dusk state of reduced metabolism – is employed. Mice living in warm nests exercising daytime torpor snuggling in groups use 2.5 times less energy at 55 degrees Fahrenheit than those that free range. The evolutionary tradeoff of night food forays to avoid the majority of their many predators (owls excepted) involves going out in the cold of darkness. To provide extra energy against freezing, white-footed mice add extra red blood cells to provide supplemental hemoglobin to transport extra oxygen for elevated metabolic rates. [15] White-footed mice are exemplars of mammalian adaptation.

The warm-blooded bundles of furry white-footed mice piled in a downy nest for a long winter’s nap are not alone. Black-legged ticks (Ixodes scapularis) have co-opted mouse sleeping dens as dining rooms for the blood meal that is necessary for tick growth and maturity. Lyme disease is caused by Borrelia burgdorferi, bacteria that are established as resident parasites in several rodents, especially white-footed mice. The virulence of the bacterium is abetted by its structure, a spiral-shaped body with whip-like flagella located between inner and outer membranes. The motility afforded by the flagella enables the bacterium to escape a host’s viscous secretions that trap and eradicate other invaders and to move about from place to place; Lyme Disease progresses from the body to the joints and eventually the brain as the bacteria multiply and relocate to more remote areas. The protected flagella arrangement is the taxonomic feature of the phylum Spirochaetes which includes the bacterium (Treponema pallidum) that causes syphilis, the scourge of the Renaissance Era. [16] Ticks are parasitic vectors that unwittingly transport bacteria from one host to another, a parasite’s parasite. After hatching, ticks require a blood meal at each of the three life stages of larva, nymph and adult. Larva obtain their first blood meal almost exclusively from chipmunks, several shrews and white-footed mice in almost equal proportions. It is the latter that is especially proficient as a B. burgdorferi host, however, as 90 percent of all feeding ticks become infected. [17] The prevalence of Lyme Disease is corelated to the mouse bacterial reservoirs that are available; it goes down when the number of mouse eating red foxes goes up. [18]

The improbable relationship between a miniscule bacterium and humans involving a rodent and an arachnid should put to rest the notion that we are the masters of the universe but should rather be humbled by it; mice are our fellow Euarchontoglires. Cats are not. They showed up in the fields of grain sown by Neolithic farmers to prey on the rats and mice that were drawn to the bounty; we took them in as they took us in. Another parasite, the protozoan Toxoplasma gondii, lives in the intestines of cats. It spreads by passing through the digestive tract to the feces where it is picked up by rodents scavenging for food. To complete the cycle and return to its cat gut home, the parasite hijacks rat brains to make them risk takers, running right into the path of a hungry cat. It is probable that T. gondii has taken up residence in humans, spread in part by those cleaning cat litter boxes, so that 30 percent of the population is infected with consequent elevation in risk-taking behavior. [19] Are we really in charge? The mouse is here to stay, no matter how hard we try to eradicate it. It is no wonder that the inventors of the first automatic controller of computer screen positioning looked at the device with a long narrow cord at the end of a box and concluded that it looked like a mouse. [20]

Notes:
1. Whitaker, J, National Audubon Society Field Guide to North American Mammals, Alfred A. Knopf, New York, 1996, pp. 403, 573.
2. Walton. M. A Hermit’s Wild Friends or Eighteen Years in the Woods, 1903, quoted by B. Heinrich in Winter World, Harper Collins, New York, 203, p.200.
3. Whitaker op. cit. pp. 574-590.
4. Drew, L. I, Mammal, The Story of what makes us Mammals, Bloomsbury Sigma, London, 2017, pp. 20-25.
5. Pennisi, E. “How life blossomed after the dinosaurs died” Science, Volume 366, Issue 6464 25 October 2019 p. 409.
6. Murphy, W. et al “Resolution of the Early Placental Mammal Radiation Using Bayesian Phylogenetics” Science, Volume 294, 14 December 2001, pp. 2348-2351.
7. Kavanau, J. “Behavior of Captive White-Footed Mice” Science, Volume 155, Issue 3770 31 March 1967, pp. 1623-1639.
8. Caton, D. Untitled. The American Naturalist. quoted in Scientific American 5 June 1869
9. https://animaldiversity.org/accounts/Peromyscus_leucopus/ An online database of animal natural history, distribution, classification, and conservation biology at the University of Michigan.
10. Graham, S. “Speedy Evolution Detected in Windy City’s Wild Mice” Scientific American 22 May 2003.
11. Emlen, D. Animal Weapons, The Evolution of Battle, Henry Holt and Company, New York, 2014, pp. 13-16.
12. Sapolsky, R. Behave, The Biology of Humans, Penguin Press New York, 2017, p. 89.
13. Whitaker op. cit. pp 659-665.
14. Beard, D. “Architecture of the American White-footed Mouse” Scientific American, 3 January 1885.
15. Heinrich, B. Winter World, Harper-Collins, New York, 203, pp.199-205
16. Tilly, K. et al “Biology of Infection with Borrelia burgdorferi” Infectious Disease Clinics of North America. 22 June 2008 pp. 217–234. Available at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2440571/
17. Enserink, M. “Lyme Disease’s Unusual Suspects” Science, 21 November 2007.
18. Chakradhar, S. “Woodland Menace” Scientific American 27 November 2013.
19. McAuliffe, K. This is Your Brain on Parasites, Houghlin, Mifflin Harcourt Publishing Company, New York, 2016, pp. 57-82.
20. Display at the Smithsonian National Museum of American History.