Evolving An Ethical Response to Mice and Rats

(Featured image: Norway rat. Courtesy Bayer CropScience UK, used under CC BY 2.0)

Originally published as an editorial in Animal People News, March 2012

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Probably the most ethically vexatious of all mammals,  if not all sentient beings,  are mice and rats – who are also by far the most numerous,  problematic,  and at times the most deadly of all non-insect pests to human beings.

From the origins of food storage,  well before the beginnings of agriculture,  mice and rats were the most ubiquitous and successful of food thieves.  We owe our long association with dogs in great part to the role of dogs as rodent hunters,  attracted not only to our refuse but to the chance to eat the mice and rats who were already feasting on it.

When our ancestors learned to store grain vertically,  to avoid spoilage,  and dogs could not climb well enough to hunt the mice and rats who soon infested the stacks,  cats were welcomed into human society.

The great feline Sphinx guarding the Giza pyramids in Egypt is believed to be a half-lion creature,  but one might instead imagine that the feline portion of the Sphinx represents Felis silvestris lybica,  the small desert cat tamed and beloved by the Egyptians.  Without this little desert cat to provide rodent control services that allowed the storage of grain in silos,  the mighty pharaonic civilizations would never have risen and flourished.

Without dogs and cats helping to control mice and rats, humans might never have been able to produce and store food in sufficient abundance to permit the existence of civilization.  Yet even with the aid of dogs,  cats,  traps,  poisons,  and firearms, humans still struggle to keep mice and rats from destroying about 15% of global grain production in a typical year,  and 25% in bad years. Regional losses of 50% are not unheard of.

Further,  while humans kill mice and rats by the multi-billion, diseases spread by mice and rats have killed humans by the multi-million,  and continue to kill tens of thousands of people per year.  The Black Death plague variant carried by black rats and other rodents killed some 75 to 200 million people in Eurasia in the 14th century, after Europe’s cat population had been decimated by religious persecution due to association with devil worship in the 13th century.  In some parts of Europe, only feral cats survived.  Rats remain the primary vectors for the fleas who in turn carry the Yersina pestis bacterium that causes plague.  Rats and mice also transmit hantaviruses,  host the spirochetes that cause leptospirosis,  and are the chief carriers of the ticks who transmit Lyme disease,  ehrlichiosis,  babeosis, Rocky Mountain spotted fever,  and a host of other disabling, sometimes fatal illnesses.

Innocuous as individual mice and rats appear to be,  and as easily befriended as they often are,  they remain collectively an authentic menace.  But the same could be said of fellow human beings. The test of ethical behavior has always been whether those who aspire to it are able to extend it to those who are problematic,  even threatening.

The challenge presented by mice and rats to the human capacity for doing toward others as we would be done by has recently been elevated by the discovery that laboratory rats themselves demonstrate sympathetic behavior.  “In repeated tests,  rats freed another trapped rat in their cage,  even when yummy chocolate served as a tempting distraction,”  summarized Associated Press science writer Seth Borenstein on December 8,  2011.  “Twenty-three of the 30 rats opened the trap by pushing in a door.  The rats could have gobbled the chocolate before freeing their partners,  but often didn’t,  choosing to help and share the goodies.”

Explained study author Peggy Mason of the University of Chicago,  “Basically they told us that freeing another rat is as important as eating chocolate.”

There is anecdotal evidence that rats at times display cross-species altruism and even exercise heroism on behalf of humans. For example,  on April 12,  1998,  in Torquay,  Devon,  United Kingdom,  an eight-month-old rat named Fido broke out of his cage and instead of running outside to escape a fire,  climbed to the second floor of a house to alert a woman named Lisa Gumbley and her daughters Megan,  9,   and Shannon,  3,  in time for all of them to escape.  This came about nine months after a hamster named Snowball alerted Meghan Holman,  7,  to a housefire in Hamden,  Connecticut. Though a hamster is not a rat,  a hamster is of the vole family, most members of whom are casually described as “rats” or “mice.”

The distinction between rats and hamsters raises the further point that the estimated 1,100-odd species commonly called “rats” and “mice” constitute about a fifth of all known mammal species,  only marginally outnumbered in diversity by the estimated 1,240 bat species.  All of them,  both rodents and bats,  are much more closely and directly related to humanity through common ancestors than are dogs,  cats,  horses,  elephants,  whales,  dolphins,  and every other order except nonhuman primates for whom arguments have been advanced for special moral consideration based on genetic similarity.

If,  as proponents of the Great Ape Protection Act argue, genetic similarity to humans should determine which animals are protected from exploitation,  rats and mice should be protected ahead of almost all animals other than non-human primates. Yet,  paradoxically,  rats and mice have always been excluded from any protection under the U.S. federal Animal Welfare Act – by regulation exempting rats,  mice,  and birds from the definition of “animal” from 1971 to 2002,  and by amendment of the law since then, to thwart a U.S. Department of Agriculture agreement with the American Anti-Vivisection Society that the definition of “animal” would be expanded to include all mammals and birds.

The human aversion to rats and mice is often extended to other rodents,  including prairie dogs,  gophers,  squirrels, beavers,  muskrats,  and nutria,  who are similarly killed en masse in various parts of the world where they are considered to be “invasive.”

But even people who would not countenance any mistreatment of squirrels,  chipmunks,  prairie dogs,  or beavers will often kill rats and mice by horrendously cruel means,  or look the other way while others do the killing,  using glue-traps,  bone-crushing spring traps,  and an array of poisons which may not be legally deployed against other species.

A case often remembered as demarcating the limits of successful prosecution for cruelty was the 1994 effort of the Associated Humane Societies of New Jersey to prosecute Frank Balun, 69,  for awkwardly bludgeoning a rat he had trapped alive in his garden.   Balun was acquitted and then-Associated Humane Societies president Lee Bernstein was ridiculed by mass media commentators for weeks.

There have in fact been many successful prosecutions of people for cruelty to mice,  rats,  and other small rodents since then,  including in New Jersey,  but typically in three specific contexts:  acts in which pet rodents were harmed to terrorize human victims,  rampage mayhem by people who break into homes or schools, and mass neglect of animals kept as pets or for business purposes. Cruelly killing or injuring mice or rats who are neither recognized as human property nor members of endangered species has apparently not been successfully prosecuted – nor even been attempted since the Balun case.  The March 2011 attempt of the ASPCA to prosecute the alleged revenge killing of a pet hamster during a family dispute in Brooklyn brought media response reminiscent of the Balun case.  The Brooklyn district attorney dropped the charges due to alleged inconsistent statements from the witnesses.

Contraceptives & rodent-proofing

As noted above,  mice and rats often do serious harm to humans as vectors of disease,  and humans have good reason to protect their property against rodent invasions.  Because mice and rats are small,  furtive,  and explosively prolific whenever they find a protein-rich food source (which triggers estrus in many species),  they are also among the most difficult of animals to defend against.

It is no more difficult to get mice and rats to consume contraceptive baits than it is to poison the mice or rats.   Every birth control drug used by humans or to contracept other animals has been extensively tested on mice and rats.  The dose levels needed to achieve contraceptive effects are thoroughly documented.  But, outside of closely controlled laboratory settings,  trying to reach the 70% of a mouse or rat population who must be reached to achieve a lasting population reduction is usually just about impossible.  Only one pregnant immigrant mouse or rat can rebuild the population to the carrying capacity of the habitat within a matter of weeks.  The rapidity of mouse and rat reproduction,  moreover,  allows nature to select for characteristics such as expedited,  delayed,  or erratic estrus cycles,  that bypass the methodology of many contraceptives.

In much of the world,  including the U.S.,  there is also warranted concern about the possible effects of food chain build-ups of contraceptive chemicals,  potentially affecting livestock, endangered species  and human health.  Rodent contraceptives have been experimentally deployed in the U.S.,  but remain far from accepted for general use.

China – which may lose the most grain to mice and rats of any nation – has much more aggressively experimented with widespread distribution of contraceptives for mice,  rats,  and other small rodents.  In 2008-2009,  for example,  forestry officials in Chanji, Xinjiang province,  spread 440 pounds of contraceptive baits over a 121,000-acre test site.  Over a year’s time the $11,400 effort effected an 8% reduction in the wild gerbil population.

Contraceptive developers might eventually discover solutions to the many difficulties that inhibit contracepting mice,  rats,  and their kin.  In the interim,  the kindest approach to thwarting mouse and rat invasions is to thwart the invasion itself,  rather than allowing an infestation to become a crisis requiring drastic measures to control.

This requires better understanding mice,  rats,  and their many close relatives.

As familiar as mice and rats are to most humans,  and have been for millennia,  few people who find themselves trying to cope with an abundance of mice and/or rats have any clear idea what they are dealing with.  Samuel Johnson in his 1755 Dictionary of the English Language defined “Mouse” as “The smallest of all beasts:  a little animal haunting houses and corn fields,  destroyed by cats.” Johnson defined “Rat” as “An animal of the mouse kind that infests houses and ships.”  For most people,  in most places and times, Johnson’s definitions are sufficient – even to ethicists,  biologists, and ecologists.  But there is much more to understand,  if humans are to evolve a less violent coexistence with these creatures,  who share virtually every human dwelling,  place of business,  and site of food production or preparation.

First,  the species usually called “mice” are actually of multiple common genus,  including house mice,  whitefooted and deer mice,  dormice,  and several different and only distantly related species called harvest mice.  Voles are even more numerous.

The species usually called “rats” include the many varieties of North American wood rats,  some of them endangered;  African and Asian cane rats,  also including some endangered species;  and only three widely distributed species who have historically been problematic to humans.  These are the Norway rat,  now the most common rat worldwide;   the somewhat smaller black rat,  originally native to Asia,  now occurring almost everywhere that Norway rats have not invaded;  and the African grass rat,  Arvicanthis niloticus,  the rat most threatening to the first human civilizations.  Among these,  the Norway rat is a voracious nest predator of  mice,  voles,  and smaller rats.  The black rat has similar habits.  Arvicanthis by contrast has little role in controlling other rodents,  other than by eating them out of house and home.

House mice,  of European origin,  are of the genus Mus,  and are the mice of most concern to most people.  A mere 30 species of house mice have conquered the world.  The Mus species who have evolved to live indoors among humans now live almost entirely in human-created habitat,  and tend to be ill-fitted for survival out in the elements,  where they may become prey of a much more diverse array of predators than dwell among humans.

Whitefooted mice and deer mice,  of whom there are 66 species,  are of the genus Peromyscus.  They inhabit most of North America,  living primarily in the wild.  They are seasonal invaders of human habitation,  chiefly in wet weather,  but rarely linger indoors.  They are,  however,  the major carriers of hantaviruses and the ticks who spread Lyme disease.

The 155 species of so-called “field mice” are actually voles. Distributed globally,  voles can be major crop raiders,  but seldom enter human dwellings.

Much smaller than most voles,  dormice and harvest mice are of similar habits,  disinclined to enter human homes but quite likely to devour grain in fields.  They typically hibernate for up to half of each year.  Dormice occur mostly in Europe and Asia;  harvest mice in Asia and North America.

Almost all houses,  anywhere,  but especially older houses,  have Mus as a constant but seldom seen presence.  Inside walls,  attics,  and basements,  mice in limited numbers can serve as a valuable check on insect larvae,   while the occasional marauding rat can help to check the mouse population.  There is a whole wildlife ecology inside walls,  between floors,  and in crawl spaces,  of which most people have little conception or knowledge.  It works the same way as any other,  with an active food chain.  Commonly perceived as the bottom of a food chain,  because almost every predator larger than a mouse eats mice,  Mus is actually more in the middle.

Mice tend to become a visible presence – and a problem – only if people leave food accessible.  An abundance of food then causes a mouse population explosion. Successfully responding to the situation requires restoring ecological balance.  Fortunately this is as easy as identifying and removing the problematic food source,  for example by storing all edibles in jars rather than bags or boxes made of paper,  cardboard,  or plastic;  and keeping food on shelves that have been mouse-proofed by caulking every hole and crack.

People who hope to evict mice without cruelty often resort to trying to live-trap and relocate them – typically a much more difficult procedure than mouse-proofing,  and usually pointless, because if a house mouse is released outdoors and is not immediately picked off by a predator,  the house mouse will find a way back into a heated building of some sort.  Often the mouse will colonize a new part of the nearest building.  Other mice will meanwhile occupy the vacated habitat niche of the mouse who has been relocated.

Mice other than Mus usually enter human homes only temporarily,  during cold or wet weather,  but leave again as soon as possible.  These species can be live-trapped and taken outdoors successfully.  Mouse-proofing,  though,  is easier and will expedite their voluntary exit by,  first,  securing food sources to prevent mice from gaining access;  then,  when there are no further signs of mice indoors,  finding and blocking the entrances that may permit mice to return indoors later.

Norway and black rats are willing and able to invade any accessible habitat with a food supply.  This includes any accumulation of edible refuse – which Norway rats,  in particular, may discover by following Mus.   Norway rat invasions typically follow an abundance of mice;  Mus nest predation fuels rat reproduction.  Suppressing the mouse population therefore suppresses the rat population as well.  Eradicating mice,  however,  does not eradicate rats after they have taken over the mouse food source.  All accessible food sources must be eliminated to cause rats to abscond.

Allowing nature to work

Beyond mouse-proofing and rat-proofing,  the most humane methods of mouse and rat control in human dwellings may be the oldest,  spelled “dog” and “cat.”  Though cats distressingly play with their food at times,  both cats and dogs are usually far quicker to dispatch rodents than any sort of trap,  and where they are present,  rodents tend to stay out of sight.

There is no humane substitute for mouse-proofing and rat-proofing grocery stores,  restaurants,  hospitals,  and other buildings attractive to rodents,  where dogs and cats are not permitted.  The most humane form of rodent control on farms and in fields tends to be allowing nature to work.  Hawks,  owls,  eagles, foxes,  jackals,  coyotes,  and snakes,  among many other natural predators,  will work for food,  or more precisely for the chance to hunt food,  and are quite willing and able to patrol against rodent infestation year round and around the clock,  if allowed to do so.

Natural predators are unfortunately often massacred to protect livestock and poultry,  a false economy,  since even predators as large as wolves are voracious mousers if mice are available.

Natural predators are also vulnerable to accumulations of pesticides and rodenticides in the food chain.  Pesticides today tend to be much more rapidly biodegradable and less likely to bioaccumulate than half a century ago,  when food chain accumulations of DDT nearly drove many rodent-eating birds to extinction,  but rodenticides are often still as deadly to rodent predators as to rodents themselves.

There is room,  in particular,  for improvement of seed coatings.  The advent of pesticide-coated seeds several decades ago made crop farming much more productive,  and permitted the replacement of broadcasting seed into deep-ploughed furrows with seed-drilling, which dramatically reduces soil erosion.  But agronomists have been slow to recognize that a seed coating which tricks burrowing rodents into believing it is inedible can be more effective than a coating that kills them.  This is because a dead rodent leaves a habitat niche for another rodent to fill,  while teaching a rodent to avoid a foul-tasting coating leaves the habitat occupied by a rodent who will not molest the seeds.

It is not certain that mice and rats are the sentient animals whom humans kill in the greatest numbers.  It is possible,  but by no means certain that humans kill more fish and chickens per year than mice and rats.  Fish and chickens,  however,  are almost entirely slaughtered for food;  mice and rats are killed just for existing.

Though fish and chickens are exempted along with mice and rats from protection by the U.S. Animal Welfare Act,   there is now global momentum in favor of improving the conditions in which chickens live and die,  while there is still only the faintest glimmering of awareness of the sentience of fish.

In November 2011,   however,  the University Grants Commission of India moved to spare an estimated three million rodents,  two million fish,  a million birds,  a million frogs,  and as many as nine million insects,  mollusks,  and arthropods per year by asking Indian universities to phase out classroom dissection in both undergraduate and postgraduate studies.  In  January 2012,  the European Union followed up a series of reductions in animal testing requirements by revising the Biocides Directive to reduce the use of animals – mostly rodents – by forty to seventy percent.

The conditions that make people respond with fear and loathing to the presence of mice and rats in homes do not apply to those who are purpose-bred and often bio-engineered for laboratory use.  The often quite costly mice and rats kept in laboratories are not unwanted and are not disease vectors,  except when deliberately infected to study diseases under close controls meant to prevent any infection of human handlers.

This suggests that the place to begin to redefine how human coexistence with mice and rats proceeds is in laboratories.  It is time for mice,  rats,  birds,  and other sentient animals to receive Animal Welfare Act coverage,  and time for more scientists to study how to resolve human conflicts with other species,  including mice and rats,  by means other than incessant massacre.