House Mouse: Belligerent Biology

“When a group of genetically identical mice lived in the same complex enclosure for three months, individuals that explored the environment more broadly grew more new neurons than less adventurous mice, according to a study. This link between exploratory behavior and adult neurogenesis shows that brain plasticity can be shaped by experience and suggests that the process may promote individuality, even among genetically identical organisms.” –Science Magazine (May 9, 2013)

Mouse individuality may play a large part pursuant to its adaptability and global survival. Injurious to man, the House mouse, Mus domesticus, is one of the most successfully pestiferous mammals. Commensal (benefit one party) interaction has allowed House mouse to thrive outside its natural environments. Neolithic revolution (agriculture and husbandry) has been essential to commensal evolution.

Along with human global expansion has come species biodiversity erosion. However, House mouse has successfully radiated with human endeavor.

Rodent synanthropy (“together with man”) has also radiated the epidemiology of infectious disease. House mouse has both obligate and facultative parasites that can have serious public heath implications. Conversely, the House mouse has been domesticated as an essential biomedical research model.

House mouse thrives under a variety of conditions. Mice are found in and around homes, commercial structures, open fields and agricultural operations.

House Mouse Taxonomy

• Kingdom: Animalia
• Phylum: Chordata
• Class: Mammalia
• Order: Rodentia
• Suborder: Myomorpha
• Family: Muridae
• Genus: Mus
• Species: domesticus

Considerable history exists between humans and mice. The Greeks built a temple to Apollo Smintheus (Mouse god) as appreciation for mice possibly chewing the leather of their adversaries’ shields in 1500 BC. During the late Greco-Roman and early Christian eras, parts of mice were included in potions designed to cure sickness.

The Chinese and Japanese have a long history with House mouse. The “Year of the Mouse” is every 12 years. The “Hour of the Mouse” is 11:00am to 1:00pm. The messenger of the “god of wealth” is the mouse.

Living 1.5 to 2.5 years, House mouse is generally non-aggressive (evades vs. confronts). Males housed together will probably fight, sometimes to the death of the opponent. “Barbering” is the phenomenon of focal loss of hair or whiskers with no wounds due to the establishment of hierarchy and defense of territory.

House mice have an adult body length of 3.0 to 3.9 inches (7.5 to 10 cm). Tail length is 2.0 to 3.9 inches (5 to10 cm). Weight is 0.4 to 0.9 ounces (10 to 25 gm). Coloration ranges from white to grey to brown to black (light on underside). Ears and tail present some hair.

Mice hind feet are 0.59 to 0.75 inches (15 to 19 mm) long. Gait is a run with a stride of 1.8 inches (4.5 cm). Vertical jumps can be up to 18 inches (45 cm).

Males and females are not easily discerned. Females have a smaller distance between their anus and genital opening. Females have five pairs of mammary glands. Males have a distinct presentation of testicles that are large compared to the rest of the body (can be retracted).

Body Exterior

• Bright eyes.                                                                                            
• Erect rounded ears.                                                                          
• Pointed snout with vibrissae.                                                                            
• A long, prehensile tail is sensitive to pain, acts as a balancer, and a thermoregulatory organ.
• Short legs.
• Feet with five toes on each.
• The first digit on the front feet (pollex) lacks a middle phalanx and is small.
• Feet have walking pads and rudimentary sweat glands open to the surface of the footpads.
• No hair upon the nose, palms, lips, and soles.
• The coat is composed of two classes of hairs (pelage and tactile).
• Pelage hairs are associated with sebaceous glands and smooth muscle (erector pili).
• Tactile hairs (larger and longer than pelage hairs) have nerve endings and are associated with venous sinuses and glands.

Body Interior

• Poor body heat regulation (can die at 99°F).
• Mice do not have true sweat glands and do not pant, but salivate some in response to high temperatures (preserves body water).
• In response to high temperature, cool shelter is sought.
• Rectal temperature is 95 to 102.5°F.
• Heart rate is 320 to 840 beats per minute.
• Blood pressure is120/80 mm Hg.
• Respiration rate is 84 to 280 breaths per minute.
• Urine is highly concentrated.
• Males have strongly scented urine (four times more allergenic than females).
• Mice excrete large amounts of protein (taurine) in urine.
• Dental formula
is incisors 1/1, canines 0/0, premolars 0/0, molars 3/3 x 2, or 16.
• A diastema or prominent space separates the incisors from the molars.
• Incisors (normally yellow in color) grow continually and are self-sharpened and wear down by mastication.
• Overgrowth may result in malocclusion (poor positioning).
• Mice cannot vomit (stomach is divided by a limiting ridge into a non-glandular proximal fore-stomach and a distal glandular body).
• Male has a baculum (penis bone, penile bone, or os penis) found within the penis of most placental mammals.
• Anogenital distance is 1.5 to 2 times greater in the male than in the female.
• The female has five pairs of nipples and mammary glands (3 thoracic and 2 inguinal). 

Lee-Boot Effect 

• When female mice are together in small groups (4-5) with male absence, there is an increase in spontaneous pseudo-pregnancies.                                         
• Overcrowded females tend to become anestrus or sexually inactive (initiated by female odors).

Whitten Effect

• If anestrus or pseudo-pregnant female mice are paired with a male, one half of the pairs will mate on the third night.
• Ninty percent of the pairs will mate within 5 days.
• Whitten described it as a post pairing synchrony of estrus (induced by a volatile substance within male urine).
• Male pheromone will accelerate puberty in exposed females.

Bruce Effect

• If pregnant mice are exposed to a strange male during the pre-implantation period (days 1 to 5, post breeding) the original pregnancy will be blocked/aborted.
• This effect is seen most frequently within the 24-48 hours post breeding.
• This effect is mediated by an androgen-dependent olfactory pheromone contained in male urine.

Reproduction                                                                            

• Puberty occurs between 28-49 days of age.
• Polyestrous is year round with spontaneous ovulation (cycle; 4 to 5 days).
• A vaginal plug is formed at copulation and contains a mixture of vesicular and coagulating gland secretions.
• A vaginal plug usually fills the vagina from the cervical canal to the vulva (persists for 18-24 hours and 80 to 90% of mice with vaginal plugs become pregnant).
• Parturition (birth) generally occurs 12:00am to 4:00am (female signals birth onset with stretching movements and delayed if lactating).
• Pups are born within 1 to 2 hours (dead pups are eaten by the female).
• Postpartum estrus is 20 to 24 hours post-parturition.
• If the female is bred during postpartum estrus there can be a period of delayed implantation that lasts 4 to10 days (gestation will be extended the same amount of time).
• Post lactation estrus is 2 to 4 days after litter evacuation.
• Females may give birth to a second litter before the first litter is weaned (the first litter is generally the smallest while the fourth or fifth are the largest).
• A litter of more than 10 pups is not uncommon.
• Breeding stops at 1 to 1.5 years in females and later in males.

Gestation

• Time is 19 to 21 days.
  • Born nidicolous (helpless).
  • Born altricial (blind and hairless).
  • Ears open in 4 days.
  • Eyes open in the second week.
  • Full hair at 14 days.
  • Require milk and weaned at 21 days of age.
  • Solid food at 11 days.
  • Do not receive antibodies in-utero.
  • Lack immunity and require maternal antibodies within lactating female’s milk.
  • Teeth lacking (incisors erupt in the second week).
  • Pups do not acquire thermoregulation capability until the end of the first week (kept warm by the mother).

Hearing

• Born deaf and hear at 21 days.
• Hearing is highly developed.
• Repeated exposure to high pitch sound causes retardation of growth and reproduction.
• Audiogenic seizures are possible.
• House mice communicate through ultrasonic vocalizations (USVs).
• It is observed that USVs are produced by a whistle mechanism within the vocal tract.
• The House mouse auditory sensitivity is highest at 15 kHz with a second subsidiary peak at 50 kHz.
• USVs are emitted by pups as distress calls (mostly due to cold), by adults during courtship (mainly males), and sexual interactions (females).

Vision

• Eyes open at 14 days.
• Retina is composed almost entirely of rods (lack cones = poor color vision).
• The retina ventral area has a greater density of UV sensitive cones.
• Poor sight.
• Harderian glands present behind the eyes produce secretions containing porphyrin (red fluorescent pigment) that is a stress indicator.

Smell

• House mouse’s olfactory system allows recognition of considerable information mediated by chemical substances of different origin and structure.
• Substances are recognized as general odorants or chemical signals via the main olfactory epithelium (MOE) and the vomeronasal organ (VNO).

Pheromones and chemical signals of the House mouse

Touch 

• Mice can sense air movements and surfaces with whiskers or thigmotaxis (migration along a gradient).

Behavior

• Mice indicate curiosity by moving toward a novel stimulus.
• Mice will rear-up or climb and push their noses high to air-sample.
• Hours are spent in grooming behavior (individually and groups).
• Areas of urination and defecation are not always well established, but communal sleeping areas are not soiled.
• Mice are generally nocturnal (cycles of activity and rest during the day and night).
• Sleep 10 to 15 minutes, wake, adjust posture, and sleep again.
• In groups, mice sleep in communal piles with individuals changing positions.
• When fighting, mice inflict bite wounds from behind the tail.
• If dominant males or females continually bite other individuals, deaths may result (lower ranking mice may lose weight under continual attack).

Nutrition

• House mouse is an omnivore (feeding ad libitum or by choice).
• Mice consume approximately 15 grams of food and 15ml of water per 100 grams of body weight (mostly at night).
• A diet of at least 20% protein is required.
• Mice store fat-soluble vitamins (synthesize vitamin C and recover vitamin B through coprophagy).
• Forefeet are used to manipulate food (incisors prepare food for mastication by molars.

House mouse belligerent biology and adaptation allows an infestation to develop very radically within vertical and horizontal architecture. In addition, mice are understood to be carriers of many diseases that present a risk to human health. And finally, mice cause significant damage to food production and property.