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Naked mole rat
Although as their name indicates, naked mole-rats do not have fur,
they are not completely hairless. They have sensory whiskers on
their faces and on their tails (they move both backwards and
forwards through their tunnel systems), and they have hairs between
their toes that allow their feet to function as brooms with which
they sweep soil. Their skin is pink and somewhat translucent on the
underside and light purplish brown on their backs and tails. This
counter shading appears to be lost with advanced age. There is no
sexual dimorphism between males and females. The queen and breeding
males are generally among the heaviest individuals in a colony, and
the queen is the longest individual. When a female becomes a queen
she actually grows longer, even though she is already an adult, by
increasing the distance between the vertebrae in her spine.
Reproduction
Only one female in a colony of naked mole-rats produces offspring;
this female is called the queen. She mates with only a few of the
colony's males, and these relationships can remain stable for many
years. All of the other individuals in the colony aid in the queen's
reproduction by caring for the pups, foraging for and providing
food, and maintaining and defending the burrow system. These
"workers" are physiologically capable of reproduction, but do not do
so as long as they remain in the colony (which in most cases is
their entire lives). The duration of gestation is approximately 70
days. A queen can produce a new litter every 80 days, and can have
up to 5 litters per year. Pups generally weigh less than 2 g at
birth. The number of pups per litter is quite variable. The average
is about 12, with a maximum of 27, the largest known litter size of
any mammal. The queen nurses the pups for about 4 weeks, although
they may begin eating solid food at as early as 2 weeks of age. Pups
also eat feces that are provided for them on demand by the workers.
This not only provides nutrition, it also inoculates their
digestive system with beneficial gut fauna.
Pups begin performing work behaviors (digging, sweeping, carrying,
etc.) at the age of 3 to 4 weeks. Maturation rate is variable, but
in general juveniles are physiologically capable of reproduction by
the age of one year. Maximum longevity in this species is as yet
unknown, since animals that have been in captivity for as long as 20
years are still currently living. This life span is unprecedented
among small rodents.
Behavior
Naked mole-rats are asocial, meaning that they live in large
colonies in which only one female breeds and the majority of workers
(both males and females) spend their entire lives working for the
colony. Colony size averages 70 individuals, but colonies of up to
295 have been observed. The animals in these groups are very closely
related. Workers are generally raising their siblings, since a
single queen may reign for many years. Dispersal is quite rare and
inbreeding is common, which results in a high degree of genetic
similarity among colony members. If a queen dies or is removed from
a colony, a few females may fight to the death in order to become
the new queen. Naked mole-rats live in complex underground burrow
systems. The tunnels are about 4 cm in diameter. Some tunnels run
just under the surface of the ground, while others can be up to 2 m
deep. There is a great deal of branching and interconnection of
tunnels, with the result that a colony's total tunnel length can add
up to 4 km. Tunnels connect nest chambers, toilet areas, and food
sources. Burrowing is the only way these animals find food, since
they do not travel above ground.
Workers dig through the hard-packed soil with their powerful
ever-growing incisors, aided by the fact that 25% of an individual's
muscle mass is involved in jaw closure. A mole-rat's lips actually
close behind its incisors, such that it can dig without getting dirt
in its mouth. They work assembly-line style, with the front animals
actually breaking through the dirt while a string of workers sweeps
the soil through the tunnel system to an opening to the surface,
where another worker kicks the dirt up onto the ground above its
head, forming a mole hill. Mole-rats seem to do the majority of a
year's digging just after the rainy season, when the normally hard
ground is softened.
These asocial rodents also cooperate to thermo-regulate. Unlike most
other mammals, they cannot maintain a steady body temperature. Their
temperatures fluctuate with the ambient temperature, making naked
mole-rats essentially cold blooded. By huddling together in large
masses, they slow their rate of heat loss. They also behaviorally
thermo-regulate by basking as needed in their shallow surface
tunnels, which are warmed by the sun.
Food Habits
These rodents eat the underground parts of
plants, particularly the succulent tubers that are formed by many of
the plant species that grow in arid areas. They obtain all the water
they need through their food; they do not drink.
In captivity they
will readily consume sweet potatoes, various fruits, baby cereal and
dog biscuits. When a group of mole-rats finds a large tuber
(sometimes more than a foot in diameter) in the wild, they generally
bore through it, eating mainly the interior flesh while leaving the
thin epidermis intact. This behavior may allow the plant to remain
healthy for some time, indeed even to continue growing, thereby
providing a long-term food resource for the colony. This diet is
high in cellulose, which is difficult to digest. Naked mole-rats
have high densities of gut fauna that aid in digestion. They also
regularly practice coprophagy, the reingestion of feces, which
allows them to maximize their uptake of nutrients from their food.
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