Mongolian Gerbils

All mongolian jirds in captivity are the offsping of 20 breeding pairs who were captured in 1935 in Eastern Mongolia and Manchuria. They were brought to Japan and were bred there. Years later, offspring of those animals came to the US and later to Europe. To enlarge the genpool, breeders have imported wild jirds in the last few years.

Mongolian gerbils live in semi-desert steppes and sand deserts ranging from Mongolia and Northern china to the south of Siberia. Temperatures fluctuate extremely in the course of a day and over the year, sommer temperatures are around 30°C/86°F and will drop to -10°C/14°F in the winter, while there is very little precipitation.
Mongolian gerbils have adapted to these changes: they will feed mainly on leaves of a variety of plants during the summer while they will gather seeds and grasses in their den for the winter.
They construct a large system of tunnels that's up to 1.5 meters/5 feet below the surface and that is constantly worked on by all members of the group. The tunnel system and the rest of their territory is marked with urine, faeces and a secretion from a gland under their belly, the ventral gland.
One group consists of a large family and is kept together by the group smell. To ensure that all members smell alike, they often smell each other's nose and mouth. They have a very distinct social behaviour and one important part is that each member of the group takes on a certain role. There are sentinels who watch over the group and will warn the others by squeaking or drumming loudly on the ground with the hind legs, causing the group to run into the tunnels.
Intruders are fought and chased away, not only other animals, but also other gerbils. If an intruder cannot run away, it will be badly hurt or even killed. Mongolian gerbils are an important part of their native ecosystem because they are the main part of the diet for many predators. Humans sometimes consider them to be an agricultural pest and they are used for medical research in laboratories.

Mongolian gerbils reach a length of 8-12 cm/3-5“ without the tail, some males will grow bigger than females. They weigh between 60 and 120 grams/2-4 ounces.
Agouti-coloured jirds are sandcoloured to lightly brown on top and a light creme on their belly. Selective breeding has created many colour morphs, from pure white to (almost) pure black and everyhting in between.
The head is round with long vibrissae, big black (or red, depending on the fur colour) eyes and small, oval ears. The overall impression is more that of a hamster that of a mouse.
The body is elongated, the hind feet are longer than the front feet and the tail is long, with dense fur and a little tuft on the end.
Mongolian gerbils have a gland in the area of the nictitating membrane (the „third eyelid“) the Harder gland, which produces are reddish secretion that is used when grooming. This can look like the jird is bleeding from its eyes, but it's normal and doesn't need to be treated (it's a good thing to ask a vet to see if he is knowledgeable about gerbils, though).
There's a area with sebaceous gland under their belly that produce a yellowish, musk-like secretion. That is used to mark their territory by dragging their belly over the ground.

If you follow a few basic rules, Mongolian gerbils are fairly easy to care for and they don't react badly to small mistakes you may make (which is not to say that you shouldn't take care to avoid them). They can be tamed easily, usually allow themselves not be handled and if you spend time with them daily, they get attached to their owner.
That makes them suitable for beginners and for people who want to handle their pets and interact with them. However, their skittishness and small size makes them unsuitable for children.

Handling them is easy because most jirds will allow you to touch them. You just need to make sure that they don't fall since you can't hold them in one hand as easily as fancy mice. Shy individuals can be caught with a pet carrier, aggressive gerbils can be handled wearing thick leather gloves.
To sex them, just hold them and turn them on their back or you put them in a glass bowl or pet carrier to take a look from beneath.
At the vet, they can be handled just the same. If you need to fixate your gerbil securely, put it on the table and hold it at the base of the tail and at the neck, that way the vet can give injections without hurting the gerbil by accident when it struggles. Escaped gerbils can be caught with a pet carrier, a live trap or a spoon net.

Mongolian gerbils are very lively rodents, active during both day and night. Even though they live in huge family groups in the wild, each pair has its own territory in the group. That's why a pair of the same sex is the most stable arrangement in captivity. The bigger a group is, the more likely it is to fall out. Groups of females are more likely to fight than groups of males.
Mongolian gerbils have a rich repertoire of different behaviours that are very interesting to watch. They groom each other to bond or fight playfully to establish and strengthen their pecking order by sitting up on their hindlegs and boxing with their paws. Soft squeaks are used to communicate, but when they fight for food, they will make louder protest sounds. Drumming with the hind legs is a sign of danger or a warning that this territory is already occupied. All other sounds are in the ultrasonic range and not audible for humans.
Mongolian gerbils will mark their territory with secretions form their ventral gland in captivity, too. You may discover a yellowish substance in some corners of the cage. The migration behaviour can still be observed with domesticated gerbils, juveniles run a lot more than adult animals. That behaviour lessens when the jirds become mature. Some jirds will start to gather food during autumn and winter.

Mongolian gerbils are best kept in pairs of the same sex in a tank measuring 100x50x50 cm/40x20x20“, that's a 50 or 75 gallon (check this website for common tank sizes). Cages, terraria or other enclosures are not suitable because you cannot fill in the bedding as high as needed without the gerbils shoving it all out again when digging. A homemade cage is an option that allows you to fill in bedding at least 30 cm/1 ft high or a tank with a homemade cage mounted on top. Heavy furniture like stones, sandbath ect. must be put on firm ground, for example a second level, so that the gerbils don't dig under it and get crushed.
Wood shavings (non-resinous wood!) mixed 50:50 with hay is suitbale for bedding because it allows the gerbilss to build stable tunnels. Cotton bedding can be added to the mix as well. Sand, hemp bedding or pellets are not suitable because the gerbils cannot dig tunnels in them. Bedding should be filled in 25-40 cm/10-15“ high. A sand bath filled with chinchilla sand is essential.
A ready-made tunnel system in or on the bedding is a good toy for the gerbils, but it mustn't be made from plastic, since that would be dangerous when chewed and cannot be ventilated well. Toilet paper rolls can be made into such a labyrinth.
Branches, roots, shingles, hiding places made from natural materials such as wood, cork, clay ect. can be used for furniture as well.
It depends on the size of the enclosure and if the gerbils are allowed outside their cage whether they need a wheel or not. If you want to offer them a wheel, make sure it's safe. It must have a diameter of at least 30 cm/12“, the running surface must be solid and one side of the wheel must be closed to avoid injuries. It's best if the wheel is made from wood, but the Wodent Wheels made by www.transoniq.com are also suitable.
A food bowl is not strictly necessary, you can just scatter the food in the bedding, the gerils will gather it. That will keep them occupied. Fresh food can also be offered outside a bowl, you need to make sure that no leftovers go bad inside the cage, though.
Fresh water is best offered in a glass bottle since plastic bottles are gnawed and water bowls are buried in the bedding.

Mongolian gerbils eat about 10-15 grams/0.3-0.5 ounces of food per day, that's about two teaspoons. You shouldn't feed them more than that to prevent them from getting fat. A good food consists mostly of small seeds, millet in particular. A mix of parakeet and canary/exotic bird food and some grass seeds (unfertilized) can be used. You can add a rodent food without pellets or a mix of cereals - both without too many oily seeds, pumpkin and sunflower seeds. Dried herbs are a good addition that supply the gerbils with minerals.
Fresh food should mainly be vegetables and lefy greens since fruit can cause or help along diabetes and other diseases caused by a too sugary diet. Some vitamins are only soluble in fat, so it's a good idea to add a few small drops of olive oil to fresh food like carrots.
Animal protein is an essential part of their diet. You can offer live or dried insects: mealworms, crickets, locusts, silkworms, butterworms. Hardboiled egg can also be given.
A mineral stone is only useful when the gerbils have a mineral deficiency for some reason. Healthy gerbils don't need it and it can cause kidney stones due to an oversupply of minerals.

Gerbils that want to mate will sniff and chase each other for hours before they actually mate. It's possible to see that the female is pregnant after two weeks, that's about the time when she will start to build a big nest.
After 22 to 29 days of pregnancy the female will give birth, often in the early hours of the morning. A litter can hold between 1 to 8 babies, sometimes as many as 12, and they weigh between 2.5 and 3.5 grams. Usually there are more females than males. The mother will be ready to mate again only hours after giving birth.
After 2 or 3 days the babies will have the first fur and their ears will open after 5-6 days. Their fur is almost complete and the eyes open when they are about two weeks old, they will begin to explore the cage then. They need to drink milk for another week and must stay with their mother to learn from her until they are at least four weeks old, but preferrably longer. The males must be separated when they are eight weeks old, since they are able to breed between the age of 63 and 84 days.
They are fully grown when they are between 6-9 months old. Females should only be bred when they are at least 70 days old, males at 84 days and females that are older than two years shouldn't have any more litters since the risk of complications is too high.
Due to excessive breeding only for colour and not for health, Mongolian Gerbils don't live as long as their wild relatives would do in captivity, most die between 3 and 5 years. The oldest gerbil known to us died at an age of almost 8 years.
Since they can have up to ten litters per year and a vast knowledge of genetics is required to breed healthy gerbils, it's better to keep pairs of the same sex. There are already more gerbils than people want and shelters and private rescue organisations often have more than they can manage. It would be irresponsible to add to this by letting your gerbils breed.

Some colour variations are susceptivle to diabetes, which the owner often notices because the gerbil drinks a lot. Diabetes can't be treated and it's best to try to avoid it by not offering any fruit or sweet, sugary treats.
Tumors are another common problem, especially for older gerbils. The tumors are usually inside the body and can only be found by palpatating the body when they have grown vast.
Inflammating of the ventral gland is common, but usually harmless and easy to treat.
Due to inbreeding and breeding without knowledge of genetics domesticated Mongolian Gerbils are not as healthy as their wild counterparts, the most obvious sign of that is the shorter life span.
You should always have a reserve of about 100 Euros for medical emergencies. Here's a list of gerbil vets in the UK.

Introducing young gerbils that are about 8 weeks old is easiest and often it's enough to let them meet on neutral ground.
Introducing adult animals can be much harder and sometimes impossible. Tried and tested methods are the introduction step by step, separating the two gerbils only by a grate and/or putting them in a very small box. Gerbils introduced with the last method need to be watched at all times to prevent them from hurting each other.
You need to allow for at least two weeks for a gerbil introduction to make sure that they really get along. If the gerbils continue to fight seriously, drawing blood, the introduction has failed. But that's no reason to believe that your gerbil is happier alone, it just doesn't get along with that particular gerbil. It will be happy with another gerbil, they have their likes and dislikes. Keeping a single gerbil is not an option.

If you have decided that gerbils are the right pet for you, you wo'nt have much problems finding a pair. They can be found in shelters and private rescue organisations and often they have already waited for years for a new home. If you google „gerbil rescue“ and add your state/hometown, you will probably find a rescue organisation near you.
If you absolutely want a gerbil of a particular colour, look for a knowledgeable, reputable breeder. There are many people who call themselves breeder, but who just put males and females together and sell whatever the results are – that's not breeding. Breeding gerbils takes a lot of knowledge about genetics, a lot of planning, time and space. In the UK, the Nation Gerbil Society is a good place to find a reputable breeder , in the US, try the American Gerbil Society who will also help with finding gerbils in shelters.
Pet shops of course sell gerbils, too, but it's not a good idea to buy there. There's a good chance that you will end up with gerbils that are sick, have parasites or are pregnant, since many pet shop employees cannot sex gerbils reliably (and may well sell you a male and a female). They also often get their gerbils from so-called breeders who operate like puppy mills.

You need to set aside between 100 and 200 Euros for two gerbils, depending on how you house them (tank/homemade enclosure, bedding, toys/furniture, food, transport cage). You can save money by buying an old fishtank, for example on http://www.freecycle.org/ or http://www.craigslist.org.
The running costs for food, bedding and bathing sand a pair of gerbils are around 20 Euros.

Mongolian gerbils are not suitable for co-housing them with other rodents due to their aggressiveness and hunting urge.

Sources
http://www.diebrain.de/re-index.html
http://www.wuesten-rennmaus.de/
http://www.uni-salzburg.at/did/bio_fuer_kids_und_teens/lebende_organismen/rennmaus.htm