Why Is One of My Gerbils Bigger than the Other?

Gerbils that are the same age and live together are usually the same size. However, it’s not uncommon for one gerbil to grow bigger than the other. There are many reasons why this could happen.

There may be a size difference because one of your gerbils is eating more than the other. Perhaps the smaller gerbil is a picky eater or has an illness affecting its appetite. Otherwise, one of your gerbils could be restricting the other’s food as a display of dominance. If your larger gerbil is female, she may be pregnant.

We’ll look at the reasons why one gerbil is larger than the other. We’ll also explain how to tell if one of your gerbils is too fat or overly thin.

What Affects the Size of a Gerbil?

Gerbil pups start small but grow quickly. Most gerbils reach their full size by 6 months of age. The average adult gerbil weighs 90g and measures 4 inches long (not including the tail).

If your gerbils are of different ages, this may explain why they’re different sizes. Some gerbils continue growing and changing shape until they’re a year old.

Even once they reach adulthood, not all gerbils end up the same size. There are four main factors that can result in gerbils being different sizes:

  • Sex. Adult male gerbils are often, but not always, bigger than females.
  • Diet. Gerbils fed nutritious diets grow bigger than undernourished or malnourished gerbils.
  • Activity level. Gerbils that don’t get much opportunity to exercise are at risk of becoming overweight.
  • Genetics. Some gerbils are genetically inclined to grow bigger than others. Their parents’ size can affect their adult weight and length.

If your gerbils aren’t related, genetics may be responsible for their size difference. But if they were littermates, that means they have the same parents and are the same age.

Two gerbils that live together will also have access to the same food and exercise opportunities. So, if you have one gerbil bigger than the other, something might be wrong.

Why Is One Gerbil Smaller than the Other?

A significant size difference among a pair of gerbils can be a cause for concern. It doesn’t necessarily indicate a serious problem, but an illness or husbandry issue could be responsible.

1) One of Your Gerbils Is Older

If you bought your gerbils at the same time, you probably assumed they were the same age. But pet stores occasionally mix gerbils from different litters and keep them in the same cage.

This is often due to a lack of space. Baby gerbils usually get along well, and they sell quickly. So there’s usually no problem with keeping unrelated same-sex gerbils in the same cage.

Even if you were told that your gerbils were the same age, it may not be true. Pet store employees can be wrong. They might receive incorrect information about the animal’s age from their supplier.

If your gerbils are still growing, wait and see how big they are in a few months. The smaller gerbil may catch up to the larger one as it grows older.

2) Your Gerbils Are Different Sexes

Gerbils are difficult to sex. Their genitalia looks similar, and there is little sexual dimorphism. While they’re pups, male and female gerbils are the same size and look almost identical.

For this reason, it’s common for pet stores and breeders to misidentify a gerbil’s gender. Your gerbils may have been labeled as two males or two females when really, they are different sexes.

This could be responsible for their size difference. When they’re adults, male gerbils are typically larger than females. This is due to their differing roles in the clan.

Wild male gerbils are responsible for defending the boundaries of their territory. They grow as large as possible so that they’ll have an advantage in a fight. The largest male is also the one who gets to mate with the females.

For female gerbils, on the other hand, there’s no advantage to being larger. Their sole job is to rear young, which they can do at any size. If you want to find out your gerbil’s gender, take a look at our guide to sexing gerbils.

3) One Gerbil is Pregnant

If one of your gerbils is female, she may be pregnant. This would explain why she’s larger than your other gerbil.

Pregnant gerbils grow larger each day throughout their pregnancy. On average, they grow by 1-3 grams per day.

As the pups develop, the mother’s abdomen will become bigger and more rounded. She’ll also put on fat in other areas. This will help her to produce more milk to feed her young.

The weight gain will speed up as the pregnancy progresses. A few days before your gerbil goes into labor, she’ll look noticeably round. This is when you’re most likely to notice the weight gain.

If both your gerbils are females, one of them may have become pregnant in the pet store. Gerbils can conceive from as young as 9 weeks old, according to Developmental Psychobiology.

A gerbil pregnancy lasts approximately 25 days. If you’ve had one gerbil bigger than the other for longer than this, it’s probably not the answer.

If you have any suspicion that your gerbil may be pregnant, take it to a veterinarian. They’ll be able to tell if your gerbil is expecting pups and sex your other gerbils at the same time.

4) One of Your Gerbils Is Lazy

To survive, wild gerbils must stay active almost constantly. On a daily basis, they have to:

  • Dig and maintain complex underground tunnels
  • Forage and gather food, sometimes traveling long distances from their burrows
  • Run away from potential predators and other dangers
  • Mark the boundaries of their territory and see off trespassers
  • Play, socialize, mate, and fight for dominance with other gerbils

Because of this, most pet gerbils have naturally high energy levels and large appetites. They’ll eat anything that’s put in front of them, especially high-calorie treats like seeds.

To make up for this, gerbils take whatever exercise opportunities they can get. Most gerbils love running on a wheel, digging, climbing, and exploring gerbil-proofed areas outside the cage.

Regular activity will allow your gerbils to maintain a trim physique. But some pet gerbils are lazy and won’t exercise as often as they should.

If you have a lazy gerbil, it will slowly put on weight and become larger than your other gerbils. This is because it’s eating the same food but isn’t burning off as many calories.

5) Your Gerbil Is a Picky Eater

Gerbils need to eat a selection of different foods to get all the nutrients they need. In the wild, gerbils’ diets are extremely varied. They eat a wide variety of plants and insects.

Most owners feed their pet store-bought gerbil food. Gerbil food mixes are well-balanced and contain proteins, fats, carbohydrates, fiber, vitamins, and minerals.

However, some gerbils are fussy eaters. They pick and choose which parts of the mixture they eat and which they leave.

The nutritionally dense, high-fat parts of the food are favored (like sunflower seeds). The low-calorie, ‘healthier’ components, such as hay pellets, get left behind. 

This is a leftover evolutionary mechanism that ensures that the gerbil gets enough calories to survive. But pet gerbils have access to more food than wild gerbils and don’t exercise as much.

So, filling up on high-calorie foods may cause your gerbil to become obese. This can result in one of your gerbils looking fatter than the other.

According to PeerJ, obese gerbils can develop health issues such as metabolic problems and fatty liver disease.

The simplest way to fix this is to buy an extruded gerbil food. It’s made by blending different foods and forming them into shapes. Each piece of food has the same nutritional value.

one gerbil bigger than other

6) One Gerbil’s Dominant over Food

When two gerbils live together, they quickly establish a hierarchy of dominance. There is always one ‘leader’ gerbil, and the other one is subordinate to them.

However, a gerbil’s time in power doesn’t last forever. Eventually, the other gerbil may try to assert itself as the dominant gerbil.

To maintain its position in the clan, the alpha gerbil frequently reminds the other that it’s the boss. It does this in many ways, such as:

  • Chasing the other gerbil
  • Mounting the other gerbil
  • Preventing the other gerbil from accessing parts of the cage
  • Restricting the other gerbil’s food.

If one of your gerbils stops the other from eating, this will lead to a size difference. The dominant gerbil will become larger, as it’s consuming more calories. The subordinate gerbil will be thinner.

You can avoid this by feeding your gerbils separately instead of putting a food dish in the shared gerbilarium. Move each gerbil to a separate box and give it food on its own.

7) Your Gerbil Is Sick or Injured

Many gerbil ailments can cause a gerbil to lose weight. This is usually because sick gerbils have less of an appetite than healthy gerbils.

If you’ve ever thought: ‘my gerbil is really small,’ this may be why. A sick gerbil will usually appear thinner and less active than its healthy cagemate. Some examples of conditions that lead to weight loss include:

  • Respiratory infections
  • Parasites such as tapeworms
  • Digestive obstructions (e.g. from eating plastic)
  • Poisoning from eating the wrong foods
  • Dental problems such as overgrown teeth
  • Tyzzer’s disease
  • Injuries, e.g. from fighting. Pain can cause gerbils not to want to eat.

The giveaway that your gerbil is sick, rather than just naturally thin, is if it’s losing weight. Healthy adult gerbils should remain roughly the same weight throughout their lives.

Always take your gerbil for a veterinary checkup if you have concerns about its health. Sick gerbils can die quickly if their affliction is not treated.

Is My Gerbil Too Fat or Too Thin?

If you have two different-sized gerbils, you’re probably wondering which gerbil is the right size. Is the smaller gerbil underweight, or is the bigger gerbil too fat?

Mongolian gerbils are not chubby-looking animals. They have short, sleek fur and toned, streamlined bodies. They’re designed to be agile and able to move quickly to escape predators.

When it stands on its hind legs, a gerbil’s body should look slim but not bony. So if one of your gerbils looks round, has a stomach pooch, or fat rolls, it may be overweight. On the other hand, if its belly is concave or you can see its ribs, it may be underweight.

How Much Should My Gerbil Weigh?

The best way to get an idea of your gerbil’s health is to weigh it.

Adult male gerbils should weigh between 80 and 135 grams. However, adult females should weigh between 70 and 110g. If your gerbil’s weight is within these guidelines, it is likely healthy. Your gerbils may naturally be different weights due to individual differences. Even gerbils from the same litter can have different size genetics.

But if your gerbil is outside of these margins, this indicates that it may be an unhealthy weight. According to Veterinary Clinics of North America, a weight of 20% above average indicates obesity.

Weigh your gerbils regularly, not just once. As an adult, a gerbil’s weight should remain stable throughout its life. A veterinarian should assess a gerbil that’s gaining or losing weight.

Your vet will be able to give you advice on how to manage your gerbil’s weight. This may involve adjusting its diet, encouraging exercise, or addressing an underlying health issue.

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