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Why do mother rats sometimes eat their babies?

Last Updated: 29.05.20

 

Rats are difficult to eliminate because they reproduce very fast – if you want to but the best poison available, check it out here. Oftentimes, not all litters of pups make it past the first 24 hours. Infanticide and cannibalism in rats are caused by malnutrition, birth defects, and abnormal deliveries as well as by stress. 

 

How common is infanticide in animals?

Infanticide in animals is not as uncommon as you might expect. In many cases, animals, females and males alike, kill their offspring. This practice has been observed in numerous species, especially in primates. However, infanticide is also practiced by birds, fish, insects, and rotifers.

Male infanticide often occurs when male adults kill their young to reduce intraspecific competition. This happens when the adult fears that the offspring will grow up to take the role of the male alpha and, thus, mate with the female.

There were also cases where the researchers noted that infanticide was committed so that the female will become fertile again. This is a common behavior that can be observed in lions. Males kill a female’s young so that she is sexually available again and ovulating.

Male mice, for instance, are known to become significantly more aggressive towards the young after fertilizing females. This type of behavior lasts for around three weeks. In this period, the male attacks, and, in some cases, kills the pups that it comes across.

Female animals also show infanticide behavior. In most cases, females do not kill their offspring but rather the young of other females. A good example is that of black-tailed prairie dogs. Because the females that commit infanticide are lactating when they do so, the specialists have argued that they might be driven to commit such acts to procure food.

 

 

Infanticide in rats

In rats, the infanticidal animal can be either the mother, a male, not the father, or another female. The justification why each of them commits infanticide is determined by different reasons. In most instances, these rodents kill newborns.

 

Maternal infanticide and cannibalism

Here are the main reasons why female rats who are new mothers decide to kill and eat their new litters of pups.

 

Deformities 

A mother rat will usually kill deformed or sick newborns that have no chance to survive. This way, she can allocate her entire attention to the offspring that are likely to live and thrive. Mothers also consume stillborn offspring immediately after birth. 

When the mother has a litter of healthy pups, the chances of her eating them are slimmer. However, if there are malformed or stillborn pups, she sometimes also consumes part or the entirety of the litter.

 

Stress-induced cannibalism

It has also been noted that, when the mother is stressed, it can also decide to kill her entire litter of pups. In these situations, it is believed that the mother perceives the environment as too dangerous for the newborns. 

Also, in some experiments conducted in 1984, researchers observed that the mothers who were transported during the pregnancy also ended up consuming their newborns.

 

 

Diet

When a female rat kills her litter, she eats all her babies. This way, she recovers some of the energy that she has invested in carrying and caring for them. Similarly, mothers who are malnourished or those who have had abnormal deliveries are also prone to commit infanticide.

Obesity is another reason why mothers commit infanticide. Females who were provided with rich diets and who, consequently, were obese during the pregnancy, were more predisposed to kill and eat their young within the first week after giving birth. 

Diet is closely related to infanticide and cannibalism in rats. For example, it has been demonstrated that mothers who were kept on a vegetarian diet throughout the pregnancy were more likely to cannibalize their pups. Other researches have shown that a deficiency in vitamin B12 might also result in cannibalism in new mothers. 

 

Infanticide depending on the sex of the pup

Another interesting aspect that researchers have observed in the behavior of mothers is in regards to sex-determined infanticide. In stressful conditions, where there is not enough food, female rats who live in polygamous colonies often kill their male newborns.

This happens because female rats have a better chance of surviving than males. If a male rat is not dominant enough, it will not mate with females and, thus, it won’t pass on its genes. If the mother rat gives birth when there is not enough food around, she will most likely kill and consume the male pups. Similar behavior has been observed in mice, woodrats, and even hamsters.

One interesting situation that deserves to be noted is that of Norway rats. In this species, if a female rat loses her young to infanticide, she can influence the sex of the new litter of pups that she conceives next. When this happens, the mother only births female pups. 

If the new pregnancy is successful and her pups are not killed again, the next litter that she births contains an equal number of females and males.

 

Abnormal deliveries and age

If the mother gives birth prematurely, she is more likely to kill and cannibalize her young after delivery. In some experiments, the pups were delivered through C-section. In most of these cases, the mother killed and ate her litter.

The age of the mother is also important when discussing infanticide and cannibalism in rats. According to statistics, young and older moms are more likely to exhibit this kind of behavior.

 

 

Infanticidal male adults

When male adults kill a litter of pups, they normally do it in an attempt to make the mother start ovulating again, and therefore, carry its young. Infanticide, however, is reduced when the males live near the pregnant female.

Researchers have also argued that those male rats that live near young show fewer tendencies towards infanticide. Males are more likely to kill strange newborns than female rats.

We must point out that infanticide is not a trait that is fixed in the way of being of male rats. The presence of testosterone in young rats, for instance, can make them commit infanticide. By comparison, studies have shown that castrated males rarely killed newborns.

Most males commit infanticide and cannibalism and, afterward, they mate with the mother. These males tend to only kill litters that they have not fathered. 

Infanticide in male adults can be reduced by cohabitation, repeated exposure to pups and sexual experience. 

 

Infanticidal adult females

Lastly, when other female rats commit infanticide, they mostly do it to gain access to a source of food. In these cases, the females not only kill the litter of newborns, but they also eat it. Once again, research shows that female rats do not commit infanticide if they cohabited with the new mother before birth. 

Moreover, related female rats rarely kill the pups of their sisters. Once both the females give birth, they also raise their offspring together. Some studies suggest that most adult females who commit infanticide have never given birth themselves. 

Some research suggests that some ways of inhibiting infanticide in female adults can be done by cohabitation with the new mother during the pregnancy and by exposing them to the pups.

 

Who is the target of infanticide?

As we have mentioned earlier, newborn pups that are one to three days old are the most likely to get killed. Pups that are ten to twelve days old are also sometimes killed. Baby rats that are 21 to 25 days of age are rarely killed.

The reason why so many newborn rats are killed and eaten by adults has to do with their appearance. It seems like adults are familiar with the taste of babies as, most of them, have killed and eaten newborns again.

Adults like consuming neonatal rats because they have no hair and because of their lack of mobility. When defenseless new pups smell the presence of another rat that is not their mother, they often stop moving and refrain from making sounds.

Also, the smell might entice them. There were documented cases where adults committed infanticide and cannibalism even when food was easily available to them.

 

 

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