AFRMA - Small vs. Large Litters

This article is from the Win-Spr 2018 AFRMA Rat & Mouse Tales news-magazine.

Small vs. Large Litters

By Nikki Kimura, Pacific Coast Rattery

Permission given to reprint article.

I see a lot of breeders favoring large litters. Some like having a lot of babies. Some like having a large variety of colors. Some seem to think bigger is better. Some simply cannot bring themselves to thin out litters due to ‘ethical’ reasons. For whatever reason others have for large litters, these are my reasons to have smaller litters.

Personally, I like having smaller litters. Small litters occur due to genetic selection or culling. While I do have larger litters time to time, I personally do not like to exceed 8 pups in a litter max. Why? Rats can have up to 15+ pups in a litter. The largest litter I have heard of was 32 babies. But for the discussion’s sake, we’ll say 13+ rats is a large litter and 6 is small.

Individuals in larger litters are born much smaller than those in smaller litters. The pups in a larger litter are smaller to fit into the same uterine space as those in a smaller litter. Of course every female is different. But really. The mother can only fit so much inside of her body during pregnancy, which is why I like selecting for naturally smaller litter sizes.

A doe has 12 nipples to feed her young. But even though she can feed 12+ babies, that doesn’t mean they will still get the proper amount or even equal amount of food and nutrition. Competition among siblings in a large litter can prevent litter mates from getting what they need to thrive. Survive? Certainly. Thrive? The chances are definitely less favorable in that aspect.

There are long term negative effects on rats that can occur if they do not get the proper nutrition as pups. That applies to any species really. Oftentimes, I see smaller or leaner pups come out of larger litters. From what I have seen in some Facebook breeder groups, I see pups that look sickly because they are stunted. These pups will typically never reach their genetic size potential due to malnutrition in their infant days. And there are scientific papers to support this observation.

Scientific studies of rat pup growth in large and small litters have shown that larger litters have a delayed growth and development. They also weigh less, contain less cells, and less fat throughout their entire lives even into adulthood. Individuals from larger litters have lower protein, DNA, and lipid content compared to individuals in smaller litters. Those in larger litters were stunted in terms of the development and appearance of opening of the eyes and fur growth. Pups in smaller litters were heavier and displayed developmental milestones earlier than those from larger litters. This is not just an animal being fatter. They are more dense. In one of the studies (shown below), pups from the smallest litter were roughly 100 grams larger than pups from the largest litter at 2 months of age.

Now, regarding the mother. Nursing depletes the mother of energy and fat reserves. That is why some females look so utterly thin during nursing and weaning. Of course providing her with a proper diet that meets her caloric and metabolic needs is a factor. But it simply doesn’t take a genius to understand that it is easier to raise fewer offspring than many offspring. A female will burn more energy to raise larger litters. She has to produce milk several times faster at greater amounts to feed more pups that will still not get ideal amounts of food. But smaller litters are less of a burden on females. Don’t get me wrong. Rats are some of the best mothers on the planet and are capable of raising larger litters to weaning. And there are many factors such as genetic, natural variation, and environmental factors that should be considered when breeding a female and how a female will raise her young. But as a breeder, I want to see my rats thrive. Not just survive. I want to give them the best chances possible to reach their greatest potential. And giving them the proper start nutritionally as pups is one of the best ways to do so.

Figures from the studies can be found in the following photos. And one photo is the physical difference between two individual rats that I have bred.

A pup from a litter of 4 that is 2 weeks (seen left) old compared to a pup from a litter of 13 that is 3 weeks old (seen right). Both have the same father, but different mothers. However, the mothers are related so there is very little genetic variation here. The 2 week old is larger and certainly more robust than the 3 week old. The developmental difference between a 2 and 3 week old rat is rather large. And in terms of these two individuals, the larger 2 week old reached developmental landmarks faster than the older pups from the larger litter. Lastly, I want to add that the pups from the smaller litter definitely have better body condition and density than their larger litter counterparts as adults.

A 2-week-old from a litter of 4 compared to a 3-week-old from a litter of 13. Photo ©Nikki Kimura.

Table 1. Relation between litter size and body weight of male rats. From Effect of Litter Size on Weight Gain in Rats Judith J. Wurtman Sanford A. Miller, The Journal of Nutrition, Volume 106, Issue 5, 1 May 1976, Pages 697–701.

Table 2. Relation between litter size and body composition of male rats. From Effect of Litter Size on Weight Gain in Rats Judith J. Wurtman Sanford A. Miller, The Journal of Nutrition, Volume 106, Issue 5, 1 May 1976, Pages 697–701.

There have been questions regarding if larger well-fed pups will become obese adults. Here are two photos of the same rat.

A) The infant photo of this rat shows how well fed he is. He is obtaining very good nutrition due to small litter size (n=5).

B) The adult photo of the rat shows that he is not obese or overweight in any way, shape, or form.

Photo A. A well-fed infant rat. Photo ©Nikki Kimura.

Photo B. ...and the baby as an adult. Photo ©Nikki Kimura.

This pup (right) is a 3 week old baby that I produced that was raised in a small litter. The pup is larger than the average baby of the same age.

The following images from Exotic Whiskers Rattery [Raterie Limoges; Tania Limoges, Québec, Canada, Ed.] are of two 3-week-old rats from separate litters born on the exact same day. The Russian Blue is from a smaller litter while the Black Rex is from a larger litter. It is very clear that even the smallest difference in litter size can impact growth.

3-week-old rats born on the same day, the Russian Blue from a smaller litter while the Black Rex is from a larger litter. Photo ©Tania Limoges.

3-week-old Black Rex from a larger litter

The same rats (from Exotic Whiskers Rattery - Raterie Limoges) at 69 days with the Black Rex at 241 grams and the Russian Blue at 277 grams. Photo ©Tania Limoges.

April 21, 2019