This is because dogs and people have very different sets of instructions in their DNA. Their genomes are simply too different to come together and create something that can live on. A female has a litter when she releases several eggs. When fertilized, these eggs produce multiple embryos.
Identical twins are the exception, but we'll talk about that later. Three eggs result in fraternal triplets, four in fraternal quadruplets. You get an idea. The dog's breed is the biggest determinant of litter size.
In general, large dogs have larger litters because, biologically, they can safely carry more puppies. For mammals, the number of teats roughly corresponds to the maximum size of their litter, although clutches generally average about half this number. Another factor involved in determining the litter size of an organism is the optimal size of its offspring. Technically, there is no difference between a litter of five and a set of fraternal quintuplets, other than the fact that human quintuplets are exceptionally rare.
In the wild, only a small percentage, if any, of the litter can survive to maturity, whereas for domestic animals and those in captivity with human care, the entire litter almost always survives. A litter is the live birth of several offspring at a time to animals of the same mother and usually to a single group of parents, particularly three to eight offspring. Female dogs produce several eggs at once, which is why they usually give birth to litters instead of individual puppies. Tailless tenrecs (Tenrec ecaudatus) have up to 29 teats and have produced litters of 32 offspring in the wild.
According to the American Kennel Club, litters born in spring are generally larger than litters born at other times of the year. Just as human siblings have different physical characteristics, puppies in the same litter often have variations in coat color, coat length, head size, ear shape, and other physical characteristics. The average litter size of a species is generally equal to half the number of teats and the maximum litter size generally coincides with the number of teats. Humans have control over certain aspects of when and how a dog is raised that can affect litter size, but there are limits.
The result of this process, known as genetic recombination or genetic reorganization, is that the actual genetic makeup of puppies in the litter varies.
