A litter is the live birth of multiple offspring at one time in animals from the same mother and usually from one set of parents, particularly from three to eight offspring. The word is most often used for the offspring of mammals, but can be used for any animal that gives birth to multiple young. In comparison, a group of eggs and the offspring that hatch from them are frequently called a clutch, while young birds are often called a brood. Animals from the same litter are referred to as littermates.
In most female mammals, the average litter size is about half the number of mammae.[1] Presumably, this enables females to successfully nurse litters even if some mammae fail to produce milk. Breeding female naked mole-rats, however, can bear and successfully rear litters that are far more numerous than their mammae because young take turns nursing from the same mammary and breeding females and pups are fed and protected by colony mates, enabling queens to concentrate their reproductive efforts on gestation and lactation.[2][3]
Factors affecting litter size
Animals with shorter gestation periods typically produce larger litters of less developed young (altricial), while those with longer gestations generally have smaller litters of more developed offspring (precocial).[1] Another factor that can affect litter size is the species of that particular animal. Smaller mammals tend to have larger litters, while larger mammals tend to have smaller litters. Animals that bear multiple young at one time often have offspring that are born at a relatively early stage of development.[3] There are some exceptions to the rule. Pigs are an exception because they do tend to have larger litter sizes and longer gestation periods. This is due to both genetics and selective breeding practices.[4]
Species
Gestation (days)
Average Litter
House mouse
19-21
6
Rabbit
28-35
7
Domestic cat
58-67
4
Domestic dog
58-68
5-6
Pig
115
10-14
Cow
283
1
Human
266-280
1
Multiple other biological and environmental factors influence litter size within and between species:[citation needed]
Maternal body size: Larger individuals within a species typically produce larger litters
Maternal age: Litter sizes often increase with age up to a point, then decline
Nutrition: Better-fed mothers produce larger litters with higher survival rates
Number of mammae: Average litter size often approximates half the number of teats
Advantages of being born into a litter
Animals frequently display grouping behavior in herds, swarms, flocks, or colonies, and these multiple births derive similar advantages. A litter offers some protection from predation, not particularly to the individual young but to the parents' investment in breeding. With multiple young, predators could eat several and others could still survive to reach maturity, but with only one offspring, its loss could mean a wasted breeding season. The other significant advantage is the chance for the healthiest young animals to be favored from a group. Rather than it being a conscious decision on the part of the parents, the fittest and strongest baby competes most successfully for food and space, leaving the weakest young, or runts, to die through lack of care.
Maternal care and rejection
Not all offspring born in a litter receive equal care. Maternal rejection occurs across many species as a mechanism to optimize reproductive investment.
Litter size exceeding the mother's capacity to nurse or care for all offspring
The presence of runts or developmentally weak individuals
Maternal stress, poor nutrition, or illness
Inexperience in first-time mothers
In wild populations, rejected offspring rarely survive without intervention.[citation needed] The "runt" phenomenon is particularly common in species with large litters, such as pigs and dogs, where the smallest individual may receive less access to milk and maternal attention.[3]
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