Weaning is the process of gradually introducing a mammal infant to what'll be its adult diet and withdrawing the supply of its mother's milk.

The process takes place only in mammals, as only mammals produce milk. The infant is considered to be fully weaned once it is no longer fed any breast milk (or bottled substitute).

Weaning in human infants

How and when to wean a human infant is a subject of much controversy. The American Academy of Pediatrics recommends feeding a baby only breast milk for the first six months of its life. However a large number of mothers find breastfeeding challenging, especially in modern times when a large number of mothers have to return to work relatively soon after the birth of their child.

The American Academy of Pediatrics, the World Health Organization, the National Health Service Choices UK, and the National Health & Medical Research Council in Australia recommend waiting until 6 months to introduce baby food. Notwithstanding a large number of baby food companies market their "stage 1" foods to children between 4 and 6 months old with the precaution that the food is meant to be consumed in addition to breast milk or formula and is just for "practice". These practise foods are generally soft and runny. Examples include mashed fruit and vegetables. Certain foods are recommended to be avoided. The United Kingdom's NHS recommends withholding foods including those "that contain wheat, gluten, nuts, peanuts, peanut products, seeds, liver, eggs, fish, shellfish, cows’ milk and soft or unpasteurised cheese" until a baby is six months old, as they might cause food allergies or make the baby ill. Notwithstanding recommendations such as these have been called into question by research that suggests early exposure to potential allergens doesn't increase the likelihood of allergies, and in a few cases reduces it.

In a large number of cultures around the world, weaning progresses with the introduction of feeding the child food that has been prechewed by the parent along with continued breastfeed, a practise known as premastication. The practise was important in human history in that it naturally gave a child a greatly improved protein source and additionally that prevents infant iron deficiency. The prechewing of food additionally gives the baby long-term immunological benefits through factors in the mother's saliva. Notwithstanding premasticated food from caregivers of lower socioeconomic status in areas of endemic diseases can result in passing the disease to the child.

No matter what age baby food is introduced, it is generally a quite messy affair, as young children don't have the coordination to eat "neatly". Coordination for using utensils properly and eating with dexterity takes years to develop. Many babies begin using utensils between 10 and 14 months, but most won't be able to feed themselves well until about 2 or 3 years of age.

Weaning conflict

At this point, the mother tries to force the infant to cease nursing, while the infant attempts to force the mother to continue. From an evolutionary perspective, weaning conflict might be considered the result of the cost of continued nursing to the mother, perhaps in terms of reduced ability to raise future offspring, exceeding the benefits to the mother in terms of increased survival of the current infant. This can come about because future offspring will be equally related to the mother as the current infant, but will share less than one hundred percent of the current infant's genes. So, from the perspective of the mother's evolutionary fitness, it makes sense for her to cease nursing the current infant as soon as the cost to future offspring exceeds the benefit to the current infant. But, assuming the current infant shares fifty percent of the future offsprings' genes, from the perspective of the infant's own evolutionary fitness, it makes sense for the infant to continue nursing until the cost to future offspring exceeds twice the benefit to itself (perhaps less, depending on the number of potential future offspring). Weaning conflict has been studied for a variety of mammal species, including primates and canines.

Age of weaning

There is significant individual and cultural variation in time of weaning.

Scientifically, one can ask various questions; a few of the most straightforwardly empirical include:

  • At what age do children self-wean?
  • At what age do various societies normatively choose to wean?
  • In comparison with additional animals, especially similar primates, by various measures.

As there are significant ranges and skew in these numbers (some infants are never nursed, or only nursed briefly, for instance), looking at the median (half-way mark) is more useful than looking at the average.

Considering biological measures of maturity, notably investigated by Kathy Dettwyler, yields a range of ages from 2 1/2 years to 7 years as the weaning age analogous to additional primates – the "natural age of weaning". This depends on the measure, for example: weaning in non-human primates is often associated with eruption of permanent molars (humans: 5 1/2 to 6 years); comparing duration of nursing to length of pregnancy (gestation time) yields a factor of about 6 in chimpanzees and gorillas (humans: 6×9 months = 54 months = 4 1/2 years); body weight might be compared to birth weight (quadrupling of birth weight yields about 2 1/2 to 3 1/2 years for humans; 1/3 of adult weight yields 5 to 7 years for humans); and similarly for additional measures.

The age at which children are normatively weaned can vary significantly between cultures, "from 6 months to 5 1/2 years". (In the book of II Maccabees, 7:27, a mother mentions giving milk to her son for three years as though this was normal.) In Islam the age of weaning is 2 years but can be less if the mother wants to.

Other studies are possible, as in psychological factors. For example, Barbara Rogoff has noted, citing a 1953 study by Whiting & Child, that the most distressing time to wean a child is at 13–18 months. After this peak, weaning becomes progressively easier and less distressing for the child, with "older children frequently wean[ing] themselves."

Weaning in additional mammals

In science, mice are frequently used in laboratory experiments. When breeding laboratory mice in a controlled environment, the weaning is defined as the moment when the pups are transferred out of the mothers' cage. Weaning is recommended at 3 to 4 weeks after parturition.

For pet carnivores such as dogs or cats, there are special puppy or kitten foods commercially available. Alternatively, if the pet owner feeds the parent animals home-made pet food, the young can be fed the same foods chopped into small pieces.