The mechanics of getting animals to breed in captivity is not always easily solved. With species that have a period of estrus or heat the female reaches a point where she will tolerate copulation for a brief period. If males and females are penned in adjacent cages the male will typically be sensitive through olfaction to when estrus occurs and will rub against the females fence and try to reach her. When the female approaches the male as well it is usually safe to allow them together but watch carefully and usually allow the male into the females enclosure or put them both into a new enclosure so territorial defense does not precipitate conflict. Female bears have been killed when introduced into a cage that the male defended as his territory. Species that exhibit a solitary existence in nature are those most apt to fight when penned together although many zoos are able to keep mates penned together continuously.

Most zoos prefer to have animals raise their own young as naturally as possible; hand reared, human-oriented young often imprint on humans and do not adapt well later to living with conspecifics. Sometimes it becomes necessary to hand-raise an animal because the baby has been rejected (or the mother is dead). Then it is best to hand-raise the youngster, wean it as soon as practical and then introduce it to the herd or group in order that it can learn natural behavior. Sometimes in the case of hoofed stock the young one is placed in with the herd or near by in an adjacent pen and bottle-fed. When the mother rejects the offspring at birth, begins killing some of the litter, dies at or soon after birth while still caring for or feeding the offspring, or if she or the infant require medical treatment hand-rearing may be prescribed.

If the delivery requires human intervention through pulling, in an assisted delivery or through caesarian section, keepers should try to get the young to suckle from the female. This is so the baby will get the colostrum, as the first milk produced by the dam after birth is called. This milk contains an abundance of antibodies and proteins that assist the infant in developing its immune system. If the baby cannot or will not suckle try to hand-milk or use a breast pump on the female to extract colostrum to feed the baby.

A baby mammal that must be hand-reared should be taken to the nursery and if cold given a warm bath followed by rubbing it with a towel and blowing it dry with hot air from a hair dryer. If the baby does not voluntarily suckle fluids may be given but a finger dipped in milk and introduced to the mouth will often initiate suckling. Depending upon the age and kind of animal different nipple and bottle sizes are required and feeding frequency has to be that appropriate for each species. The fed formula must be that suitable for each different species. Baby seals require milk that is nearly half oil. The American Association of Zoological Parks and Aquariums has published an Infant Care Diet Handbook on hand-raising techniques for mammals, including feeding schedules and milk formulae.

Where necessary vitamin injections may be prescribed and in areas where selenium deficiency is present all young hoofed babies are given a selenium injection shortly after birth. Young carnivores may require vaccination against feline and canine distemper and rabies.

Babies should be kept in warm, draft free areas for at least a few days. If a baby animal has not received colostrum from its mother, cow or goat colostrum extracted soon after the birth of the calf or kid and kept frozen until needed can be used. If the mother can accept the baby for feeding try to encourage them to be together at least for that time. That allows you to monitor the condition of both the mother and the baby.

Sometimes young altricial birds may be abandoned in the nest or adults die before the young can care for themselves. Hand-raising altricial birds, especially insectivorous ones is difficult and time consuming as they need to be fed every half hour to an hour during the day. Freeze dried hamburger can substitute for insects but it should be hydrated and mixed with vitamins before being fed. A human simply is not as efficient at collecting insects as parent birds are.

Young precocial birds raised from eggs hatched in the incubator do not require as much attention as altricial birds. However, keepers need to ensure that the young learn how to feed, sometimes by picking up bits of food and dropping it. The diet needs to be watched so growth is not too rapid or leg bending and deformed wings may result. Young precocious birds do not need to feed for the first day or two after hatching as they have an internal yolk reserve. However, they are not hatched with good temperature control so must be kept warm for the first week or more. A sudden rainstorm that soaks and chills young precocious birds may appear to have killed them but if they are brought inside, dried with a hair dryer and placed under warm - not hot - heat lamps they may miraculously recover. DO NOT TRY TO DRY OR WARM ANY ANIMAL IN A MICROWAVE OVEN!

Newly emerged reptiles and amphibians also retain an internal supply of yolk and thus do not need to feed immediately. Amphibians with their prolonged larval period require daily monitoring of food and water quality. Cage mates may quickly consume smaller animals, so try to separate them by size.

Extra effort may be needed to get newly emerged reptiles to feed. Be certain they are warm, then, try a number of different prey items. Sometimes it helps to rub earthworms or a frog over the proffered item, as reptiles seem quite sensitive to smell. Some snakes prefer dark objects and will refuse white mice others prefer baby hairless rodents for their initial meal. Some reptiles will only feed when alone, others seem to be stimulated when littermates are present. Although baby reptiles may require extra attention the satisfaction of success is well worth the effort.