Delayed Implantation or Embryonic Diapause is a reproductive strategy used by approximately 100 different mammal

Mammal

Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s in seven or eight different orders

Order (biology)

In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

. In embryonic diapause, the embryo

Embryo

An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...

 (blastocyst

Blastocyst

The blastocyst is a structure formed in the early embryogenesis of mammals, after the formation of the morula. It is a specifically mammalian example of a blastula. It possesses an inner cell mass , or embryoblast, which subsequently forms the embryo, and an outer layer of cells, or trophoblast,...

) does not immediately implant in the uterus

Uterus

The uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...

, but is maintained in a state of dormancy

Dormancy

Dormancy is a period in an organism's life cycle when growth, development, and physical activity are temporarily stopped. This minimizes metabolic activity and therefore helps an organism to conserve energy. Dormancy tends to be closely associated with environmental conditions...

. Little to no development takes place while the embryo remains unattached to the uterine wall. As a result, the normal gestation

Gestation

Gestation is the carrying of an embryo or fetus inside a female viviparous animal. Mammals during pregnancy can have one or more gestations at the same time ....

 period is extended for a species-specific period of time.
While much of the molecular regulation involved in activating dormant blastocysts has been characterized, little is still known about entry into diapause, and the conditions which enable a blastocyst to remain dormant.

Some mammals that undergo embryonic diapause include rodent

Rodent

Rodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....

s, bears, mustelids (e.g. badger

Badger

Badgers are short-legged omnivores in the weasel family, Mustelidae. There are nine species of badger, in three subfamilies : Melinae , Mellivorinae , and Taxideinae...

s), and marsupial

Marsupial

Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...

s, (e.g. kangaroo

Kangaroo

A kangaroo is a marsupial from the family Macropodidae . In common use the term is used to describe the largest species from this family, especially those of the genus Macropus, Red Kangaroo, Antilopine Kangaroo, Eastern Grey Kangaroo and Western Grey Kangaroo. Kangaroos are endemic to the country...

s). Some groups only have one species that undergoes embryonic diapause, such as the roe deer

Roe Deer

The European Roe Deer , also known as the Western Roe Deer, chevreuil or just Roe Deer, is a Eurasian species of deer. It is relatively small, reddish and grey-brown, and well-adapted to cold environments. Roe Deer are widespread in Western Europe, from the Mediterranean to Scandinavia, and from...

 in the order

Order (biology)

In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 Artiodactyla.

Purpose

Mammals may undergo diapause to avoid the risk to their own lives during unfavourable and/or environmental conditions.
Mammals use embryonic diapause to time the birth

Birth

Birth is the act or process of bearing or bringing forth offspring. The offspring is brought forth from the mother. The time of human birth is defined as the time at which the fetus comes out of the mother's womb into the world...

 of their offspring

Offspring

In biology, offspring is the product of reproduction, of a new organism produced by one or more parents.Collective offspring may be known as a brood or progeny in a more general way...

 for favorable metabolic and/or environmental conditions. Reproduction has a large energy cost and it is beneficial to have ideal conditions (e.g. available food, mild weather, previous offspring weaned) to ensure the offspring

Offspring

In biology, offspring is the product of reproduction, of a new organism produced by one or more parents.Collective offspring may be known as a brood or progeny in a more general way...

 survives before giving birth.

Facultative diapause

Facultative diapause is also known as lactational delayed implantation due to its regulation via lactation

Lactation

Lactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process occurs in all female mammals, however it predates mammals. In humans the process of feeding milk is called breastfeeding or nursing...

. If a female copulates while still lactating for a previous litter, the sucking stimulus will cause the embryos to enter into diapause. This is known to occur in some rodent

Rodent

Rodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....

s, insectivore

Insectivore

An insectivore is a type of carnivore with a diet that consists chiefly of insects and similar small creatures. An alternate term is entomophage, which also refers to the human practice of eating insects....

s and marsupial

Marsupial

Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...

s.

Obligate diapause

Obligate diapause is also known as seasonal delayed implantation and is a mechanism that allows mammals to time the birth of their offspring for favorable environmental conditions. This mechanism occurs as a regular part of the reproductive cycle in mammals such as armadillo

Armadillo

Armadillos are New World placental mammals, known for having a leathery armor shell. Dasypodidae is the only surviving family in the order Cingulata, part of the superorder Xenarthra along with the anteaters and sloths. The word armadillo is Spanish for "little armored one"...

s, all species of pinniped

Pinniped

Pinnipeds or fin-footed mammals are a widely distributed and diverse group of semiaquatic marine mammals comprising the families Odobenidae , Otariidae , and Phocidae .-Overview: Pinnipeds are typically sleek-bodied and barrel-shaped...

, many mustelids, all ursids, one species of fruit bat

Fruit Bat

Fruit Bat can refer to:* Megabats, a species of bat which eats fruit* Les "Fruitbat" Carter, guitarist of Carter the Unstoppable Sex Machine* Fruit Bats , an American band...

, and the roe deer

Roe Deer

The European Roe Deer , also known as the Western Roe Deer, chevreuil or just Roe Deer, is a Eurasian species of deer. It is relatively small, reddish and grey-brown, and well-adapted to cold environments. Roe Deer are widespread in Western Europe, from the Mediterranean to Scandinavia, and from...

.

Remarks

The term Diapause

Diapause

Diapause is the delay in development in response to regularly and recurring periods of adverse environmental conditions. It is considered to be a physiological state of dormancy with very specific initiating and inhibiting conditions...

is also used to refer to the temporary, partial, or complete metabolic suspension in various life stages of insect

Insect

Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

s and plant

Plant

Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

s. This can cause problems in terminology. Delayed implantation or embryonic diapause is a specific type of diapause which developed independently of other instances of the phenomenon. A uterus is required for implantation; therefore, the terms "delayed implantation" and "embryonic diapause" may only be used in describing the phenomenon in mammals.