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Why does delayed implantation occur?

Delayed implantation is a reproductive strategy seen in over 100 mammal species, including bears, seals, and mustelids like mink and ferrets. In delayed implantation, females mate and the fertilized egg divides to form a blastocyst, but the blastocyst does not immediately implant in the uterus. Instead, it remains dormant in a state of suspended animation for a period of time before implanting and resuming normal embryonic development.

What is delayed implantation?

Delayed implantation is a type of embryonic diapause, where the development of the embryo pauses at the blastocyst stage. After mating, the fertilized egg undergoes several divisions to reach the blastocyst stage. The blastocyst is made up of an inner cell mass, which will become the fetus, surrounded by an outer layer of cells called the trophectoderm. Under normal circumstances, the blastocyst would implant in the endometrium (uterine lining) within days or weeks after fertilization. However, in delayed implantation, the blastocyst remains free-floating in the uterus in a state of dormancy.

The length of delayed implantation varies by species. In black bears, implantation may be delayed for up to 5 months, while in American mink it may last only 25 days. During this arrest, the blastocyst does not grow or advance developmentally, though the inner cell mass remains alive. A signal will eventually trigger the dormant blastocyst to implant in the uterine wall and resume a normal pregnancy.

What triggers the end of delayed implantation?

The exact mechanism that terminates delayed implantation is not fully understood, but likely involves a combination of maternal and embryonic signals. Rising estrogen levels, progesterone levels, and other hormonal changes may help create a uterine environment receptive for implantation. The blastocyst itself goes through molecular changes that allow it to resume growth and finally implant in the uterine lining.

In some species, the presence of a fertilized egg alone does not seem to terminate delayed implantation. Rather, the physical stimulation of mating or the presence of certain proteins in seminal fluid appear necessary to trigger implantation. This may help ensure that implantation does not occur unless the female has mated and environmental conditions are favorable for bearing young.

Key factors in ending delayed implantation

  • Rising estrogen levels
  • Changes in progesterone levels
  • Other hormonal changes in the female
  • Molecular changes in the blastocyst
  • Physical stimulation of mating
  • Proteins in seminal fluid

Why does delayed implantation occur?

Delayed implantation is thought to confer several evolutionary advantages that enhance reproductive success under challenging conditions:

1. Allows mating at times optimal for fertilization

Some species only come into estrus at times when harsh environmental conditions or scarce food supplies would make pregnancy risky. Delaying implantation allows females to mate at times best suited for fertilization, while waiting for more favorable conditions to give birth and raise young.

2. Synchronizes births with optimal seasons

By delaying implantation for months, females can time births to seasons with mild weather and abundant resources. Spring and summer births maximize food availability and offspring survival.

3. Extends the window for mate selection

Females may have more opportunities to assess potential mates and select optimal partners by mating early but delaying fertilization of eggs. This may lead to fitter offspring.

4. Provides flexibility in response to resource availability

If environmental conditions deteriorate after mating, delayed implantation may allow pregnancies to be paused until circumstances improve. This prevents committing resources to bearing young that are unlikely to survive.

5. Reduces risk of embryo loss

Delayed implantation reduces chances of losing embryos to environmental stressors in early pregnancy. The dormant blastocyst is more resilient than an actively growing embryo.

What animals exhibit delayed implantation?

Over 100 mammal species are known to undergo delayed implantation. Some examples include:

Animal Length of Delay
Bears (polar, black, brown) 4-5 months
Weasels, mink, ferrets 10 days – 1 month
Seals 3-4 months
Spotted hyenas 1-2 months

Delayed implantation tends to occur in carnivores like bears, seals, and weasels. It also occurs in some hoofed mammals and marsupials. The phenomenon is especially common in animals that inhabit northern latitudes with harsh winters.

Why these animals exhibit delayed implantation

Several shared traits may explain why delayed implantation is advantageous for these groups:

  • Breeding is restricted to short seasons due to climate
  • Gestation and lactation require high energy reserves
  • Offspring need extended maternal care after birth
  • Unpredictable food supplies
  • Benefit from synchronizing births with seasons of abundance

What happens during delayed implantation?

During the delayed implantation period, the blastocyst remains dormant and does not increase in size. However, some important developmental changes do occur:

Persistence of the blastocyst’s inner cell mass

The cells that will become the fetus remain alive but do not multiply or differentiate. This preserves the embryonic cells.

Continued secretion by trophectoderm cells

The outer trophectoderm layer of the blastocyst secretes small amounts of estrogen, progesterone, and proteins. These secretions maintain the uterine lining in a prepped state for later implantation.

Loose attachment to the uterine lining

The blastocyst is not fully implanted, but may be loosely associated with the uterine wall. This keeps it anchored in position until ready for implantation.

Metabolic changes

The blastocyst may enter a relatively inactive state, but some minimal metabolism allows the embryo to persist. Its metabolic requirements are met by uterine secretions.

Genetic activity

Some gene transcription occurs at low levels to preserve the blastocyst’s readiness to resume development.

How does the blastocyst implant after delayed implantation?

When the period of delayed implantation ends, the dormant blastocyst must implant in the uterine lining in order to resume normal growth and development. This process involves:

Activation of the blastocyst

Molecular changes in the embryo allow it to emerge from dormancy. Cellular changes prepare it for implantation.

Hatching from the zona pellucida

The blastocyst hatches from the protective zona pellucida layer surrounding it. This allows it to directly contact the endometrium.

Apposition and adhesion

The blastocyst loosely associates with the uterine lining (apposition). It then firmly sticks and adheres to the endometrium.

Invasion

The blastocyst burrows into the uterine lining until it is completely embedded. This establishes connections with the maternal blood supply.

Reinitiation of cell growth

The blastocyst resumes cell proliferation and differentiation. The embryo progresses through normal developmental stages.

Conclusion

Delayed implantation is an intriguing reproductive adaptation that allows certain mammals to separate mating from active pregnancy. Pausing embryo development prior to implantation helps synchronize breeding and births with optimal ecological conditions. Carefully regulating the delay and implantation process is essential for successfully carrying pregnancies to term after a dormancy period. While not fully understood, numerous physiological changes in the uterus and embryo appear involved. By elucidating the mechanisms behind delayed implantation, we can better understand how some mammals reproduce under challenging environmental constraints.