What animals can get pregnant while pregnant?
Only a select few species possess the extraordinary ability to become pregnant while already carrying a fetus: the swamp wallaby and the European brown hare. This rare reproductive strategy is known as superfetation.
Understanding Superfetation
Superfetation is a unique biological phenomenon where a second conception occurs during an existing pregnancy, resulting in two fetuses of different gestational ages developing simultaneously in the womb. This is distinct from having twins or multiple offspring from a single conception and is a highly specialized adaptation.
The Swamp Wallaby: A Master of Perpetual Pregnancy
The swamp wallaby (Wallabia bicolor), native to eastern Australia, is a prime example of superfetation. This marsupial is remarkable for its near-constant state of pregnancy.
- Unique Timing: A female swamp wallaby typically mates and conceives about a day before giving birth to her current joey. This means she becomes pregnant with a new embryo while the previous one is still developing internally and nearing full term.
- Embryonic Diapause: To manage this overlapping pregnancy, the newly conceived embryo enters a state of suspended development known as embryonic diapause. It remains dormant in the womb until the older joey leaves the pouch, at which point it resumes development. This ensures a continuous cycle of reproduction, allowing the mother to always have offspring developing.
The European Brown Hare: A Similarly Rare Case
The European brown hare (Lepus europaeus) shares this unusual reproductive trait. Like the swamp wallaby, female brown hares can conceive a second litter while already pregnant with the first. This adaptability allows them to maximize their reproductive output, which is particularly beneficial in environments with limited breeding seasons.
Distinguishing from Other Marsupials
It's important to differentiate superfetation from the reproductive strategies of other marsupials. For instance, kangaroos also have remarkable reproductive capabilities, but they do not become pregnant while already pregnant. Instead, a female kangaroo can mate and conceive approximately a day after giving birth. The resulting embryo also undergoes embryonic diapause, waiting for the pouch to become available. While this strategy also ensures a continuous reproductive cycle, it does not involve concurrent pregnancies within the womb in the same manner as the swamp wallaby or European brown hare.
Animals Capable of Superfetation
| Animal Species | Unique Reproductive Trait |
|---|---|
| Swamp Wallaby | Can conceive a new embryo while still pregnant with an existing fetus, often about a day before birth. The new embryo enters diapause until the previous joey vacates the pouch, leading to a state of near-continuous pregnancy. |
| European Brown Hare | Capable of conceiving a second litter while already pregnant with the first. This allows for overlapping gestations and helps maximize reproductive output within their breeding period. |
| Contrast: Kangaroo | Mates and conceives after birth, not while pregnant. The new embryo enters embryonic diapause, waiting for the pouch to be free. While this maintains a continuous reproductive cycle, it differs from superfetation as it does not involve concurrent pregnancies in the womb at the same time. |
