Why Are Roe Deer Pregnant for So Long? Delayed Implantation

How long is a roe deer pregnant?

From mating to birth, a roe deer pregnancy lasts about ten months, roughly 290 days. That is far longer than you would expect for a deer weighing only 20–25 kg (44–55 lb). The larger red deer, several times heavier, carries its calf for about eight months.

The roe deer pregnancy has two distinct phases:

In other words, only about half of the roe deer’s pregnancy is spent actually building a fawn.

What is delayed implantation?

In most mammals, the fertilized egg travels to the womb, divides into a hollow ball of cells called a blastocyst, and then quickly attaches, or implants, in the lining of the uterus. A placenta forms and the embryo develops steadily until birth.

In delayed implantation, also called embryonic diapause, the process stops at the blastocyst stage. The tiny embryo lives free inside the uterus, nourished by fluids secreted by the womb, and its development almost halts. Scientists have found that the roe deer blastocyst keeps growing very slowly during this phase rather than stopping completely, then speeds up dramatically shortly before implantation.

The signal to restart is linked to the changing seasons, especially day length, which influences the mother’s hormones. The exact chemical triggers in roe deer are still being studied.

Why do roe deer delay their pregnancy?

The pause lets roe deer separate the best time to mate from the best time to give birth.

The best time for birth is late spring. Fawns born in May and June arrive when fresh leaves, herbs and grass are most abundant. The mother needs this rich food to produce milk, and the fawns have the whole summer and autumn to grow before their first winter. Tall grass and crops also provide hiding places for newborns.

The best time to mate is summer. In July and August, bucks and does are in good body condition after feeding on spring growth. Bucks have finished growing their antlers and established territories. Mating in summer may also avoid an energy-draining rut in autumn, right before winter.

Without diapause, a summer mating followed by a five-month pregnancy would produce fawns in midwinter, when they would be unlikely to survive. The pause bridges the gap. Biologists still debate why roe deer evolved this solution when other deer simply mate in autumn, but the result is a reliable spring birth season.

Are roe deer the only hoofed animals with delayed implantation?

Yes, as far as is known. Roe deer, both the European species and the larger Siberian roe deer, are the only hoofed mammals (ungulates) known to have true embryonic diapause. Other deer, cattle, antelopes, horses and pigs all have continuous pregnancies.

This makes the roe deer unusual among its relatives. The red deer, for example, ruts in autumn, carries its calf for about eight months without a pause and gives birth in late spring. The end result, a spring birth, is the same, but the route is very different.

Other animals with delayed implantation

Embryonic diapause is known in more than 100 species of mammals. Some well-known examples:

• European badger. Badgers can mate at almost any time of year, but implantation happens in winter, and cubs are born mainly in February. See the European badger.
• Bears. Brown bears and black bears mate in late spring or early summer, but the embryo does not implant until the bear enters its winter den. Cubs are born in midwinter. More in the brown bear.
• Seals. Many seals mate soon after giving birth, and a delay of a few months lets the next pup arrive at the same season a year later.
• Stoats, minks and other weasels. Stoats have one of the longest delays, with implantation postponed for many months.
• Nine-banded armadillo. Mating happens in summer, implantation in late autumn, and the four identical young are born in spring.
• Kangaroos and wallabies. Many macropods carry a paused embryo while an older joey is in the pouch, and restart it if the joey dies or leaves.

For other long pregnancies across the animal kingdom, see animals with the longest pregnancies.

A pregnancy that tricked early naturalists

For centuries, the roe deer’s breeding season confused European naturalists and hunters. Because no visible embryo could be found in the womb during autumn, many assumed that the summer chases were not real mating and that roe deer must mate later, in late autumn or early winter.

In the mid-1800s, German anatomists studying roe deer embryos showed that mating does happen in summer, and that the hidden, tiny blastocyst simply rests in the womb for months. The roe deer became one of the first mammals in which delayed implantation was described scientifically.