Pregnant deer? Extended gestation? 

Sometimes the looks are unbelievable when you talk about the deer’s reproductive strategy.

The mating – in Germany “Blattzeit” (Blatt = leaf, Zeit = period)  in July and August and birth – setting time – in May?

At the very beginning of the deer’s development, a zygote is formed from the fusion of egg and sperm. The cells continue to divide and the blastocyst that follows development enters a resting phase at a stage of around 20-30 cells. It does not implant in the uterus. At this cell stage, the blastocyst does not need any further energy supply to continue living; it is an optimal time to stop growth.
Around November, the blastocyst is reactivated and begins cell division.

From a 100 cell stage at the end of December/January, the reactivated blastocyst implants after the 5 month resting phase and normal fetal growth takes place.

69 mammals (including 3 bat species, 4 insectivores, 18 rodent species, 41 carnivores and 1 even-toed ungulate) show this delayed development.

2 forms are described.

On the one hand, the facultative diapause caused by circumstances such as breastfeeding/suckling, metabolic stress, social stress, overpopulation, etc. in rodents and insectivores.

On the other hand, the obligatory diapause, which is based on environmental factors in armadillos, martens, deer, flying foxes, bears and seals, among others.

The cause lies in the ice ages and the associated long winter period. Thanks to extended gestation, the animals were able to build up a good physical condition despite fertilization and thus also save energy costs.

The end of diapause is initiated by a cascade of environmental conditions, including day length, temperature, and metabolic changes.

Viewed from today’s perspective, diapause no longer represents an evolutionary advantage, as our climatic extremes no longer occur in the same way as during the Ice Age.

https://www.rug.nl/research/gelifes/tres/publications/_pdf/ya_eaqrevbiol06.pdf
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3848826/
https://www.ncbi.nlm.nih.gov/pubmed/23279480
Bischoff TLM (1854) Entwickelungsgeschichte des Rehes. TLM Bischoff1854Entwickelungsgeschichte des Rehes.J. Ricker’s Buchlandlung, Giessen. J. Ricker’s Buchlandlung, Giessen. A