Published online 3 February 2009 | Nature | doi:10.1038/news.2009.77


Fossil of pregnant whale found

Position of fetus suggests that early whales gave birth on land.

Fossilised pregnant  whaleFossils of a female whale and fetus (blue) suggest that early whales gave birth on land.University of Michigan Museum of Paleontology

Scientists have discovered the first known fossils of a pregnant early whale and her unborn calf, and with them evidence that these ancient creatures may have given birth on land.

The 48-million-year-old whale fetus is positioned for a head-first delivery typical of land mammals, says palaeontologist Philip Gingerich of the University of Michigan in Ann Arbor, who led the study. The findings lend credence to the idea that early whales — protocetids — were amphibious animals that fed in the oceans but came ashore to sleep, mate and give birth.

The report is "fascinating", says Ewan Fordyce, a palaeontologist at the University of Otago in Dunedin, New Zealand, who was not involved in the work. "These protocetid whales truly were transitional between fully marine and fully amphibious," he says.

Head first

Protocetids, which lived 49 million–37 million years ago and were roughly 2-5 metres long, had four legs with elongated flipper-like feet and small hooves that allowed them to roam on shore. They are thought to have evolved from land mammals called artiodactyls, which today include animals such as goats and cows.

Gingerich and his colleagues searched for preserved skeletons in a rock formation called Habib Rahi in Pakistan. The area, now inland, was once beneath a sea in which early whales may have made their transition from land to water. The researchers found the fossil fetus nestled in its mother's ribcage, as well as a nearly complete skeleton of what they believe to be a male nearby. The fossils belong to a newly identified whale species, which the researchers have dubbed Maiacetus inuus.

The fetus, estimated to have been about 66 centimetres long when alive, appeared to be near term and was oriented for a head-first delivery, the researchers report in PLoS ONE1. Land mammals are normally born in this way, presumably to allow the young to breathe immediately, whereas mammals that live in the sea tend to be born tail-first, perhaps to avoid drowning.

Well preserved

The fossils are "beautiful and very informative", says Hans Thewissen, a whale palaeontologist at Northeastern Ohio Universities College of Medicine in Rootstown and a former student of Gingerich's. Although fragments of young whales have been found before, he says, this fossil fetus is unusually well preserved.

Male whale fossilThe second adult fossil is probably male.Philip Gingerich, University of Michigan Museum of Paleontology.

Because of the presence of the fetus, the team could confidently identify one fossil as female. The other adult fossil was 2.6 metres long and the animal's weight when alive was estimated to be 390 kilograms. This specimen was designated as male on the basis of its dimensions, large teeth and pelvic structure and the development of the teeth and bones suggested it was fully grown. Because the male was only 12% longer than the female, the males probably did not encounter intense competition that would have favoured bigger individuals, Gingerich says. That, in turn, could mean that food in the area was spread out, preventing males from staking claims over congregated females.

Jonathan Geisler, a palaeontologist at Georgia Southern University in Statesboro, cautions that it is too early to conclude that these whales gave birth on land. Modern hippos, which are closely related to whales, deliver babies both on land and in water, and published accounts describe births as feet-first, he says. Fetal position and birth environment may not have a "one-to-one correlation", says Geisler. 

  • References

    1. Gingerich, P. D. et al. PLoS ONE doi:10.1371/journal.pone.0004366 (2009).
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