Principal Investigator: J. Craig George, Ph.D.
Collaborators: AEWC, Ray Tarpley and Daniel Hillman, Todd O'Hara, Raphaela Stimmelmayr, Hans Thewissen, Robert Suydam, Gay Sheffield
Summary: The purpose of this study was to describe the structure and function of the bowhead whale reproductive system. Most of the work was focused on the female reproductive system. Results indicate that female bowheads have a reproductive system similar to other baleen whales; however, their calving rate is somewhat lower. Recent studies suggest that females remain reproductively active over 100 years of age and that females give birth at three to four year intervals. Studies continue on fetal development (see poster below).
Bowhead whale immature female reproductive tract. Analysis conducted by Ray Tarpley and Daniel Hillman in 1999.
Bowhead whale fetus from 1999 harvest. Photo: Craig George
A remarkable CAT scan of a mid-term, ~ 5 ft long, bowhead whale fetus, recovered during the fall harvest. (Image credit: Ray Tarpley and Dan Hillmann at Texas A&M University)
This study reviews and characterizes fetal developmental landmarks of bowhead whales, Balaena mysticetus. The subsistence harvest of bowhead whales in Northern Alaska takes place during the spring and fall, and, occasionally, pregnant females are taken. Bowheads gestation is approximately 14 months, and hence, prenatal specimens collected from this harvest sample three developmental periods: around 2 months, 8 months, and near full-term.
These specimens illustrate the morphological development that is reminiscent of the evolution of cetaceans. For instance, the tail of the smallest specimens in our collection is circular in cross-section, similar to the tail of land mammals as well the tail of ancestral whales such as Pakicetus In slightly older fetuses, the tail expands laterally, and forms a diamond-shape. This is not a shape that is found in in fossil whales, where some early cetaceans, such as Kutchicetus, had a long, narrow, and flattened tail. Fetuses caught in fall as well as full term fetuses have a triangular fluke, similar to postnatal animals. In evolution, this shape of fluke originated approximately 45 million years ago, in the family Protocetidae.
Spring caught bowhead whale fetuses show that more than 40 tooth buds are present in each jaw. These tooth buds develop and are probably mineralized (as indicated by other baleen whales), but are then resorbed. Adults of the fossil mysticete Aetiocetus polydentatus had similar numbers of teeth when it lived 35 million years ago. After the tooth buds disappear, baleen forms, and bowheads are born with baleen approximately 10 cm long.
In most cetaceans, hind limb buds are formed early in development. Our early fetuses already show the presence of internal cartilaginous precursors of pelvis, femur, and tibia. In adult bowhead whales, pelvis and femur ossify (or mineralize), and a synovial joint occurs between them, whereas the tibia usually remains cartilaginous. Occasionally, in postnatal bowhead whales, the hind limb remnants are visible on the abdominal skin, which may show a low welt, or an unusual pigmentation pattern. Hind limbs were fully developed in most Eocene cetaceans, and underwent a quick reduction in size and numbers of elements in the late Eocene basilosaurids.
Poster presented at the 2017 Alaska Marine Science Symposium, Anchorage, Alaska, and the AEWC Mini-convention, Barrow, Alaska, February 2017. Abstract
Tarpley, R.J., G.H. Jarrell, J.C. George, J. Cubbage, G.G. Scott. 1995. Male Pseudohermaphroditism in the Bowhead Whale, Balaena mysticetus. The Journal of Mammalogy 76(4):1267-75.
O'Hara, T.M., et al. 2002. Sexual maturation in male bowhead whales (Balaena mysticetus) of the Bering-Chukchi-Beaufort Seas stock. Journal of Cetacean Research and Management 4(2):143-148.
Koski, W.R. et al. 2008. A calf index for monitoring reproductive success in the Bering-Chukchi-Beaufort Seas bowhead whale (Balaena mysticetus) population. Journal of Cetacean Research and Management 10(2):99-106.
Kellar, N.M. et al. 2013. Variation of bowhead whale progesterone concentrations across demographic groups and sample matrices. Endangered Species Research 22:61-72.
Hirt, S.J. 2015. Sexual dimorphism, the big-mother hypothesis, and analyses of hormones, minerals, and stable isotopes to determine reproductive events in bowhead whales (Balaena mysticetus). Doctor of Philosophy Dissertation, Auburn University, Auburn, Alabama. 99 pp.
Tarpley, R.J. et al. 2016. Morphometric correlates of the ovary and ovulatory corpora in the Bowhead Whale, Balaena mysticetus. The Anatomical Record 299:769-797.