Why do humans possess a twisted birth canal?
Extraordinary shape makes births more difficult, but guarantees stability


Date:
October 29, 2021
Source:
University of Vienna
Summary:
The relatively narrow human birth canal presumably evolved as a
'compromise' between its abilities for parturition, support of the
inner organs, and upright walking. But not only the size of the
birth canal, also its complex, 'twisted' shape is an evolutionary
puzzle. Researchers now present new insights into why the human
birth canal evolved to have this complex shape. They suggest that
the longitudinally oval shape of the lower birth canal is beneficial
for the stability of the pelvic floor muscles.



FULL STORY ==========================================================================
In most women, the upper part, or inlet, of the birth canal has a round
or transversely (left-to-right) oval shape, which is considered ideal for parturition, but it is unknown why the lower part of the birth canal has a pronounced longitudinally (front-to-back) oval shape. This twisted shape typically requires the Baby to rotate when passing through the narrow
birth canal, which further increases the risk of birth complications.


==========================================================================
In comparison with humans, apes have a relatively easy birth pattern
that does not require rotation of the baby thanks to the longitudinally
oval shape of the birth canal both at its inlet and the outlet. "For
giving birth, it would be much easier to have a uniformly shaped
birth canal also in our species," says Katya Stansfield, a specialist
in biomechanics. Instead, the twisted human shape requires a complex, rotational birth mechanism: The baby needs to rotate to align the longest dimension of its head with the widest dimension of each plane of the
birth canal. Misalignment can lead to obstructed labour and result in
health risks for both mother and baby.

A research team of evolutionary biologists and engineers from the
University of Vienna, the Konrad Lorenz Institute for Evolution
and Cognition Research in Klosterneuburg and the University of
Porto hypothesised that the support function of the pelvic floor
muscles, which are suspended across the lower pelvis and also play an
important role in sexual function and continence, may have influenced
the evolution of the shape of the birth canal. The team carried out
extensive biomechanical modelling of the pelvic floor and found that
the highest deformation, stress, and strain occur in pelvic floors
with a circular or transverse-oval shape, whereas a longitudinally oval elongation increases pelvic floor stability. "Our results demonstrate
that the longitudinally oval lower birth canal is beneficial in terms
of stability," says Katya Stansfield.

"However, this outcome prompted us to ask why the pelvic inlet in humans
is not also elongated longitudinally," elaborates Barbara Fischer,
an evolutionary biologist.

Traditionally, it has been assumed that the transverse dimension of the
human pelvis is constrained by the efficiency of upright locomotion. "We
argue that the transverse elongation of the pelvic inlet has evolved
because of the limits on the front-to-back diameter in humans imposed by balancing upright posture, rather than by the efficiency of the bipedal locomotion," says Philipp Mitteroecker, who was also involved in this
study. A longitudinally deeper inlet would require greater pelvic tilt and lumbar lordosis, which would compromise spine health and the stability
of upright posture. These different requirements of the pelvic inlet
and outlet likely have led to the evolution of a twisted birth canal,
requiring human babies to rotate during birth.

========================================================================== Story Source: Materials provided by University_of_Vienna. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Ekaterina Stansfield, Barbara Fischer, Nicole D. S. Grunstra,
Maria Villa
Pouca, Philipp Mitteroecker. The evolution of pelvic canal shape
and rotational birth in humans. BMC Biology, 2021; 19 (1) DOI:
10.1186/ s12915-021-01150-w ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211029114010.htm

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