Why is childbirth in humans such an agonizing and difficult process? The answer, as you might expect, lies in our evolutionary history. David K. Jordan, professor of anthropology at the University of California San Diego, believes that the human pelvis, and mammalian pelvises in general, has been shaped by competing evolutionary forces.
On one hand, a narrow pelvis promotes bipedalism by increasing the efficiency of locomotion, safeguarding the pelvic bones from breakage, and conserving bone matter. However, in females, a narrower pelvis also makes it more difficult for a newborn baby to squeeze its way out of the birth canal.
Another strike against Homo sapiens, at least when it comes to childbirth, is our extraordinarily gigantic heads as babies. Our large brains, and by extension our large heads, make it quite difficult for us as newborns to fit through the pelvises of our mothers.
As Dennis O’Neil, professor of anthropology at Palomar College, puts it: for a human baby to pass through the birth canal, he or she must partially rotate laterally twice to accommodate his or her large head and broad shoulder. This stunning acrobatic feat unfortunately translates to a hard and danger-ridden journey for both the child and mother.
Due to the difficulty of childbirth, human mothers generally prefer the assistance of a midwife or an obstetrician in modern times. In contrast, non-human primates take more of a do-it-yourself approach.
So, when it comes to giving birth, how does the non-mammal department fare? Our backbone-less friends, the invertebrates, have long been known to do it the asexual way, but scientists are increasingly discovering examples of asexual reproduction in vertebrates as well.
For example, Komodo dragons, the world’s largest living lizard, have demonstrated the ability to reproduce through parthenogenesis, a form of asexual reproduction where the growth and development of embryos occur without fertilization. These so-called virgin births can also occur in sharks, where it has been witnessed in at least two captive individuals, in birds such as chickens and turkeys, and in snakes such as pit vipers and boa constrictors.
Since most of these virgin births were witnessed in captive animals, scientists believed for a while that asexual reproduction was an oddity among vertebrates. However, the recent discovery of facultative parthenogenesis, where a normally sexual species engages in asexual reproduction, in wild female pit vipers, suggests that virgin births are more common in nature than previously expected.
The study, published on Sept. 12 in the journal Biology Letters, also revealed instances of virgin birth in two close relatives of the pit viper snake: Agkistrodon contortrix, also known as the copperhead, and Agkistrodon piscivorus, commonly known as the cottonmouth.
The research team, led by Warren Booth, a molecular ecologist at the University of Tulsa in the Oklahoma Department of Biological Science, collected genetic samples from long-term studies of snakes. With the help colleagues at the Copperhead Institute, Wofford College and San Diego State University, the researchers were able to collect samples of mothers and their offspring from 22 litters of copperheads and 37 litters of cottonmouths.
Through DNA analysis, they confirmed that in one litter from each species, the babies were solely the product of the mother. In other words, in these litters, the father contributed zilch in terms of genetic material - probably because there was no father.
For a phenomenon that was once considered to be a captive syndrome, or at the very least, an evolutionary novelty in the vertebrate world, the frequency of parthenogenesis comes as quite a shock. According to Booth’s calculations, these findings suggest that between 2.5 and as much as five percent of litters in the copperhead and cottonmouth populations may be a result of parthenogenesis.
Pit vipers, like much of the animal kingdom, carry out meiosis, a form of cell division that results in the production of haploid gametes, or sex cells. Unlike in humans, where babies are created from the union of the male and female gametes, commonly known as the sperm and egg, in the pit viper, the sex cells of the mother can pair up to create an embryo.
While this recipe for producing babies only possess genetic material from the mother, it is different from asexual reproduction in that the viper offspring are not miniature clones of their mother. The key point of differentiation is that they were not made using identical halves of her genome.
So now we come to the million-dollar question: how common, really, is virgin birth, and can we humans do it as well? Well, humans clearly do not engage in obligate parthenogenesis, which is the primary baby-making strategy in species that have few or no members of the male sex. An example of a species that does this is the New Mexico whiptail, Cnemidophorus neomexicanus. Booth believes that obligate parthenogenesis may have resulted from ancestral interbreeding between species.
The origin of facultative parthenogenesis, on the other hand, is much less clear. Booth says that a common trait of facultative parthenogens is the lack of genomic imprinting, a process in which the mother provides a specific set of genes, and the father provides a second set, resulting in the development of an embryo.
With the exception of the monotremes, consisting of the platypus and echidnas, genomic imprinting occurs in all mammals. This explains why facultative parthenogenesis does not occur in mammalian species without experimental interventions.
Booth and colleagues originally believed that virgin births might happen in the absence of potential mates. However, over the years, they have witnessed six captive female boa constrictors give birth through parthenogenesis, even when males were hanging around them during their breeding cycles (rejection – ouch).
The frequency of these virgin births pretty much rules out the possibility of a one-shot accident. Now the team is investigating other possible causes for these births, such as genetics, viruses, tumors, and bacteria – all the good stuff.
In the future, the research group hopes to investigate the occurrence of virgin births in other species, such as water snakes in Oklahoma, along with how well the offspring of these births survive and reproduce. It would be quite cool — and scary, if you happen to be a ophidiophobe — if these virgin mothers could generate entire populations of snakes by themselves. Hey, at least cockroaches can’t do it! At least, I hope not...