The sawfish virgin births are not unnatural – they are a survival strategy
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Version 0 of 1. The news this week that sawfish have been found to have had virgin births in the wild is not a niche occurrence. For many of the planet’s animal species, reproduction is not always what it seems. Some, such as bdelloid rotifers have successfully reproduced for millions of years without sex. In others, such as stingless wasps, little fire ants and some fish, the need for sex to reproduce and even their gender, can be changed by fungal infection or bacteria. Related: Sex and the sawfish: the world of wildlife virgin births One virgin-birth-inducing bacterial infection has been found in more than 75% of arthropods, including 80 insect species, 17 isopods (a category of crustaceans that includes the woodlouse), many spiders and a type of mite. It is also likely there is an equivalent proportion of infections among nematode worms. If those creepy-crawlies sound insignificant, bear in mind that arthropods and nematode worms comprise something like 99.99% of the world’s animal species. However, the sawfish is a much larger – and endangered – species. It may have been employing a strategy previously seen in bonnethead and blacktip sharks, and in Komodo dragons, who appeared to have become pregnant without mating. Yet it was thought the dragons and sharks had virgin births because theirs was an artificial situation: in captivity they had no males with which to mate. However, all the zoo animals recorded to have had virgin births were also endangered. Poaching and human encroachment have decimated the population of Komodo dragons. And in the north-west Atlantic, there has been a decline of more than 75% in large coastal and oceanic shark populations over the past 20 years or so. This indicates that the situation in nature may not be so different from the living arrangements for these animals in captivity. Until the new report on the sawfish virgins, however, all of this was just a hypothesis. For obvious reasons, it is extremely difficult to spot a virgin birth in a wild animal population. However, in wild animal species that have no males, females have been known to employ a variety of strategies to reproduce, such as simulating the sex act to kick-start eggs into becoming embryos; or borrowing sperm from males of related species, while ensuring their daughters inherit only the genetic code of their mothers. It is notable that in all virgin births recorded in captivity – and with these sawfish – virgin productivity is very low, compared to the number of offspring normally produced. But it’s not such a bad option to have just one shark daughter, instead of six, if the alternative is not reproducing at all. Knowing that it is perfectly possible to create life from eggs alone – a strategy that females of many species have long exploited – has also been explored, in a more domestic context, by people. After the second world war, for example, the US department of agriculture and British animal research units sponsored some interesting experiments in improving the efficiency and sustainability of animal farming. Using selective breeding programmes to intensify a “virgin-birth trait” in turkeys, the hunt for the ultimate breeder led to the creation of the Beltsville small white. The programme increased the percentage of virgin births in turkeys from nearly 17% to around 45% in a decade: females and, weirdly, males could transmit the trait to their offspring, who continued to be fertile. But that’s the birds and the bees. And the snakes, the fish and the lizards. In fact, we humans are something of an aberration when it comes to virgin births. The biological impossibility of an individual mammal replicating herself has only been circumvented through cloning (which is illegal in humans) and tangentially, when in 2004 the first mammal (a mouse named Kaguya) was created using two eggs and no sperm. Even though she had no father Kaguya was fertile and went on to have pups of her own in the usual way. Because what the sawfish females have done is the ultimate form of inbreeding. If their strategy is to have any hope of changing the species’ endangered status, their virgin-born daughters will need to grow up to be healthy, fertile adults. |