Snakes, like most reptiles in the world, are oviparous. This means that rather than producing live young ones, the way mammals do, snakes produce an immature single cell — an egg. Most snakes follow through on this reptilian trend except for a select few species; these rule-breakers spice things up with their reproductive tendencies when it comes to baby (snake)-making.

The reasons behind this difference in reproductive strategies are not yet fully understood. However, it is believed that environmental factors, such as variations in temperature and availability of suitable nesting sites, may play a role in the evolution of different reproductive methods.

What is oviparity?

Oviparity is a reproductive strategy common to most lower vertebrates. Oviparous animals lay eggs, typically in a safe location such as in nests, underground burrows, or even within leaf litter. For instance, animals like snakes are generally oviparous.

It isn’t just snakes that fill up the reptilian quota of egg-laying animals. A group of reptiles — known as squamates — comprised of lizards, turtles, crocodilians, and snakes, are all overwhelmingly oviparous. However, even within squamates, not all of them are oviparous. A small minority (20 percent) of them give birth to live young ones.

Is it just a reptilian thing?

Oviparity isn’t just a reptilian tendency. From fish to birds; four of the five classes of vertebrates are generally oviparous. As a matter of fact, mammals are the only vertebrate class to have a majority of viviparous species — animals that give birth to live ones. So, why is oviparity such a common reproductive strategy in the animal kingdom? Well, it’s got a few advantages to it.

What are the advantages of oviparity?

There’s a reason why oviparity and viviparity are known as reproductive “strategies”; they each have pros and cons that help certain animals maximize their evolutionary fitness.

For instance, there’s one particular advantage oviparous animals have over viviparous ones: numbers. Take snakes for instance, oviparous snakes don’t just produce one egg at once; they produce multiple. Unlike viviparous mammals like elephants or humans, snakes hedge their (reproductive) bets by laying broods of numerous eggs — anywhere from 1 to a hundred or even more — at once.

By producing massive broods at once, oviparous animals do not need to expend considerable parental care into just a handful of young ones to ensure their survival. Rather, with a brood, even if a handful amongst a hundred survives till adulthood; it can still be considered a reproductive success.

Similarly, oviparous animals have another trick up their sleeve. Time. Again, let’s consider snakes. Unlike invertebrates like bees or lower vertebrates such as some fish and amphibians, snakes do not die after giving birth.

Vertebrates generally live relatively long lives. For those that employ oviparity, they live long lives with relatively short incubation periods. They also expend way less energy than viviparous animals do. This is because egg-laying animals do not develop embryos fully in vitro, in stark contrast with viviparous embryos. Animals, like snakes, lay eggs that contain immature embryos that develop fully outside the female body.

Wait…so why are mammals viviparous?

It’s a valid question that pops up: if oviparity has so many advantages, why (and how) did mammals peg their existence to viviparity? Again, it all comes back to reproductive strategy.

Think about it this way: consider chimps. Chimps are viviparous mammals. Mother chimps carry their pregnancy to term and give birth to live babies. Unlike snakes, mother chimps do not give birth to hundreds of offspring. The task is literally inconceivable.

Rather, chimps give birth to just one baby for every incubation (gestation in mammals) period that it has. This one baby chimp however is advantaged in ways reptilian babies just aren’t. For starters, chimps are social mammals that live in amiable family units. The chimp isn’t just privy to parental care but also the care of an extended social unit.

Weighing up viviparity versus oviparity is kinda like… weighing up offense versus defense; it’s a fruitless debate. Both strategies have their merits and research into the evolutionary history of squamate reproduction modes backs this up.

There are some notable exceptions though: monotremes. In contrast to the viviparous marsupials and placentals, monotremes are oviparous. The five species of mammals that lay eggs include the duck-billed platypus found only in eastern Australia, the short-beaked echidna found in Australia and New Guinea, and the three species of the long-beaked echidna found only in New Guinea.

Oviparous for now

A study conducted by scientists from George Washington University shed light on the history of oviparity amongst reptiles, specifically squamates. As part of their research, scientists analyzed the evolutionary tree of squamates — the group of reptiles that snakes and reptiles belong to. Using DNA sequencing technology, the team categorized thousands of lizards and snakes to track down the common ancestors of the reptiles.

The team was able to conclude that squamate ancestors didn’t always lay eggs but rather also gave birth to live ones. The finding was a shocking one as live birth or viviparity has always been assumed a modern (read: less primitive) reproductive strategy, as opposed to oviparity — which is seemingly seen as “ancient”.

According to Dr. Alex Pyron, assistant professor of biology at George Washington University and one of the researchers involved with the study, the concept of snakes giving birth is quite a conflicting one.

“This is a very unusual and controversial finding and a major overturn of an accepted school of thought,” Pyron said in a press release.

The study also revealed that reptilian ancestors switched back and forth quite a bit between oviparity and viviparity — seemingly sticking with the reproductive strategies that offered the best edge at that time. The concept again challenges the popular assumption that once a species gains access to a viviparous mode of reproduction, it very rarely reverts to laying eggs.

“Before, researchers long assumed that the ancestor of snakes and lizards laid eggs, and that if a species switched to live birth, it never reverted back. We found this wasn’t the case,” explained Dr. Pyron.

The team’s findings were sufficiently backed up by fossil records from the Cretaceous Period — the last era of the dinosaurs. Fossils of ancient marine reptiles like plesiosaurs and mosasaurs — that look like they gave birth to live offspring — add hefty weight to the team’s conclusions.

Ovoviviparity: a secret third mode of reproduction

It is common knowledge that most snakes lay eggs (oviparity). It’s lesser known that a few snakes give birth to live ones (viviparity). But what’s a bit of a hidden secret is that there is a third mode of reproduction — ovoviviparity.

What is ovoviviparity?

If the name didn’t clue you in: ovoviviparity is a blend of the two — oviparity and viviparity — modes of reproduction. Ovoviviparous animals lay eggs, as oviparous animals do, but retain those eggs inside their bodies for longer. Remember: oviparous animals lay immature eggs and incubate them outside their bodies in nests. In ovoviviparous animals, rather than laying immature eggs, the females incubate the eggs inside their bodies till they’re ready to hatch.

Where does viviparity kick in?

As for the viviparity in ovoviviparity, the eggs hatch internally — inside the reproductive tract of the female — and the young ones then emerge; fully developed and ready to live independently from their mother. It’s reasonable to think of ovoviviparity as a bridge between the two popular modes of reproduction.

This mode of reproduction is overwhelmingly characteristic of a few genera of amphibians like frogs and salamanders. Apart from amphibians, cases of ovoviviparity have been observed in a few genera of insects, fish, and reptiles.

Snakes that give birth to live babies

Around 70 percent of all known species of snakes are oviparous. Popular species like ball pythons, kingsnakes, and copperheads, are a few of the popular snakes that make up the long list of egg-laying snakes. However, a third of all snakes aren’t oviparous. These non-oviparous snakes are either viviparous or ovoviviparous and include commonly known species like some boas.

Boa constrictors

Well known for their neck-crunching moves, boas are one of the largest extant snakes alive today. These big boys and girls, native to the Americas, call a wide range of habitats their home. Depending on the species, boas can be found anywhere from scrub (Whitaker’s sand boa) and rainforests (Emerald tree boas) to open and arid grasslands (Egyptian sand boa) and woodlands (rainbow boa).

Most species of boas, including anacondas (yup, anacondas are a kind of boa constrictor), don’t lay eggs. Many species, including the quintessential boa constrictor, are viviparous. They give birth to live young. Viviparous boas give birth to fully developed baby snakelets. The babies are attached to a yolk sac and are covered by a thin and clear membrane.

The small minority of boas that aren’t viviparous are either ovoviviparous or oviparous. Ovoviviparity (although the degree of ovoviviparity is often highly contested) is more common than oviparity in boas. The Boidae family is exceptionally diverse. Snakes within the taxon exhibit a wide range of reproductive strategies; from viviparity and oviparity to even…asexual reproduction. Yep, boas can reproduce asexually if required.

Virgin birth in boas

In 2010, researchers from the North Carolina State University reported the first known incidence of virgin birth in boa constrictors. Before this report, parthenogenesis — making babies without doing the nasty — was widely assumed to be impossible in snakes like boas.

In a press release, Dr. Warren Booth, a post-doctoral scientist and entomologist at NC State University, highlighted the unique nature of the event. According to Dr. Booth, such parthenogenic birth, especially in primitive snakes like boas, forces scientists to re-imagine any preconceived notions about reptilian reproduction.

Before this, it was popular belief that such virgin births occurred only in highly controlled laboratory environments. Specifically, it was generally accepted that this type of asexual reproduction could only be observed in fish and amphibians, and had never been witnessed in reptiles.

The world’s first…and second

For those who might have missed the news the first time, the very same female mama boa astonishingly gave birth to live offspring — again, through asexual reproduction — within just two years. If anyone found the first occurrence exceptional, the second event might truly astound them. Not only did Mama Boa give birth to one virgin brood, but she produced two!

The virgin mother boa didn’t quite live in isolation. Not only was the female in the presence of sexually mature male boas, she was actively courted as well. She even mated with some of them previously.

So why did the boa suddenly go asexual? According to Dr. Booth, it might just be a freak event.

“Reproducing both ways could be an evolutionary ‘get-out-of-jail-free card’ for snakes. If suitable males are absent, why waste those expensive eggs when you have the potential to put out some half-clones of yourself? Then, when a suitable mate is available, revert back to sexual reproduction,” explained Dr. Booth in the same press release.

Dr. Booth’s association with the boas didn’t just end with the study either. As an enthusiastic herp himself, Dr. Booth ended up adopting one of the females from the mama’s brood.

More viviparous snakes

Other species of snakes follow the viviparous pattern of reproduction. For instance, rattlesnakes, venomous snakes found in various habitats throughout North and South America also give birth to live young. Often found in habitats like grasslands, forests, deserts, and rocky areas; many species of rattlesnakes, like the Eastern (Crotalus adamanteus) and Western diamondbacks (Crotalus atrox) as well as the timber rattlesnakes (Crotalus horridus) are viviparous.

Giving birth to live offspring isn’t reserved for terrestrial snakes. In aquatic environments, sea snakes such as the Pacific and Indian Ocean’s tiger sea snake (Hydrophis curtus) are viviparous. Gravid sea snakes haul their bodies to safe and sheltered locations — like shallow waters near the coast or in coral reefs — to birth their young ones.