A person could easily fill hours of airtime talking about bizarre mating habits of animals, from the white-spotted puffer fish that draws intricate patterns in the sandy ocean floor to attract a mate, to spotted hyena females whose psuedo-penis, which is sometimes larger than the male’s actual penis, is also their birth canal. No, today we’re going to focus on the latter two-thirds of the process, gestation or incubation and birth. Even limiting the topic, there’s still a lot I won’t get to, like how certain reptiles determine the sex of their offspring by the exact temperature in the nest.
The gestation periods of animals is a matter of scale. The larger an animal is, the longer it takes to make a new one, despite the resources of the mother being large as well. If all of the dimensions of a given animal were doubled, that animal would now have eight times the volume and hence eight times the weight of before. But the thickness of the umbilical cord, through which all of the growth nutrients flow, is only four times as large. So, all else being equal, it would take twice as long for the necessary nutrients to go through. If you want to get super math nerdy about it, and you’re welcome to, the volume and therefor weight of an animal is proportional to the cube of the scale so scale is proportional to the cube root of the weight of the animal.
We can scale back the maths by simply looking at examples. Humans have a gestation period is 40 weeks, one week short of 9 months, while human’s best friend has a gestation period of 2 months. For small animals such as rabbits the period is about 33 days and for mice about 20 days. The medal for shortest gestation of a North American mammal belongs to the possum, which finished pregnancy, soup to nuts, in under two weeks. This may have less to do with their size than the fact that their average lifespan is around 3 years, so replacements are needed constantly. Elephants are pregnant for a long time. Like really, really long. 95 weeks, in fact, nearly two years. This marathon baby-building is one reason that female elephants usually don’t have more than four offspring during their lifetimes. Who has the time? A giraffe needs almost 15 months to form its 150lb/68kg baby, which starts life off with a bang. Giraffes give birth standing up, so it de rigueur for babies to fall 6ft/2m to the ground in the process of being born. There are exceptions to our easy-to-follow big babies mean long pregnancies guidelines, of course. A 110lb/50kg baby hippo is ready to debut in only about 8 months, even faster than a 7lb human baby. Black bears are pregnant for 30 weeks, but their cubs only weigh about a pound or half a kilo.
One thing that list of gestation periods didn’t take into account was pregnant pauses, by which I mean pausing pregnancies. That’s right, some animals have the ability to say, “You know what, now is not a great time to have babies. Let me just halt this embryo in its tracks and turn my uterus into a sci-fi stasis chamber while I wait for conditions to improve.” Since it was discovered in the 1850’s, more than 130 species of mammal have been found to have this ability. The pause, called conveniently diapause, can last anywhere between a couple of days and nearly a year. In most species, this happens when the embryo is still a tiny ball less than 100 cells that has yet to attach to the uterus. Pausing pregnancy isn’t the sole domain of any of one class or family. It’s been found in certain kinds of bats, bear, seals, rodents, deer, and armadillos, among others. More than a third of the species that take a breather during gestation are from the capital of strange nature, Australia. Of the twenty or so species of kangaroo and wallaby, there are only three that *can’t do it. In fact, it’s the tammar wallaby that can put embryos on hold for up to 11 months.
There are a few mechanisms at work. Some animals mate against right after giving birth. It’s like a back-up plan in case something happens to the newborn. If nothing bad happens and the newborn is nursing, the physical tax of lactation stalls the understudy fetus. Once the extant offspring is weaned, the fetus begins developing again. The second way is to pause every pregnancy until the time is right, usually to do with the weather. For example, minks mate around the start of March but put the embryos on pause until after the spring equinox, when the days are growing longer in their northern hemisphere homes. This ensures that the young are born in spring when conditions improve, and not in winter. Some herbivorous animals will pause pregnancies in time of drought, hoping the rains will come back to keep plant life growing. The tammar wallaby combines these two methods to ensure the joey is ready to leave the pouch in the spring rather than the middle of a hot Australian summer.
Diapause was first identified in 1854 after hunters in Europe noticed that pregnancy in roe deer seemed to last a lot longer than normal. Since then scientists have been fascinated by this process and it has helped us understand more about basic reproductive processes in all mammals. But how the process worked at the molecular level is still a mystery. Until recently, there seemed to be no connection between which species used it and which didn’t; and there didn’t seem to be a unifying mechanism for how pregnancy was paused. Even the hormones controlling diapause are different between mammal groups. Researchers in Poland were able to pause embryos in a sheep, a non-diapause species, by transferring them into a mouse uterus and then back into the sheep with no ill effects. This indicates the potential for diapause could lie in all mammals, including humans, but I would still take my birth control pill if I were you. What experiments with diapause could do for us is improve our understanding of how to make and select healthy embryos for IVF, as well as to create better stem cells that could be used to fight cancer. The first stem cells ever isolated by scientists came from a mouse embryo in diapause.
Whether or not the pregnancy was paused, once the baby or babies come out, they need to eat, and for mammal babies, that means milk. A mother’s milk contains a concoction of nutrients, fats, proteins and carbohydrates that are essential for a baby’s development, as well as a cocktail of protective factors to effectively supply the baby with an immune system until it develops its own. All mammals produce milk, but they don’t all produce the same milk. To give you a baseline for comparison before I start throwing out numbers, cow milk is 3.5% protein and 5% carbs, while human milk is about 1% protein and 7% carbs, while both are 88% water. You won’t find that much water in the milk of the hooded seal. Their milk is more than 60% fat, more like a milkshake than milk. This high-fat milk is crucial for the seal pups, born into the freezing waters of the North Atlantic and Arctic oceans. Pups also only nurse for about four days, having been born on floating ice, an environment that is both unstable and unreliable, so the seal mama needs to pack a lot of energy-dense fat into her milk. The pups can consume about over 16lbs/7kg of milk every day. Their body weight doubles in the first week of life. Conversely, in the savannahs of Africa, the black rhinoceros has the skimmest milk going, with only 0.2% fat. They also nurse for almost two years, which would only be possible with thin milk, given how much resources lactation demands.
Tammar wallabies, who are quickly stealing today’s show, produce milk for their joeys with Willa Wonka levels of sugar at 14%, double the amount present in human milk and one of the highest levels among mammals. For my money, the most interesting thing about marsupial mammarries is that they can produce two different milks at the same time. Wallabys and kangaroos can conceivably, no pun intended, have one embryo in their uterus, a basically fetal joey in the pouch, and an older joey who still come in to nurse. The tiny joey gets milk rich in sugars, while the older one gets milk higher in proteins and fat. Marsupials are the lobby Keurig machines of the animal kingdom.
The most protein-rich milk comes from one of the last animals you would think of if the mention of protein conjures up images of large ripply men lifting heavy things–it’s the eastern cottontail rabbit. The 15% protein comes in hands as they have to leave their young unattended for long stretches to go out and forage, returning to the nest only once or twice a day. Such a rich diet means the little bunnies are able to fend for themselves after only nursing for a few weeks.
Contrary to what Ben Stiller says, you don’t have to have nipples to give milk. Take for instance the echidna. Similar to the platypus, the echidna is a mammal that lays eggs, and that, combined with its porcupine spines, bird beak, kangaroo pouch for the females and four-headed penis for the male, is arguable proof that God was high as balls during the creation. The platypus and four species of echidnas are monotremes, which lay eggs through the orifice they use for both their urogenital and digestive tracts. After mating, a female echidna lays a single, soft-shelled egg, about the size of a dime, into her pouch. Ten days later, the baby echidna hatches. Instead of feeding the baby, with the ridiculously cute name of puggle, via a nipple, the have special glands in their pouches called milk patches that secrete milk, sort of like having a saucer of milk instead of a bottle.
Mammals may have a monopoly on milk, but some birds, like pigeons, produce a milk-like substance for their babies, too. And unlike mammals, both male and female pigeons produce this milky substance to feed their young squabs. Pigeon parents produce what is known as crop milk, which is secreted into a small sac at the base of their throats that normally stores and moistens food. Once a squab is born, the pigeons regurgitate fat and protein rich crop milk into the baby bird’s mouth. Flamingoes and emperor penguins are also known to produce crop milk for their young.
Emperor penguins are a shining example of males helping with the young, since the male penguin will tend the egg and the subsequent chick for two months while the female is off hunting. They don’t actually sit on the egg, but keep it warm with a roll of skin called a brooding pouch. Even as great as emperor penguin dads are, they had stiff competition for that World’s Greatest Dad mug. For almost every species on Planet Earth, the female holds the majority of the responsibility when it comes to pregnancy, childbirth, and looking after the young until they can survive on their own. *Almost every species. The seahorse, pipefish, sea dragons, and other members of the Syngnathidae family, which means “fused jaw,” are the exception that proves the rule.
Seahorse baby-making starts with a dance. For several days prior to the actual mating, the two fish (yes, they are fish) will intertwine their tails and swim together. When the time is right, the two swim in a sort of snout-to-snout embrace, and the female uses an organ called an ovipositor to transfer eggs from her body to the male’s brood pouch. You can track the progress of the transfer by watching the female get skinnier and the male get bigger as it goes on, like a really bizarre progress bar. When it’s done, the female swims off on her merry way, though she does pop in on the male each day to say hi. The male finds a choice piece of plant life to anchor himself to while he fertilizes the eggs. Two to four weeks later, depending on the species, as many as 1,000 my little sea-ponies, called fry, spew forth. The sheer number and the fact that the male will be ready to mate again in a matter of hours is helpful because the survival rate for the fry is really bad. They’re basically plankton, floating around, waiting to be eaten for the first few weeks.
The process is similar for leafy sea dragons, which look at seahorses in a Carnivale parade, except the eggs are stuck to a brood patch on the sea dragon’s tail, rather than being in a pouch, and sea dragon fry are better able to care for themselves. The seahorse’s uncurled cousin, the pipefish, uses a slightly different approach. Like seahorses, female pipefish drop their eggs into the male’s brood pouch, which is on their belly, but kinda close to their heads. There, they fertilize the eggs and carry them until the offspring, usually between 5 and 4, hatch a couple weeks later. What’s interesting about pipefish is their ability to use resources differently for broods of offspring. Males of this species have been known to not take care of offspring that come from a mother they aren’t particularly fond of. Males put more resources into broods from larger, more attractive females. They also don’t put as much effort into ensuring survival in a brood if it just hatched a successful, healthy brood.
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Syngnathids aren’t the only underwater co-parents. The African cichlid does what they do, without benefit of a brood pouch. After the female has laid her eggs, preferably on a smooth rock, and the male has fertilized them, the female scoops the eggs up in her mouth, where they stay until hatching. This process is called mouthbrooding and several species of fish do it. Some are maternal mouthbrooders, some are paternal, while others trade off, which is called biparental mouthbrooding. Regardless of who’s parenting, the point is to keep the eggs safe until the little fry can swim away, though some species also let tiny fry back into their mouth for a while, like a kangaroo with a joey.
What’s normal for cichlids looks like an opportunity for the cuckoo catfish. Like the namesake bird, these catfish are brood parasites. Rather than going through all the fuss and bother of making a nest and tending their offspring, they get another animal to do it, without that animal’s knowledge. The cuckoo catfish barge in on the cichlids mating, eating some cichlid eggs and depositing some of their own in the confusion as the cichlids try to run them off. Catfish eggs develop faster and hatch sooner than cichlid eggs, which start to hatch at about the time the catfish need to start feeding. The catfish intruders use their wide jaws and extra teeth to devour the new hatchings headfirst. If the catfish run out of cichlid hatchlings, they start to chow down on each other.
It might be better down where it’s wetter, but there’s mouth-brooding on dry land, too and it goes by the name Darwin’s frog. Named for American writer and activist Mike Darwin–no of course it wasn’t, it was named for and by Charles Darwin–this South American frog has a head that looks like a curling dead leaf to help it hide from predators. And inside that leafy head is the next generation of their froggy family line. When a female Darwin’s frogs lay her eggs, her mate keep a careful watch until the tadpoles hatch. Then, like a temperate and well-intentioned Chronos, the male basically swallows his young, squirreling them away to grow safely in his vocal sac, until they have grown legs and are ready to make a go of it. Unfortunately, one of the two species of Darwin’s frogs hasn’t been spotted in the wild since 1980 and researchers are nearly certain the species is extinct. The other species is in a precipitous decline and for once humans aren’t the primary reason why. Instead, blame rests with the chytrid fungus, which is deadly to amphibians.
What if, rather than kinda swallowing the eggs, Darwin’s frog totally actually swallowed them? Well then we’d be talking about the Australian gastric brooding frog. (Why is it always Australia?) The Australian gastric brooding frog, thought to be extinct since 2002, had a bizarre method of maternal care. The female swallowed her fertilized eggs and incubated her young in her stomach for about six weeks. Her babies go through their tadpole stage, safe inside mom’s actual stomach, to later emerge from her mouth as fully developed froglets. Wait, if they are in her stomach, why don’t they get digested? The mother frog not only stop eating but also stop producing stomach acid. There seems to be a chemical released by the eggs and hatchlings that inhibited the production of acid, though scientists don’t know what. The mother would brood about two dozen eggs at a go, which, by the time they were ready to be spat out, could make up almost 40% of her total body weight. That would be like a woman who weighed 150lbs/68kg carrying 60lbs/27kg of baby in her stomach. It’s not especially comfortable for the frog designed to do it, either. The young frogs stretched the mother’s stomach to the point that it completely compressed her lungs, forcing her to breathe through her skin. The baby frogs would normally emerge one or a few at a time over a period of days as they became ready. If the mother sensed danger, though, she would give birth by vomiting them out. Researchers once observed a female expel six young frogs together, shooting them about 40in/1m in the air.
All of these animal mothers have had one thing in common–a male mate. But a partner is strictly optional for animals capable of parthenogenesis, sciency speak for virgin birth. It’s not that the females are fertilizing their own eggs; the eggs are able to develop without having been fertilized. Some animals, like whiptail lizards, are fully asexual and do not need a male to give birth, while there are also animals that *can mate with a male, but do not always do–everything from brine shrimp to wasps to sharks and even domestic turkeys–and they are the ones we are going to look at.
Scientists initially thought that parthenogenesis only happened with animals in prolonged captivity, that animals wouldn’t have a need to if they weren’t isolated, such as the case of a python in the Louisville zoo who had never had contact with a male of her species, but produced six baby snakes over four years. They’ve since learned that virgin births can happen in nature even when there are males around, like with the Australian stick bug, who will drive away males she finds wanting and jolly well do it by herself. In the 1800s, reports started appearing of virgin births among chickens. For those who need a basic farmyard primer, poultry lay eggs regularly, but you need to have a male, generally speaking, if you want baby poultry. Researchers started studying similar events among turkeys, finding that the turkeys were laying unfertilised eggs that produced live chicks. In most documented instances of parthenogenesis, the offspring is female, a half-clone of the mother. The turkey poults, however, were always male, which was put down to a quirk of bird genetics where male sex chromosomes are dominant. Soon a parthenogenetic strain of the domestic turkey was developed in which most males appeared normal and reproduced successfully. Scientifically speaking, the turkeys were regarded as a curiosity, something that only happened because of human-imposed conditions.
In the past few decades, though, more and more cases of virgin births have been reported in snakes, lizards, and fish, including sharks. Just what we needed, infinitely-replicating sharks. Do they people who green-light SyFy original movies know about this? In 2001, one of three captive adult female bonnethead sharks gave birth to a healthy female pup. The sharks had all been caught at not yet breeding age, yet one of them had clearly given birth. Genetic tests confirmed that no males had been involved, and since then, the phenomenon has been discovered in four other shark species. In 2006, scientists reported that two different Komodo dragons, giant carnivorous lizards with toxic spit, had also had virgin births at two different UK zoos. Then scientists documented different snake species, including boas and pythons, giving birth in the absence of males. That left two questions: how and why?
One possible answer may lie with the whiptail lizard. Certain sub-species are entirely female; males have been completely cut out of the reproductive process. Each female produces asexually, creating new generations of females, and so on. Creating such an exclusive ‘no boys allowed’ club has benefits– if any of these lizards were left stranded, they could still pass on their genetic material, the ultimate goal of all animals. If you had to wait around for a male to show up, your DNA might end with you. That’s thought to be the reason for the virgin snake birth in Louisville. The snake’s name is Thelma, by the way. Thelma lived in a climate controlled environment and had plenty of food, optimal conditions to undertake solo parenthood. There’s a catch, though. Asexual reproduction is the ultimate form of inbreeding – there is no way to create genetic diversity. So animals that clone themselves leave their lineages vulnerable to disease and other threats, which they lack the genetic variety to counter. For that reason, after the virgin birth of the Komodo dragons, scientists recommended that the species, which is endangered, not be kept in isolation. The dragons would need genetic diversity to stand a chance at long-term success.
Some scientists believe that parthenogenesis may actually be an ancient mode of vertebrate reproduction. The species who do it well, the pythons and sharks for example, are also some of the oldest. More recently evolved species, such as cobras, fare less well, producing only one or two babies via a virgin birth, which then often die. Unfortunately, that’s not something we can tell by looking at the fossil record. It’s also pert near impossible for us to truly know how many wild species actually reproduce on their own. There are too many fish in the sea to be able to tell if they’re giving virgin birth and many shark species are endangered to the point that it would be unethical to catch them for study.
So why do animals reproduce alone, when asexual reproduction has so many down sides and where males are plentiful? Genetic diversity isn’t the end all and be all. Baby sharks born to virgin mothers are less genetically diverse than those born to two parents, but they also appear just as healthy, having been “purged of all the deleterious recessive genes”, according to one researcher. Females may also choose to reproduce alone because the act of sexual reproduction–finding a partner, having partners fight over you, the act of mating itself– can use up a lot of resources. Or perhaps something other than evolution is at work. Perhaps virgin births are triggered by some outside factor; a hormone, or hormonal imbalance? Or even a pathogen, such as a virus, or parasite. There is a species of wasp, for example, that starts reproducing asexually when infected with a certain bacteria.
And that’s where we run…. Back, npi, to the suriname toad. If you have trypophobia, an aversion to clusters of small holes, thanks for listening and I’ll see you next week. For the rest of us, suriname toad reproduction goes like this: the female toad lays her eggs, then the male toad fertilizes the eggs and scoops the eggs up onto the female’s back. Her skin grows over the eggs, protecting them until they hatch. Imagine a honeycomb, but each cell, rather than holding delicious, delicious honey, holds a tiny black tadpole or froglet. After a few weeks, tiny baby toads squirm and burst out of holes in her skin, ready to swim and look for food. It’s weird and gross and amazing. Remember…Thanks…