Evolutionary success is not about the number of one's children, but one's grandchildren: the children need to survive and pass on their genes.
David Attenborough, asked a few years ago by journalist Joanna Nikodemska about the animal he finds most interesting, answered after some consideration that he's most fascinated by a three-year-old human child, whose potential for development and adaptation are simply limitless.
The same journalist and I have been verifying this opinion for over eight years now – indeed, observing the development of a juvenile representative of the Homo sapiens species is a continuous, fascinating adventure.
More r or more K?
It's a fact that evolutionary success is determined not by the length of adult individuals' lives, but by the number of their offspring that carry their genes into subsequent generations. More precisely, it's not about the number of one's children, but one's grandchildren: the children need to survive and pass on their genes. Of course, in order to have children, one must beget them, or at least somehow initiate the development of the egg, as it happens in parthenogenetic species, where females don't bother with guys at all, or only rarely. But I've already written before about various original methods of completing that first stage, so let's focus on what happens later.
Ecology differentiates two strategies of reproduction: 'r-selection' and 'K-selection'. The symbols are taken from a complicated formula illustrating population dynamics developed in 1838, which systematized our thinking about animal success for the rest of the 19th century and for almost all of the 20th century. It was developed by Pierre François Verhulst (1804–1849), and its simplified version is as follows: dN/dt = rN (1 – N/K), where N is the population, r is its maximum growth rate, K is the carrying capacity of the local environment, and dN/dt is the rate of change of the population with time. According to this model, species that engage in r-selection produce as much offspring as quickly as possible, while K-selection involves an investment in quality rather than quantity. So we either have masses of children that we're not too worried about, hoping that things will work out and some of them will survive; or we have few, but we invest a lot in them and we try to make sure that they do as well as possible. Of course, as is often the case, in nature it's more of a continuum, where not only different species, but also different individuals from the same species, function somewhere between these extremes and we can only say that one is 'more r' or 'more K'.
For example, guppies – small fish from South and Central America, popular with both aquarists and evolutionary biologists – are very flexible in this regard. Researchers have been studying them for years in Trinidadian streams and it turns out that their strategies vary widely depending on the presence of predators, sometimes within the space of a few metres. In the upper reaches of the streams, where rocks make it impossible for bigger fish to get through, guppies have fewer, but larger and better-fed young, so they're 'more K', and their offspring grow up peacefully in calm waters. Below the rocks (sometimes literally one boulder is all it takes) they choose a strategy more closely aligned with r-selection – their offspring are smaller, but they're much more numerous, because in the face of the constant risk of being eaten it makes sense to have as many as possible. So, although science is currently leaving this classic model behind, speaking more often about the diversity of survival strategies, my opinion is that – with some reservations – these two letters make it easier for us to describe a complex reality.
Still, no matter how much offspring there is to be, they need to be brought into the world somehow. Here, there are fundamentally two methods. You can lay an egg with a yolk (the evolutionary equivalent of a packed lunch) from which after a while, with more or less assistance from the parents, your kids will hatch; or you can nourish the offspring within your own body and give birth to them ready-made. It's an easy guess that apart from oviparity and viviparity there's also a third option: ovoviviparity. It refers to embryos that develop in eggs that hatch while still in the mother's body, which the young leave later.
All the eggs in one basket?
Let's start ab ovo. The egg must be encased in something, so that it can protect the embryos at least a little from outside danger. Species that lay their eggs in water usually don't have to worry that they'll dry out, so for them a jelly-like membrane is usually enough; it means the contents of the egg stay where they should, instead of sloshing around. But if you live on land, you must – like many insects and arachnids, and all reptiles and birds, as well as mammals such as the platypus and the echidna – invest in something more watertight. The hard shell of a bird's egg also protects it from at least some predators. For example, the shell of an ostrich egg – incidentally, the largest single cell in the world – is so thick and strong that even lions have trouble breaking it.
Photo by Anna Sjöblom on Unsplash
Still, whatever the eggs are encased in, they all have a better chance of surviving if someone looks after them. We automatically associate incubating eggs with birds; indeed, they either take care of their clutch themselves or, like cuckoos, frame someone else into doing it. But other animals also provide many examples of parental dedication. Female octopuses spend the last weeks of their lives defending their eggs, tucked away in some underwater nook, oxygenating them and cleansing away algae and parasites. This work uses up all the time and energy they've got left after the enormous effort of producing and laying the eggs in a suitable place. When the young octopuses finally hatch, their mum is either already dead or about to die. Although this strategy seems to suit cephalopods, we owe our current position in the world to it – I suspect that if a mother octopus could pass her knowledge and experience to her offspring, Earth would be a very different place. As it is, despite their astonishing intelligence, each octopus must re-invent the wheel. Considering that their intelligence precedes ours by a few million years, I really think that if they could accumulate experience from generation to generation, I'd be writing this text for an eight-legged editor-in-chief, had she even been interested in the opinion of an organism as inferior as a human.
Although the sacrifice of the cephalopod mum is impressive, some invertebrates go further. Perhaps the most extreme form of parental devotion is matriphagy, or the consumption of the mother by her newly-hatched offspring. This phenomenon can be observed in some arachnid species: after laying the eggs, the female starts to dissolve the tissues of her body with digestive juices, so that when the adorable spider babies hatch, their mother is nothing more than an eight-legged chitin container filled with nutritious juice. The tots just need to bite through her skin and they can lap it up. Among insects, apart from the obvious examples of the Hymenoptera (i.e. ants, wasps and bees) and termites, earwigs provide another example of exemplary parental care. The Japanese species Anechura harmandi is the only insect known to science in which the mother also dies before the young hatch, to become their first meal. Even the common earwig is no stranger to motherly sacrifice. The females of these rather unpopular fearless vanquishers of aphids and silverfish frequently gather into groups to care for their clutches together, and then to feed their young and bravely defend them from predators.
The mixed method
Laying eggs has its obvious advantages. If they require no care, you can not only produce many, but also expect that they will spread around the world on their own. But carrying their offspring in their own bodies makes it easier for parents to provide suitable conditions for development. No wonder, then, that some animals (including many species of shark and the common European adder) have chosen the compromise of ovoviviparity during their evolution. In others – like in the viviparous lizard – one or the other method of reproduction dominates depending on environmental conditions. In Southern Europe these lizards, like most lizard species, lay eggs. But in cooler areas the females give birth to their young. Thanks to this flexible strategy, they can live in environments that are inaccessible to many other species, like high up in the mountains and the far north of Europe. It is the only reptile on our continent that also lives beyond the polar circle, although vipers – the northernmost of our snakes – reach almost as far north as that.
Another interesting issue is laying your eggs in someone else's body, although I'm not sure if that still counts as ovoviviparity. The most banal and drastic example are the many species of parasitoids – animals that exploit their host completely, living in it for a time, before killing it like the Alien from the famous science fiction film. Many wasps paralyse their victim (usually a caterpillar or a spider) and lay their eggs in that living larder; the larvae will later gradually bite their way out of it. But laying eggs into the body of one's own partner is even more interesting.
This is what happens in the Hippocampus, or the slowly moving fish known as seahorses. After their mating dance and successful consummation of the relationship, the female lays the fertilized eggs into a special pouch on the male's front. From then on, they will be in his care, so that one day he can give birth to hundreds of miniature seahorses, which he will still take care of after the birth.
But since early childhood, I have been fascinated by another organism. The common Suriname toad – a tailless amphibian (i.e. frog-adjacent) from the northern part of South America with the charming Latin name Pipa pipa – appeared in my life in the form of an illustration in an ancient animal atlas, and it immediately hopped onto the pedestal as one of my favourite species of all times. Just after the female lays the eggs, the male gathers them up and distributes them evenly on her sticky back. Her skin grows spongy, and the eggs sink into it and develop relatively safely; after a time, fully formed little frogs leave her back. It is undoubtedly one of the most interesting births in nature.
The strongest bond
If the young isn't separated from its mother's organism by the egg shell, she usually nourishes it via a placenta. This is, of course, the case in a substantial majority of mammals, but not exclusively. The placenta can also be found in some sharks and lizards, but true viviparity has evolved independently at least 150 times and occurs in many species of fish, amphibians, insects and arachnids. One of these unexpectedly caring parents is the infamous tsetse fly: the female flies around for nine months with a single, increasingly large larva in her abdomen, feeding it with a nutritious milky liquid. A more macabre version of feeding one's young can be observed in some Gymnophiona from the family of common caecilians. Their embryos have special teeth that allow them to feed on the epithelium of the mother's oviduct. After they're born, young common caecilians switch over to the female's outer epithelium and literally flay her, although fortunately she regenerates quickly.
After leaving the mother's body – one way or another – many young animals still need constant care. Because the physical connection is no longer there, persuading the parents to continue to provide food and shelter requires initiating a psychological bond. The parents must like their newly born or hatched children to keep taking care of them.
And so evolution has equipped young animals with a whole arsenal of signals that leave their carers helpless. In birds, it's frequently a lurid colouration of the inside of the beak and the area around it, visible when it is fully open. Adult birds find this irresistible and stuff food down the open, begging mouth, even if it doesn't belong to their children but, for example, to a fish taking advantage of the situation. It is due to our own primitive instincts that most of us also feel tenderness and an urgent need to take care of young animals (or ones that look young). What's more, the recipients of that care don't even need to be cute, pretty bunnies – I still remember how touched I was when, as a student, I discovered a wryneck nest in one of the nest boxes I was checking. The chicks of this woodpecker, with their thin, twisty necks and flat heads, look like mould-infested hallucinogenic mushrooms and they're certainly not pretty, but it works. Their relatively big eyes and squeaky sounds are all it takes. Of course, if the animal meets our criteria of beauty, the effect is even stronger. Cats blatantly exploit this – the charm of their small faces, large eyes and the meowing that emulates the voice of a human baby turns out to be so strong that even my geologist friend is unable to resist them. Although due to his profession he is used to communing with nature through the means of a hammer, he can't stop himself and constantly regales everyone with photos of his feline charges on social media.
There's no doubt, however, that in animals such as birds and mammals it's not only the case of a simple reflex. For some time now, researchers have been claiming more and more boldly that other animals also experience feelings and emotions, like fear, anger, boredom and love. And love for one's offspring is probably the easiest to observe. It is the simplest explanation for such dramatic examples as the behaviour of a killer whale called Tahlequah who, two years ago, carried the body of her dead child with her for 17 days. Parental love can also be the explanation – because there is no other – for more prosaic and happy examples of behaviour, such as the fact that I'm about to walk my daughter to school, even though I've spent all night writing this text.
Translated from the Polish by Marta Dziurosz.
The tiny swimmers appear to transmit signals that "persuade" the female body to have a baby.
The following article was originally published on our sister site, Freethink.
New research suggests that sperm play a bigger role in pregnancy than previously believed, not only fertilizing the egg, but also "persuading" the female body to accept it.
Revisiting sex ed: Human reproduction typically starts with sperm traveling via seminal fluid to an egg and fertilizing it. That creates a zygote, which can then attach to the wall of the uterus and begin the process of developing into a baby.
Implantation isn't a given, though. The woman's immune system may reject the fertilized egg as something "foreign" and prevent her from becoming pregnant.
Persuasive swimmers: Past research has shown that seminal fluid contains proteins that dampen this immune response, encouraging the woman's body to accept the zygote and the pregnancy.
Now, researchers at the University of Adelaide have discovered that sperm send their own signals designed to persuade the woman's body.
"This overturns our current understanding of what sperm are capable of," project leader Sarah Robertson said in a press release. "They are not just carriers of genetic material, but also agents for convincing the female to invest reproductive resources with that male."
The study: For a new study, female mice were mated with males who produced sperm and males who'd been vasectomized (meaning they produced seminal fluid without sperm).
Based on changes in the expression of different genes in the females' uteruses, the researchers determined that the sperm-producing mice prompted a stronger immune tolerance in their mates than the vasectomized rodents.
By introducing male sperm to female cells in the lab, the researchers were able to confirm that the sperm was directly responsible for the changes.
Why it matters: Infertility is a common issue, affecting tens of millions of couples across the globe, and problems such as recurrent miscarriage, preeclampsia, and stillbirth have all been linked to the female immune response during reproduction.
Now that we know that sperm can affect that immune response, researchers may be able to identify new ways to treat infertility — potentially helping some couples conceive and give birth to healthy babies.
Freethink would love to hear from you! If you have a comment about this article or if you have a tip for a future Freethink story, please email us at firstname.lastname@example.org.
The island rule hypothesizes that species shrink or supersize to fill insular niches not available to them on the mainland.
- Brookesia nana, the nano-chameleon, may be the smallest vertebrate ever discovered.
- The "island rule" states that when new species migrate to islands, they may shrink or grow as they evolve to fill new ecological niches.
- It remains unclear whether the island rule can explain the nano-chameleon or nature's other extreme miniaturizations.
The newly discovered nano-chameleon (Brookesia nana) is the latest contender for the title of the world's smallest reptile and amniote vertebrate. Found in a mountainous region in northern Madagascar, the males of this diminutive species sport a body size of 13.5 mm, meaning one could comfortably stand on the end of your finger.
Its wee challenger is the Jaragua dwarf gecko (Sphaerodactylus ariasae). These pocket-change-sized geckos—the genus is often pictured snogging the minted portraits of past presidents—come in at 16 mm from nose to tail. They were discovered in 2001 on Isla Beata, a small, forested Caribbean island just south of the Dominican Republic.
The title of the world's smallest, however, is difficult to award thanks to sexual size dimorphism. As Dr. Mark Scherz, herpetologist and evolutionary biologist, pointed out on his blog, nano-chameleon females are significantly larger than their male counterparts or Jaragua dwarf gecko females. "As a result, whether or not the new species is considered the smallest amniote in the world depends on whether we define that based on the male or female body size, or the midpoint of the two. It turns out this is quite a common problem in other species with size dimorphism as well, such as frogs," Scherz writes.
Beyond their shrimpy stature, these and other miniaturized species have another thing in common: They live on islands. That fact may explain why evolution has pushed them to shrink in a world full of giant competition.
Bigger isn't always better
The New Zealand little spotted kiwi evolved to be small to fill an ecological niche. Before the arrival of humans, its island ecosystem contained no land mammals to prey on these flightless birds.
Credit: Wikimedia Commons
Because of their geographic isolation, islands can have powerful effects on the evolution of their residential species. The massive Komodo dragon prowls its namesake island. The Barbados threadsnake is thin enough to slither through a straw. And the fossil record recounts a history of unusually sized and bedecked creatures who established homes far from the mainland, such as the Hoplitomeryx of the Mikrotia fauna.
One hypothesis for evolution's insular experimentation is "the island rule." The rule states that after establishing themselves on an island, smaller species will tend to evolve into oversized versions of their mainland ancestors. Meanwhile, larger species will tend to evolve into smaller variations. These processes are known as insular gigantism and insular dwarfism, respectively. They do this to fill the ecological niches available to them, which often differ from those they filled on the mainland.
The rule was first formulated by evolutionary biologist Leigh Van Valen and based on a 1964 study by mammologist J. Bristol Foster—which is why it is also known as Foster's rule. Since then, many observational studies have corroborated the island rule, and there is even evidence to suggest that new species introduced to islands will, for a time, evolve more rapidly to fill available niches.
A flock of migrant birds, for example, may find an island's lack of mammalian and reptilian predators opens the ground-living niche once forbidden to them. Such birds would then be free to grow larger, forage below the canopies, and lose the ability of flight.
This appears to be the origin story for New Zealand's flightless birds including the giant moa, which, at six-feet tall, is the tallest bird on record. This megafauna enjoyed all the benefits of being large and in charge: fewer predators, wider ranges, access to more and varied foods, and the ability to better survive trying times. The species enjoyed island life until roughly 600 years ago, when humans arrived on the scene and hunted them to extinction.
Conversely, large species may find island living restrictive as there's less room or food when compared to their mainland nurseries. Because of this, evolution may select for smaller body sizes as such bodies require less energy, and therefore fewer resources, to survive and reproduce.
This is the theory behind the miniaturization of the Channel Islands pygmy mammoths. As the story goes, in the search for food, a herd of Columbian mammoths embarked on a journey to the super island Santaroasae. Over time, the island was cut off from the mainland. Food became scarce, and smaller mammoths had an easier time surviving and reproducing, thus passing on their Shrinky-Dink genes. Thanks to a lack of oversized predators, such evolution proved fruitful, and in less than 20,000 years, the giant Columbian mammoths evolved into a new species—the (relatively) pint-sized, 6.5-foot-tall pygmy mammoths.
To be clear, the island rule doesn't state that any species that washes ashore must go either Lilliputian or Brobdingnag. It only states that if an ecological niche becomes available and improves survival and reproductive success, then such a change is likely.
Thanks to that island living?
Such constrained growth may be the cause of the Jaragua dwarf gecko's bantam evolution. The gecko eats tiny insects and may be filling a niche that's unavailable on the North American continent with its many, many insectivores. In fact, the island rule may explain why islands are so rich with endemic species—particularly the Caribbean, which is considered a biodiversity hotspot.
Of course, scientific rules are only provisional, and scientists are prepared to revise or completely disregard a hypothesis should new evidence appear. In a field as new as biogeography, the question of whether the island rule is truly a "rule" remains an open and hotly debated question.
One systematic review found empirical support for the island rule to be low, while another analysis argued the rule is simply a recognition of "a few clade-specific patterns." The latter's authors conclude that "[i]nstead of a rule, size evolution on islands is likely to be governed by the biotic and abiotic characteristics of different islands, the biology of the species in question and contingency."
That brings us back to the newly discovered nano-chameleon. While it seems to follow the island rule—Madagascar being an island known for its rich biodiversity—there is a wrinkle. The species' closest relative lives right next door. Brookesia karchei is near twice the size of the nano-chameleon but ranges in the same mountains on mainland Madagascar.
If the nano-chameleon evolved to fill an ecological niche, why didn't those same environmental pressures miniaturize the karchei chameleon? If not the island rule, what did lead to the nano-chameleon's smaller size? As is often the case in science, further evidence may one day answer these questions.
Imagine poisoning your rival and yourself and giving only yourself the antidote.
- The t-haplotype alleles play dirty when it comes to reaching the egg first.
- In order for their nefarious scene to work, just the right amount of a certain protein has to be present.
- Experiments with mouse sperm reveal the whole complicated story.
In the life-or-death scramble to fertilize an egg, not all sperm are alike. A new study of mice by researchers from the Max Planck Institute for Molecular Genetics (MPIMG) in Berlin identifies a genetic factor called "t-haplotype," whose tag-team act with the protein RAC1 helps a spermatozoan speed straight to the prize.
The study is published in PLOS Genetics.
The weird power of the t-haplotype
Credit: ibreakstock/Adobe Stock
The researchers conducted experiments with mouse sperm to learn more about the properties of the t-haplotype, a group of genetic alleles that are known to appear on Chromosome 17 of mice.
Comparing the movement of mouse sperm with the t-haplotype against sperm without it, the researchers, led by first author Alexandra Amaral of MPIMG, definitively demonstrated the difference t-haplotype makes. Sperm with the gene factor progressed quickly forward, while "normal" sperm didn't exhibit the same degree of progress.
While most genes operate cooperatively with others, some don't. Among these "selfish" genes are the t-haplotype.
"Genes that violate this rule by unfairly increasing their chance of transmission can gain large fitness advantages at the detriment of those that act fairly. This leads to selection for selfish adaptations and, as a result, counter-adaptations to this selfishness, initiating an arms race between these selfish genetic elements and the rest of the genome." — Jan-Niklas Runge, Anna K. Lindholm, 2018
"The trick is that the t-haplotype 'poisons' all sperm," he explains, "but at the same time produces an antidote, which acts only in t-sperm and protects them. Imagine a marathon in which all participants get poisoned drinking water, but some runners also take an antidote."
The t-haplotype distributes a factor that distorts, or "poisons," the integrity of genetic regulatory signals. This goes out to all mouse sperm that carry the t-haplotype in the early stage of spermatogenesis. Chromosomes split as they mature, and half the sperm that retain the t-haplotype produce another factor that reverse the distortion, neutralizing the "poison." These t-sperm hold onto this antidote for themselves.
Even the t-haplotype needs a friend
RAC1 acts as a molecular switch outside the sperm cell. It is known to be a protein that guides cells to different places in the body. For example, it directs white blood cells and cancer cells towards other cells that are putting out specific chemical signatures. The study suggests that RAC1 may point sperm toward an egg, helping it "sniff" out its target.
In addition, the presence of RAC1 seems to help the t-sperm carry out their sabotage. The researchers demonstrated this by introducing an RAC1 inhibitor to a mixed population of sperm. Prior to its introduction, the t-sperm in the group were "poisoning" their normal neighbors, causing them to move poorly. When the inhibitor neutralized the populations' RAC1, the t-sperms' dirty trick no longer worked, and the normal sperm began moving progressively.
However important RAC1 may be to t-sperm, too much or too little is problematic. Says Amaral, "The competitiveness of individual sperm seems to depend on an optimal level of active RAC1; both reduced or excessive RAC1 activity interferes with effective forward movement."
When females have two t-haplotypes on Chromosome 17, they are fertile. When sperm have one t-haplotype, their motility may be negatively affected, but when they have two, they are sterile. The researchers discovered the reason: They have much higher levels of RAC1.
At the same time, the study finds that normal sperm who aren't being held back by t-sperm stop moving progressively when RAC1 is inhibited, meaning that too little RAC1 also results in low motility.
It’s a jungle in there
Herrmann sums up the insights the study offers:
"Our data highlight the fact that sperm cells are ruthless competitors. Genetic differences can give individual sperm an advantage in the race for life, thus promoting the transmission of particular gene variants to the next generation."
Previous research suggesting it's all about prolactin may be missing the mark.
- Men and other male creatures need time to recover between ejaculations, and scientists have assumed it has to do with an increase in the hormone prolactin after coitus.
- A new study finds that manipulating prolactin levels in mice makes no difference in their sexual behavior.
- The authors suspect more complex interactions may be at the heart of the wait for round two.
For some time, scientists have suspected the reason men require recovery time between ejaculations has to do with the hormone prolactin. During the "post-ejaculation refractory period" (PERP) following orgasm, levels of prolactin spike, and since high prolactin levels have been linked to a lack of sexual desire, it's been thought that this surge has to subside before men are ready for another go. It takes a little while for this to happen, though there's no consensus on exactly how long a wait is necessary.
A new study from researchers at the Champalimaud Research Center for the Unknown in Portugal involving mice suggests that prolactin may not be the only, or even the main, factor in post-coital downtime. Its first author Susan Lima says prolactin's presence during the PERP may have been misinterpreted: "This means it was just correlation. Causation was never tested," she tells Inverse.
The study's finding was a bit of surprise, in fact, says Lima in a press release: "When we started working on this project, we actually set off to explore the theory. Our goal was to investigate in more detail the biological mechanisms by which prolactin might generate the refractory period."
The research is published in the journal Communications Biology.
Credit: Julian Hochgesang /Unsplash
From an evolutionary standpoint, as the study puts it, "The PERP is thought to allow replacement of sperm and seminal fluid, functioning as a negative feedback system where, by inhibiting too-frequent ejaculations, an adequate sperm count needed for fertilization is maintained." The length of time involved appears to be influenced by a range of factors, including age and the excitement associated with having a new sexual partner.
Prolactin itself serves a variety of functions in the human body for both sexes. Its most well-known role is to promote lactation—it's released by the female body during nursing. Estrogen triggers its production by the pituitary gland, while dopamine restrains it.
Though prolactin's other roles remain under investigation, it's also believed to be involved in behavior regulation, and in maintaining the immune, metabolic, and reproductive systems.
No smoking gun
The authors write that "the sequence of sexual behavior in the mouse is very similar to the one observed in humans, making it an ideal system to test this hypothesis."
Therefore, for the study, Lima and her colleagues studied prolactin's role during and after sexual activity for two types of male mice—one type required several days to recover from ejaculation while the other had a relatively short PERP.
The researchers took blood from the males before they were introduced to female partners from whom they'd been kept separated. Blood was again taken after a preliminary mounting, again after a number of mounts that depended on the male's PERP—five mounts for the slow-recoverers and three for the males with the shorter turnaround time. Finally, blood was taken after ejaculation, which was fairly easy to discern since it was accompanied by what the study calls "stereotypical shivering" in the males, who also fell over afterward.
The researchers did find that the males' recovery was accompanied by higher levels of prolactin. However, during subsequent experiments in which the scientists boosted prolactin levels prior to sex—which, if the prevailing theory was correct, would have reduced their interest in copulation—no change in their sexual behavior was observed. Says Lima, "Despite the elevation in prolactin levels, both strains of mice engaged in sexual behavior normally."
Repressing prolactin levels after ejaculation also failed to reduce the males' PER interval. "If prolactin was indeed necessary for the refectory period," says Lima, "males without prolactin should have regained sexual activity after ejaculation faster than controls. But they did not."
Lima does caution that there are some differences between mice and men when it comes to prolactin dynamics, so more study is warranted.
So, what is going on?
Lima suggests that there's likely some complex interaction between the two systems involved in ejaculation: the central brain system that manages desire and the peripheral system that handles the physical aspects of ejaculation.
At the very least, the research suggests that we don't yet know why men experience their mandatory time-out. "Our results indicate that prolactin is very unlikely to be the cause," Lima summarizes. "Now we can move on and try to find out what's really happening."