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."
Monogamy is often considered a key component of traditional marriages, but it's only half the story.
- Depending on who you ask, monogamy is either essential to a successful marriage or it is unrealistic and sets couples up for failure.
- In this video, biological anthropologist Helen Fisher, psychologist Chris Ryan, former Ashley Madison CEO Noel Biderman, and psychotherapist Esther Perel discuss the science and culture of monogamy, the role it plays in making or breaking relationships, and whether or not humans evolved to have one partner at a time.
- "The bottom line is, for millions of years, there were some reproductive payoffs not only to forming a pair bond but also to adultery," says Fisher, "leaving each one of us with a tremendous drive to fall in love and pair up, but also some susceptibility to cheating on the side."
Positive, romantic thoughts could produce positive, romantic outcomes while dating.
- Fear of rejection, self-doubt, and anxiety are just some of the obstacles humans need to overcome to make a meaningful, romantic connection with another person.
- According to a 2020 project by a group of psychologists at the University of Rochester (and the Israeli-based Interdisciplinary Center Herzliya), humans see possible romantic partners as a lot more attractive if they go into the interaction with a "sexy mindset."
- Across three separate studies, this team discovered that this sexual activation helps people initiate relationships by inducing them to project their desires onto prospective partners.
What encourages us to seek out potential partners? What interactions encourage us to keep dating, despite the possibility of being rejected? The sexual behavioral system of humans has evolved over millennia and has been the topic of many scientific studies over the years. The concept of dating and pursuing romantic partners has been a curiosity to everyone, it seems, with lists like this one from Mental Floss detailing what dating was like throughout the centuries.
Ultimately, romantic "success" depends on our ability to target the right potential partner whom we not only find attractive but who is also attracted to us. Fear of rejection, self-doubt, and anxiety are just some of the obstacles humans need to overcome to make a meaningful, romantic connection with another person.
Being in a frisky mood improves your chances with potential romantic partners
The right mood could land you the right date, according to a new study.
Credit: BlueSkyImage on Shutterstock
According to a 2020 study by a group of psychologists at the University of Rochester (and the Israeli-based Interdisciplinary Center Herzliya), humans see possible romantic partners as a lot more attractive if they go into the interaction with a "sexy mindset."
Harry Reis, professor of psychology and the Dean's Professor in Arts, Sciences & Engineering at Rochester, and Gurit Birnbaum, a social psychologist and associate professor of psychology at the IDC (Interdisciplinary Center Herzliya) have dedicated decades of their lives to studying the intricate dynamics of sexual attraction and human sexual behavior.
In a previous study, the pair discovered that when people feel greater certainty about a romantic partner's interest, they put more effort into seeing that person again. Additionally, this study found people will rate the possible partner as more "sexually attractive" if they knew the person was interested in seeing them again.
For this project, Reis and Birnbaum, along with their team, examined what would happen if a person's sexual system is activated by exposing them to brief sexual cues that induced a thought process that included the potential for sex or heightened attraction.
Across three separate studies, the team discovered that this sexual activation helps people initiate relationships by inducing them to project their desires onto prospective partners.
Study one: Immediacy
In the first study, 112 heterosexual participants (between the ages of 20-32) who were not in a romantic relationship were randomly paired with an unacquainted participant of the opposite sex. Participants introduced themselves to each other (speaking about their hobbies, positive traits, career plans, etc.), all while being recorded.
The team then coded the recorded interactions and searched for nonverbal expressions of immediacy (such as close proximity, frequent eye contact, smiles, etc.) that could indicate interest in starting a romantic relationship.
In the study, the team determined that the participants exposed to a sexual stimulus before the meeting (versus those exposed to a neutral stimulus) exhibited more immediacy behaviors towards their potential partners and also perceived the partners as more attractive and/or more interested in them.
Study two: Interest
In the second study, 150 heterosexual participants (between the ages of 19-30) who were not in a romantic relationship served as a control for the potential partner's attractiveness and reactions. All participants in study two watched the same pre-recorded video introduction of a potential partner of the opposite sex. They then introduced themselves to the partner while being filmed themselves.
The researchers found that the activation of the sexual system led to participants viewing the potential partner as more attractive as well as more interested in them.
Study three: How it all ties together
In the third and final study, the team investigated whether a partner's romantic interest could explain why sexual activation impacts how we view other people's romantic interest in ourselves.
In this study, 120 single heterosexual participants (between the ages of 21-31) interacted online with another participant who was actually an attractive opposite-sex member of the research team. This was a casual "get-to-know-you" kind of interaction. The participants rated their romantic interest in the other person as well as that person's attractiveness and interest in them.
Again, the team found that sexual activation increased a person's romantic interest in the other person, which, in turn, predicted that the other person would then be more interested in a romantic partnership as well.
The takeaway: Positive, romantic thoughts could produce positive, romantic outcomes.
The basis of this multi-study theory is simple: Having active sexual thoughts arouses romantic interest in a prospective partner and often leads to an optimistic outlook on dating.
"Sexual feelings do more than just motivate us to seek out partners. It also leads us to project our feelings onto the other person," said Reis to Eurekalert.
Reis goes on to explain, "...the sexual feelings need not come from the other person; they can be aroused in any number of ways that have nothing to do with the other person."