Rabbits are, of course, adorable. We kvell over awww-inducing pictures of the little cuties who look like they couldn't hurt a fly. (Fun fact: Male rabbits are incredibly fierce when they fight each other and will actually fight to the death.)
Rabbits have been around for a long time, and they don't exhibit the same variations in size as for example, rodents. Even a big rabbit is not as big as, say, a capybara that can weigh from 60 to 200 pounds. Likewise, there is no rabbit anywhere near as tiny as a pigmy mouse of sub-Saharan Africa, which can weigh as little as 3 grams or as much as a zaftig 12 grams.
Researchers at Kyoto University's Primate Research Institute wondered why there are no horse-sized — or for that matter, tiny — lagomorphs. The lagomorph order includes rabbits, pikas, and hares.
The curious scientists recently published a paper titled "Why aren't rabbits and hares larger?" in the journal Evolution. It suggests the answer to this question may say something about the factors that most profoundly influence a species' evolution.
Size limited by competition
Credit: Victor Larracuente/Unsplash
First author Susumu Tomiya says, "The largest living wild lagomorphs weigh only about 5 kg on average, a tenth of the largest living rodent, the capybara."
There are breeds of domestic rabbits that can be somewhat large. Tomiya notes "some breeds of domestic rabbits and other extinct species can weigh up to 8 kg [about 17.63 pounds]. We were surprised by this, and so began to investigate what sort of external forces keep wild lagomorphs across the world from evolving larger body sizes."
After analyzing the available fossil record to explore how lagomorphs have fared through time, the team came to suspect that their size tended to be constrained by competition for food with larger herbivores.
Blame the sheep
As the researchers investigated the ecosystems in which lagomorphs lived, creatures of a different order, ungulates, came to their attention. Ungulates are an order of hoofed mammals including horses, rhinoceroses, and pigs. It also includes cloven-hoofed animals like cows and sheep.
It seems that ungulates were more than mere neighbors of rabbits, hares, and pikas. Similarly sized ungulates probably were their competitors.
The researchers calculated that large lagomorphs would require an excessive amount of energy — food, water and so on — to thrive, considerably more than smaller rabbits and hares. As a result, Tomiya says that when they compared "how much energy is used by populations of lagomorphs and ungulates relative to their body sizes," they found "that lagomorphs weighing more than six kilograms are energetically at a competitive disadvantage to ungulates of the same size."
To confirm their theory, the researchers next looked at the fossil record of North America. They found that the smallest hoofed animals predicted the size of the largest bunnies.
It remains true to this day, he says. "We see this pattern today across numerous eco-regions, suggesting that there is an evolutionary 'ceiling' placed on lagomorphs by their ungulate competitors."
The red queen versus the court jester
Tomiya says the study may help resolve biologists' ongoing "red queen" vs. "court jester" debate over the type of forces that most affect a species' evolution.
The red queen represents biotic forces and the court jester abiotic factors. ("Biotic" refers to other organisms that live in the same ecosystem as a species being studied, while "abiotic" refers to non-living factors such as climate, light, the quality of water, and so on.)
"For some time," says Tomiya, "the court jester model — ascribing diversity to abiotic forces such as the climate — has been dominant, due to the difficulty of studying biological interactions in the fossil record." He says the study's findings show that the red queen shouldn't be counted out when it comes to influencing evolution.
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
"Sperm with the t-haplotype manage to disable sperm without it," says corresponding study author Bernhard Herrmann, also of MPIMG.
"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
Credit: Emw/Wikimedia
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."
In the early 19th century, Napoléon Bonaparte was perhaps best known for leading successful military campaigns and serving as the Emperor of the French for nearly a decade. But today, the ruthless French leader is probably best remembered in the popular imagination for his short height, a trait that inspired what many now call the Napoleon complex.
The Napoleon complex is a popular belief that describes an inferiority complex in which short men tend to compensate for their small stature through behavior, such as increased aggression or gossiping. From an evolutionary perspective, it makes sense that short men might try to compensate; research shows that tall men are more likely to hold positions of power, attract mates and be perceived as higher status by their peers.
A new study published in the journal Psychological Science uses economic games to examine the Napoleon complex, providing some of the first results on the importance of height in competition between men.
In an economic experiment called the dictator game, participants were asked to divide a sum of money between themselves and an unseen opponent. Each participant could divvy up the money however he wished. Interestingly, the participants who tended to keep the most money for themselves in this version of the game weren't necessarily shorter—they were people who reported that they often felt small.
The researchers then conducted the same game in a competitive setting, in which two male opponents met face to face, had their heights recorded and read aloud (along with other physical and strength measurements), and were asked to enter separate cubicles. Again, the participants had to divide a sum of money. The allocator could choose to give any amount, or nothing, to his opponent, who was the receiver. The researchers told the participants that one person would play the allocator and the other would play the receiver, but in reality every participant played the allocator.
The results showed that, on average, relatively shorter men kept more money for themselves.
Next, the participants played an ultimatum game in which an allocator divides a sum of money, keeping some of it and offering a portion of his choice to the receiver. But if the receiver perceives the offer to be unfair, he can reject it and both parties get nothing.
Unlike the dictator game, height didn't seem to play a significant role in influencing how much money participants chose to keep in the ultimatum game.
In another experiment, two male opponents once again played a dictator game. This time, however, each participant also had to choose an amount of hot sauce their opponent would have to consume, which was, in theory, a measure of aggression. But the results showed that shorter men were not significantly more likely to make their opponents eat more hot sauce.
Men show flexible behavior in competitions
The study suggests that shorter men are more likely to show indirect, rather than direct, aggression toward taller men in competitions for resources. For shorter men, the researchers wrote that these indirect strategies represent safer options than physical combat. Also, the results suggest that the Napoleon complex is most likely to manifest in situations where the shorter man has all the power, and the taller man can't retaliate.
"The results imply that participant height is most important in predicting competitive behaviors in an absolute-power situation (the dictator game), regardless of opponent height," the researchers wrote. "This is not surprising as shorter and taller men likely have different life experiences that may influence their decision making in behavioral experiments."
It's not just competitions between men that bring out the Napoleon complex, the researchers noted.
"In terms of underlying mechanisms, the Napoleon complex may also be shaped by intersexual selection forces—shorter men could use behavioral strategies to impress females, such as risk taking, generosity, or showing commitment (e.g., Griskevicius et al., 2007; Iredale, Van Vugt, & Dunbar, 2008)."
The researchers suggested it'd be interesting to see whether men would behave differently in these kinds of economic games if an attractive female were also involved.
"For further studies, it would be of great interest to add a potential mating opportunity to the paradigm to see how intersexual competition affects the Napoleon complex. The presence of an attractive female could exacerbate other kinds of overcompensating behaviors in short men—for example, an increased propensity toward risk taking to impress women."
The study, "The Napoleon Complex: When Shorter Men Take More", was authored by Jill E. P. Knapen, Nancy M. Blaker, and Mark Van Vugt.
