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Japanese honeybees surround a giant hornet. Image courtesy of Masato Ono, Tamagawa University

For millions of years, Japanese honeybees have been locked in a deadly battle with the Japanese giant hornet, a fierce predator with an appetite for bee larvae. With a two-inch-long body and a 3-inch wingspan, the hornet is enormous – many times larger than the bees. But the honeybees have evolved a unique defense mechanism: When a hornet invades a honeybee hive, as many as 500 bees gang up and form a tight ball around the attacker. The heat from the bees’ vibrating wings and the carbon dioxide they respire proves a deadly combination. In less than an hour, the hornet is dead.

The attack unfolds like this: When a hornet approaches a honeybee hive, bee guards posted at the entrance fiercely shake their abdomens. In a paper published last month, researchers argue that this abdomen shaking represents an “I see you” signal, something that is advantageous to both predator and prey. “The prey avoids attack, the predator avoids chasing a prey that has been alerted,” the researchers write. If the waggling doesn’t deter the hornet, the guards alert the rest of the hive. Some of the worker bees exit the nest and wait outside. If the hornet moves to attack, these bees surround it, forming a “hot defensive bee ball.”

Hot bee ball. Image courtesy of Masat Ono, Tamagawa University.

A new study, published last week, examines what goes on in the honeybees’ brains while they are in this ball. The researchers, including Takeo Kubo of the University of Tokyo and Masato Ono of Tamagawa University, first identified a gene whose expression could be used as a marker of brain activity. They then used a live hornet tied to a wire to spur the formation of a bee ball. When they inserted the hornet into the hive, the bees swarmed and the researchers managed to extract the bee ball and place it in a beaker. That enabled them to pluck individual bees from the pile at different time points and examine their brains for increased expression of the target gene. (See a video of the process here.)

The balling behavior seemed to prompt activity in particular neurons found in bee brain regions called the mushroom bodies, which are involved in learning and memory. Heat exposure alone led to increased activity in these same neurons. What this means isn’t yet entirely clear. The researchers speculate these neurons may help the bees monitor how hot the ball gets and avoid overheating.

One thing is clear: The balling behavior seems vital to the bees’ survival. European honeybees, which were introduced in Japan more than a century ago, have not evolved any defense mechanisms against giant hornets. Hornet attacks can devastate their hives; a group of 20 to 30 hornets can slaughter a 30,000-bee colony in just a few hours.

The endangered pygmy hippopotamus reproduces well in captivity. Photo courtesy of Dr. Gabriela Galateanu

For most animals, the number of males in a population is about the same as the number of females. And that makes sense, evolutionarily. If a population were skewed toward females, for instance, males would become a hot commodity and each one would have a better chance of mating than would a male in a balanced population. Eventually, parents who had boys would accumulate more grandchildren, and the genes for producing boys would spread until the sex ratio evened out.

But that explanation, known as Fisher’s Principle, is too pat. There are many species that, for a variety of environmental and social reasons, wind up with an imbalance of males and females. Typically, researchers have said that the female—usually the one that invests more time and energy into her offspring—is responsible for skewing the ratio depending on her needs. But a new study in pygmy hippos, published today in Nature Communications, shows that males can influence sex ratios, too.

In the wild, pygmy hippopotamuses live long and solitary lives in the swamps and forests of West Africa. Males compete, often to the death, for control of territory. Males and females meet only for mating, and then go their separate, reclusive ways.

Because the animals are nocturnal and males and females look exactly the same, researchers don’t know much about how pygmy hippos mate in the wild. In zoos, though, the hippos reproduce easily and, intriguingly, make more girls than boys. Of animals born in captivity, just 42 percent are male.

To find out why, researchers analyzed semen samples from 10 male pygmy hippos. In hippos, like humans, the mother’s egg always contributes an X chromosome to the offspring, whereas the father’s sperm cell will hold either an X or a Y chromosome. In a study published this morning, researchers used colored dyes to distinguish X-carrying sperm from Y-carrying sperm. They found that the average proportion of Y-carrying sperm was 43 percent—almost exactly the proportion of male hippos at birth.

The $64,000 question of course is why it would be an advantage for male hippos to have more daughters than sons. The researchers can’t say for sure, and the balance probably changes with environmental conditions, but they speculate that it’s a survival strategy in times of high population density (which the hippos may feel in captivity). When there are too many males, competition for territory will spike, leading to death matches between two brothers or between father and son—an evolutionary dead end.

A cane toad is highly toxic and should not be eaten or even licked. Photo from Wikicommons.

American cane toads (Rhinella marina), native to Central and South America, are an invasive species in Australia. These toads contain a substance called “bufotoxin” that makes a lot of predators ill, sometimes fatally. (Warning: This is very poisonous stuff. Do not even lick a cane toad!)

Australian animals that eat this toad are typically poisoned by it, but one animal, the bluetongue skink (Tiliqua scincoides), appears to be able to eat the toad with few or no ill effects. Or, more exactly, some bluetongue skinks can eat the cane toads, depending on where they live.

Many animals and plants produce complex molecules (like bufotoxin) that have been shaped by natural selection to be toxic to predators. Some of our favorite spices, such as basil, chili peppers and other aromatic plants, owe their culinary properties to these molecular adaptations to herbivory. Only a few mammals produce molecular toxins, but many frogs and toads do.

The bluetongue skink. Note the blue tongue. Photo from Wikicommons.

If a weapon evolves in nature, there is a certain chance that a counter-weapon will also evolve. Many insects that feed on toxic plants have evolved the ability to sequester the poisonous molecules produced by those plants, rendering them harmless to the insect, and in some cases concentrating the undesirable substance in the insect’s own body to be used as a defense against insect-eating animals (usually other insects). Many mammals have enzymes in their digestive tract that detoxify plants that would otherwise be harmful. The evolution of toxicity and the evolution of anti-toxin strategies is considered an arms race between the eaten and the eaters.

So, it would be reasonable to suspect that the bluetongue skink has evolved a physiological mechanism to combat the bufotoxin produced by the cane toads. But it turns out that the explanation for the ability of some skinks to snack on the toxic toads is a little more complicated.

Another invasive species found in Ausralia is the ornamental “mother-of-millions” plant, a Bryophyllum from Madagascar. This plant produces a toxin that is chemically similar to bufotoxin. Why is it chemically similar to bufotoxin? This is probably a coincidence. If you have a large number of animals and plants producing toxins, sometimes there are going to be accidental similarities.

Mother-of-millions plant. Image from Wikicommons.

The mother-of-millions plant is invasive and found in the wild in certain areas of Australia, but it is not common everywhere. Bluetongue skinks that live where mother-of-millions is common appear to have adapted to eating them, and as such posses the ability to neutralize bufotoxin-like molecules. When these skinks encounter cane toads, they eat them without consequence. In fact, the skinks living in these area regularly eat both the mother-of-millions plants and the cane toads.

This research was was carried out by scientists at the Richard Shine Lab at the University of Sidney.

Price-Rees, Samantha J. Gregory P. Brown, Richard Shine, 2012. Interacting Impacts of Invasive Plants and Invasive Toads on Native Lizards. Natural History Editor: Craig W. Benkman. Published online Jan 25, 2012

One of the most interesting large-scale patterns in evolution is parallelism. For example, flight has evolved many times, in parallel, from numerous non-flying organisms; many species of vertebrates that are not fish have evolved swimming, in parallel. One study discovered parallel evolution in body armor among freshwater stickleback fish from numerous saltwater ancestors.

Another interesting thing about evolution, which has only been appreciated in recent decades, is the fact that there is not a simple correspondence between genes and traits. Rarely does one gene determine one trait, and rarely does one trait vary because of one gene. There are dozens of examples of simple gene-trait relationships, many of which were discovered years ago. Because these relationships were relatively easy to find and describe, our textbooks are full of them and our thinking about genetics was for a long time based on them. But this is a little like basing our conception of how all vehicles work by deeply understanding the workings of a toy wagon. The mechanics and engineering of a little red wagon will not help us understand escalators, submarines, or Apollo lunar launch systems. We now think that most genes affect multiple traits and most traits are affected by multiple genes, and that it is all very complex.

A recent study looking at stickleback behavior seems to be an example one gene affecting multiple traits.

Sticklebacks are members of the Gasterosteidae family of fish, with species that live in salt and fresh water. The freshwater sticklebacks evolved from saltwater ancestors who were landlocked less than about 17,000 years ago at many locations across the Northern Hemisphere. For this reason, differences among freshwater and saltwater sticklebacks represent recent and rapid evolution among a well-known group of species and are thus especially interesting to scientists.

Saltwater sticklebacks have up to 36 bony plates associated with a smaller number of sharp spines. These plates and spines protect the fish from predators, but they are costly to produce and maintain. The bony plates require extra calcium, which is rare in some environments, and they restrict the body movements of the fish.

Freshwater sticklebacks tend to have fewer spines and bony plates. Some have a gap in the row of plates (this is called a “partial morph”) while others have only a few plates at the back end of the fish (“low morph”). Fresh water has less calcium than salt water, so this may be an adaptation to a limiting resource. Also, freshwater environments tend to have fewer predators than saltwater environments, so the protective features of the bony plates may be less important in fresh water; perhaps there was relaxed natural selection on this armor, and over time it was lost in many different populations in parallel.

Top: The ninespine stickleback, Pungitus pungitus, is typical of the saltwater form. Bottom: A freshwater form of stickleback with fewer bony plates and fewer spines. Image based on drawings from the Queensland Government Fisheries

In a 2005 study, scientists looked at a gene (Eda) that determines the growth of the bony plate and found that freshwater sticklebacks had a variant of the gene that caused fewer plates to form in those populations. The gene Eda probably serves a regulatory function, so it could determine one of a range of phenotypes from the fully armored saltwater version to the two lesser armored versions found in fresh water. A combination of genetic and population analysis led the researchers to discover that most freshwater sticklebacks in the Northern Hemisphere which exhibit a loss of bony plates do so because they all inherited a variant of Eda that is rare in the original saltwater populations. So the trait evolved in parallel in many lineages, all of which came from different saltwater populations, but it also evolved from a single pre-existing form of the gene. However, it was also found that one or more of the Northern Hemisphere sticklebacks with reduced bony plates got this trait from an entirely different genetic change.

This trait is thus an example of a feature determined by more than one gene, and an example of parallel evolution occurring by more than one means.

A second study just reported at a scientific meeting looks at what seems to be an entirely different question about stickleback evolution. Most sticklebacks form schools, which is a common adaptation among fish, following the principle that there is safety in numbers. But there is one population of freshwater sticklebacks that does not form schools. The sticklebacks of Paxton Lake, in British Columbia, Canada swim around alone most of the time. Rather than forming schools, they hide out in thick vegetation on the bottom of Lake Paxton.

The research team led by Anna Greenwood of the Fred Hutchinson Cancer Research Center in Seattle devised a machine to test for and measure schooling behavior in sticklebacks. This consists of a mobile-like cluster of fake fish which move together as a robotic school in a circle around a large aquarium. When fish from a schooling population of sticklebacks were placed in the water with this machine, they joined the fake fish and swam around with them. When fish from the non-schooling population were placed in the water with this machine, they did not school. These two populations are so closely related that they can interbreed. The researchers tested offspring of the schooling and non-schooling fish to see which behavior each fish would exhibit. As expected, some schooled, and some did not. Once the hybrid fish were sorted out, their genes were examined to see if there was a particular signature that went with schooling versus solitary swimming.

It turns out that the gene that seems to control schooling behavior in these fish is none other than Eda, the same gene that controls the number of bony plates.

So the sticklebacks not only give us a great example of how parallel evolution can arise, but also a great example of a gene affecting more than one trait. But how does that work? The fish that do not develop bony plates also do not develop a fully functioning lateral line. A lateral line is a sense organ many fish have that allows fish to detect movement elsewhere the water. Some predatory fish use the lateral line to find their prey, other fish use the lateral line to detect predators and thus avoid becoming prey, and schooling fish use the lateral line to keep track of the other fish in the school. Apparently, the sticklebacks with the poorly developed lateral lines can’t school because they can’t properly sense the other fish with whom they would need to coordinate their movements.

Sources:

Colosimo, Pamela F., Kim E. Hosemann, Sarita Balabhadra, Guadalupe Villarreal, Jr., Mark Dickson, Jane Grimwood, Jeremy Schmutz, Richard M. Myers, Dolph Schluter, and David M. Kingsley. 2005. Widespread Parallel Evolution in Sticklebacks by Repeated Fixation of Ectodysplasin Alleles Science 25 March 2005: 307 (5717), 1928-1933. [DOI:10.1126/science.1107239]

Pennisi, Elizabeth. 2012. Robotic Fish Point to Schooling Gene. News and Analysis. Science 335(6066):276-277. DOI: 10.1126/science.335.6066.276-b

A dragon statue in Ljubljana, Slovenia. Photo courtesy Wikimedia Commons.

Around the world, people are celebrating the Chinese New Year and the start to the Year of the Dragon. This got us wondering: Where did the myth of the dragon come from in the first place? Scholars say that belief in dragons probably evolved independently in both Europe and China, and perhaps in the Americas and Australia as well. How could this happen? Many have speculated about which real-life animals inspired the first legends. Here’s our run-down of the likeliest suspects.

Dinosaurs. Ancient people may have discovered dinosaur fossils and understandably misinterpreted them as the remains of dragons. Chang Qu, a Chinese historian from the 4th century B.C., mislabeled such a fossil in what is now Sichuan Province. Take a look at a fossilized stegosaurus, for example, and you might see why: The giant beasts averaged 30 feet in length, were typically 14 feet tall and were covered in armored plates and spikes for defense.

The Nile Crocodile. Native to sub-Saharan Africa, Nile crocodiles may have had a more extensive range in ancient times, perhaps inspiring European dragon legends by swimming across the Mediterranean to Italy or Greece. They are among the largest of all crocodile species, with mature individuals reaching up to 18 feet in length—and unlike most others, they are capable of a movement called the “high walk,” in which the trunk is elevated off the ground. A giant, lumbering croc? Might be easy to mistake for a dragon.

The Goanna. Australia is home to a number of species of monitor lizards, also referred to as Goannas. The large, predatory animals have razor-sharp teeth and claws, and they are important figures in traditional Aboriginal folklore. Recent studies even indicate that Goannas may produce venom that causes bite victims’ wounds to develop infections after an attack. At least in Australia, these creatures may be responsible for the dragon myth.

Whales. Others argue that the discovery of megafauna such as whales prompted stories of dragons. Ancient humans encountering whale bones would have no way of knowing that the animals were sea-based, and the idea of such gargantuan creatures might well have led people to assume that whales were predatory. Because live whales spend up to 90 percent of their time underwater, they were poorly understood for most of human history.

The Human Brain. The most fascinating explanation involves an unexpected animal: the human. In his book An Instinct for Dragons, anthropologist David E. Jones argues that belief in dragons is so widespread among ancient cultures because evolution embedded an innate fear of predators in the human mind. Just as monkeys have been shown to exhibit a fear of snakes and large cats, Jones hypothesizes that the trait of fearing large predators—such as pythons, birds of prey and elephants—has been selected for in hominids. In more recent times, he argues, these universal fears have been frequently combined in folklore and created the myth of the dragon.