April 30, 2013
Baby animals may seem irresistibly adorable, but in reality many of them are calculating killers. Hyena, wolf or even dog litter runts are pushed aside by their larger siblings and left to go hungry; fuzzy white egret chicks will kick their weaker clutch mates out of the nest to certain doom; and baby golden eagles sometimes go so far as to snack on their smaller brothers and sisters while their mother looks on.
Perhaps most disturbing of all, however, is the case of the baby sand tiger shark. While sharks may not be the most snuggly animals to begin with, the sand tiger shark sets a new precedent for fratricide. This species practices a form of sibling-killing called intrauterine cannibalization. Yes, “intrauterine” refers to embryos in the uterus. Sand tiger sharks eat their brothers and sisters while still in the womb.
Even by nature’s cruel standards, scientists admit that this is an unusual mode of survival. When sand tiger sharks develop in their mother’s uteri (females have both a left and right uterus), some–usually the embryo that hatched first from its encapsulated, fertilized egg–inevitably grow faster and larger than others. Once the largest embryos cross a certain size threshold, the hungry babies turn to their smaller siblings as convenient meals. “The approximately 100 mm hatchling proceeds to attack, kill and eventually consume all of its younger siblings, achieving exponential growth over this period,” a team of researchers who investigated the phenomenon wrote this week in Biology Letters.
From what began as two uteri full of a dozen embryos results in just two dominating baby sand tiger sharks coming full term. What’s more, once the unborn babies consume all of the living embryos, they turn to their mother’s unfertilized eggs next, in a phenomenon called oophagy, or egg-eating. By the time those two surviving babies are finally ready to be introduced into the big, bright world, all of the pre-birth inner feasting has paid off. They emerge from their mother measuring in at about 95 to 125 centimeters long, or a bit longer than a baseball bat, meaning fewer predators can pick them off than if they had shared food with siblings and were smaller.
This peculiar situation has implications for the genetic makeup of the species. Female sand tiger sharks, like many animals, mate with multiple males. Oftentimes in nature, females determine which males will sire the next generation by selectively choosing to mate with the most impressive bachelor (or bachelors) around. If mating with multiple males at any given time–as sharks, insects, dogs, cats and many other animals sometimes do–the babies that the female eventually produces share the same womb with siblings that may have different fathers.
In this case, however, there are two modes of selection at work. Females may choose mates, but that does not guarantee those males’ genes will make the cut. The embryos the males sire will also have to survive the subsequent frenzy of cannibalism going on inside the female’s body.
To find out whether some males are mating but missing out on actually producing offspring, the authors of this new study undertook microsatellite DNA profiling of 15 sand tiger shark mothers and their offspring. The researchers collected the sharks from accidental mortality events near protected beaches in South Africa between 2007 to 2012. By comparing the embryo genetics, the researchers could determine how many fathers were involved in fertilizing the eggs.
Nine of the females, or 60 percent, had mated with more than one male, the researchers found. When it came to which embryos hatched and grew large first (and thus would have survived if their mothers hadn’t have been killed), 60 percent shared the same father. This means that even if a female mates with more than one male, there is no guarantee that the male has been successful in passing on his genes. Rather, he could have just provided a convenient entree for another male’s offspring.
This also explains some male sand tiger shark behavior and physiology. Male sand tiger sharks often guard their mates against other males just after copulation. Males of this species also produce a conspicuously large amount of sperm compared to other sharks. Both of these characteristics increase the likelihood that the embryo fertilized by that male will successfully implant in the female’s uterus earlier, giving it a significant head start for developing more quickly than its siblings, which makes it more likely that the recent mate’s offspring will eat the others that may come along.
As for the females sand tiger sharks, some researchers think they actually may not have much of a choice when it comes to mating with multiple males. It could be that females just give in to some amorous partners because the energetic cost of resisting those advances outweighs the cost of just conceding to the act–a behavior biologists call the convenience polyandry hypothesis. In this case, however, females may still get the final laugh since the males they first mated with and most likely preferred will have the greater chance of actually triumphing as the father of their children. “[Embryonic cannibalism] may allow female sand tigers to engage in convenience polyandry after mating with preferred males without actually investing in embryos from these superfluous copulations,” the researchers speculate.
While the females did invest in initially developing those doomed embryos, those investments are much smaller than what would be required to bring multiple embryos to full term. Those smaller embryos also represent resources allocated to the stronger, dominate embryonic winners, which thus have a better chance of surviving and passing on their mother’s genes than if she had spent the energy to instead birth multiple, weakling babies. In a way, the mother shark is providing nourishment for her strongest babies by producing multiple embryos that the most robust can eat.
“This system highlights that competition and sexual selection can still occur after fertilization,” the authors write. For example, the first embryo to implant may not end up being the the one that survives the gladiator arena of the sharks uterus. While this new research still needs to delve into the details of the competition that takes place within the uterus, a picture is emerging based upon these initial findings: Females may chose which males to mate with or may be coerced into reluctantly mating, but male sperm fitness and the quality of the embryos they produce could also carry significant weight in which animals ultimately wind up as winners in this system.
“This competition can play an important and probably under-appreciated role in determining male fitness,” the authors conclude.
March 20, 2013
For centuries, monsters of the deep sea captivated the imagination of the public and terrified explorers–none more so than the many-tentacled kraken. In 13th century Icelandic sagas, the Vikings wrote of a terrifying monster that “swallows both men and ships and whales and everything that it can reach.” Eighteenth century accounts from Europe describe arms emerging from the ocean that could pull down the mightiest ships, attached to bodies the size of floating islands.
Today, we’re fairly confident that a tentacled beast will not emerge from the depths to swallow up a cruise ship, but the enduring allure of such creatures lingers. None of the ocean’s massive animals, perhaps, are as intriguing as the giant squid.
Now, scientists have come one step closer to unraveling the mysteries behind this rare animal. As it turns out, contrary to some squid enthusiasts’ former hypothesis, all giant squid belong to a single species. What’s more, those animals are extremely similar genetically.
To arrive at these findings, researchers from the University of Copenhagen’s Natural History Museum of Denmark along with collaborators from 7 other countries genetically analyzed bits and pieces of 43 of the animals–which can grow more than 40 feet long and weigh nearly 2,000 pounds–recovered from all over the world.
Their results indicated that, unlike most marine animals, giant squid harbor almost no genetic diversity. Remarkably, individuals as far apart as Florida and Japan, from a statistical standpoint, shared almost the same DNA. The giant squid’s genetic diversity turned out to be 44 times lower than the Humboldt squid, another large species, and seven times lower than the diversity of a population of oval squids living in a restricted area and thus prone to inbreeding. In fact, the giant squid’s diversity was lower than all other measured oceanic species, save the basking shark, which scientists believe recently underwent a severe population bottleneck in which most animals died and only a few individuals survived and repopulated the species.
The researchers can only speculate about this finding’s underlying reasons–the giant squid’s genetic data alone cannot provide a plausible explanation. Perhaps something about the giant squid makes it advantageous to cull mutations from its genome? Alternatively, the animals may have undergone a recent bottleneck, similar to what happened to the basking sharks, meaning that all giant squid following that event are closely related. Or perhaps a few foundered squid somehow wandered in new stretches of ocean, so when they populated these new habitats their offspring shared the same squid family tree. The short answer, however, is that the researchers simply do not know.
“We cannot offer a satisfactory explanation for the low diversity, and this requires future studies to resolve,” they write in a paper published this week in Proceedings of the Royal Society B.
This has been a big year for giant squid. In January, a Japanese team released the first footage of a giant squid interacting in its natural environment. Yet much still remains to be learned about these enigmatic creatures. For example, researchers still have no idea how large of a range the adult squid patrol, how long they live, how quickly they grow and whether problems such as climate change affect their populations.
For the imagination’s sake, however, perhaps it’s best if some mysteries endure.
“Despite our findings, I have no doubt that these myths and legends will continue to get today’s children to open their eyes up–so they will be just as big as the real giant squid is equipped with to navigate the depths,” said lead researcher Tom Gilbert in a statement.
March 18, 2013
Some scientists investigate the universe’s biggest mysteries, like the Higgs boson, the mysterious particle that endows all other subatomic particles with mass.
Other researchers look into questions that are, well, a bit humbler—like the age-old puzzle of whether roosters simply crow when they see light of any kind, or if they truly know to crow when the morning sun arrives.
Lofty or not, it’s the goal of science to answer all questions that arise from the natural world, from roosters to bosons and everything in between. And a new study by Japanese researchers published today in Current Biology resolves the rooster question once and for all: The birds truly do have an inner circadian rhythm that tells when to crow.
The research team, from Nagoya University, investigated via a fairly straightforward route: They put several groups of four roosters in a room for weeks at a time, turned the lights off, and let a video camera running. Although roosters can occasionally crow at any time of day, the majority of their crowing was like clockwork, peaking in frequency at time intervals roughly 24 hours apart—the time their bodies knew to be morning based on the sunlight they’d last seen before entering the experiment.
This consistency continued for about 2 weeks, then gradually began to die out. The roosters were left in the room for 4 weeks in total, and during the second half of the experiment, their crowing began occurring less regularly, at any time of day, suggesting that they do need to see the sun on a regular basis for their circadian rhythms to function properly.
In the experiment’s second part, the researchers also subjected the roosters to alternating periods of 12 hours of light and 12 hours of darkness, while using bright flashes of light and the recorded crowing of roosters (since crowing is known to be contagious) to induce crowing at different times of day. When they activated these stimuli near at or near the dawn of the roosters’ 12-hour day, crowing rates increased significantly. At other times of day, though, exposing them to sudden flashes of light or playing the sound of crowing had virtually no effect, showing that the underlying circadian cycle played a role in the birds’ response to the stimuli.
Of course, many people who live in close proximity to roosters note that they often crow in response to a random light source turning on, like a car’s headlights, no matter what time of day it is. While this may be true, the experiment shows that the odds of a rooster responding to a car’s headlights depend on how close the current time is to dawn—at some level, the rooster’s body knows whether it should be crowing or not, and responding to artificial stimuli based on this rhythm.
For the research team, all this is merely a prelude to their bigger, more complex questions: Why do roosters have a biological clock that controls crowing in the first place, and how does it work? They see the simple crowing patterns of the rooster as an entry point into better understanding the vocalizations of a range of animals. “We still do not know why a dog says ‘bow-wow’ and a cat says ‘meow,’” Takashi Yoshimura, one of the co-authors, said in a press statement. “We are interested in the mechanism of this genetically controlled behavior and believe that chickens provide an excellent model.”
March 6, 2013
In 1975, a team of Russian archaeologists announced that they’d made a remarkable find: From a cave in the Altai Mountains of Siberia, they’d unearthed a 33,000-year-old fossil skull that resembled a wolf. In 2011, an anatomical analysis suggested that the fossil was a hybrid of a wolf (with its large teeth) and a dog (with its shortened snout), raising the possibility that it was a partly domesticated wolf—in other words, one of the oldest ancestors of the modern dog ever discovered.
At the time, though, DNA analysis was needed to make certain that the fossil came from an ancestor of man’s best friend. A paper published today in the journal PLOS ONE confirms that fact, indicating that the creature was more closely related to modern dogs than wolves, and forcing scientists to reconsider the dog’s evolutionary family tree.
To come to the finding, a team led by Anna Druzhkova of the Russian Academy of Sciences sequenced mitochondrial DNA taken from one of the skull’s teeth. This type of genetic material comes from an organelle inside each cell called the mitochondria, which has a distinct type of DNA that’s separate from the cell’s normal chromosomes. For each individual, mitochondrial DNA is inherited directly from one’s mother without any modifications and thus remains relatively constant over generations, except for the gradual effect of mutations. Similarities found in such DNA collected from various animals helps scientists understand the evolutionary relationships between species.
The research team compared their sample of mitochondrial DNA from the ancient skull with samples from 70 different modern breeds of dog, along with 30 different wolf and 4 different coyote DNA samples. Their analysis found that the fossil’s DNA didn’t match any of the other samples perfectly, but most closely resembled the modern dog breeds, sharing the most similarities with Tibetian Mastiffs, Newfoundlands and Siberian Huskies in particular.
Scientists know that dogs evolved as a result of the domestication of wolves, but the specific time and location of this domestication is still poorly understood—and this discovery further complicates that picture. Most experts agree that dogs predate the invention of agriculture (which happened roughly 10,000 years ago), but some say that domestication may have occurred as long as 100,000 years ago.
This finding—and the previous radiocarbon dating of the skull which established its age—set that event to at least 33,000 years ago. However, dogs may have been domesticated from wolves multiple times, and this breed of Siberian dog may have actually gone extinct, rather than serving as an ancestor for modern dogs. Archaeological evidence indicates that, with the onset of the last glacial maximum (around 26,000 years ago), humans in this area of Siberia may have stopped domesticating dogs, maybe due to food scarcity. In that case, an independent domestication elsewhere may have led to the dogs of today.
On the other hand, domestication in the vicinity of the Altai Mountains, as evidenced by this finding, may have led to the geographic spread of dogs elsewhere in Asia and Europe, even if they died out in Siberia. Previously, many have suggested that the first domestication occurred in the Middle East or East Asia, but this skull could force scientists to rethink their theories. The research team behind the analysis notes that finding more ancient dog remains will help us in putting together the puzzle.
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November 19, 2012
Stereotypically, people experiencing a mid-life crisis desperately seek to justify their lives through superficial means, perhaps by buying an expensive sports car or getting into a relationship with a younger romantic partner. Although their behavior looks rather different, a new study says that chimpanzees and orangutans go through a mid-life nadir in overall well-being and happiness that roughly resembles our own.
A team led by psychologist Alexander Weiss of the University of Edinburgh asked zookeepers and researchers around the world to keep track of the well-being of resident chimpanzees and orangutans—508 animals in total. The results of all that record-keeping, published today in the Proceedings of the National Academy of Sciences, show that, like humans, these great apes generally experience a U-shaped pattern of happiness and well-being, starting off with high ratings for happiness as adolescents, declining gradually during middle age (bottoming out in their late 20s or early 30s), and then rising back up again in their elder years.
Although popular conceptions of human mid-life crises focus on material acquisitions, psychologists believe they’re driven by an underlying decline in satisfaction and happiness as we go through middle age, and reflected by increased antidepressant use and suicide risk. In this sense, the primates studied went through a similar pattern:
Of course, unlike with humans, no one can directly ask chimps and orangutans how they are feeling. Instead, the researchers relied upon surveys, filled out by zookeepers and caretakers, that rated the animals’ mood and how much pleasure they took from certain situations. They acknowledge the ratings are necessarily subjective, but they feel that the size of the dataset and consistency in the trends as reported from the different zoos with different animals suggests that the pattern is legitimate.
Weiss’ group originally embarked on the ape study to answer the question of why mid-life dissatisfaction is so common in humans. “We hoped to understand a famous scientific puzzle: why does human happiness follow an approximate U-shape through life?” Weiss said in a statement.
Although many are apt to blame external cultural factors such as disappointing careers or mounting bills as the cause, Weiss felt it was something more fundamental. By showing that a similar pattern exists in other primates, he argues that his team has dispelled the notion that these types of external factors are solely responsible. “We ended up showing that it cannot be because of mortgages, marital breakup, mobile phones or any of the other paraphernalia of modern life,” he said. “Apes also have a pronounced midlife low, and they have none of those.”
Instead of these cultural factors, Weiss suggests that this pattern is rooted in biological or evolutionary factors. It might have been the case, for example, that the human ancestors who had an innate tendency for happiness and satisfaction at the stages of life when they were most vulnerable (youth and old adulthood) might have been less likely to venture into risky and potentially harmful situations in the pursuit of more resources.