Tag Archives: Size

Food availability acts as a cap for whales’s maximum size

Whale size may be held in check by the availability of prey, a new study reports. While baleen whales have evolved to leverage size as an advantage while feeding — which put them on an “energetic knife’s edge” — toothed whales instead stand to benefit from being less massive.

Humpback whale and her calf.
Image credits National Marine Sanctuaries.

Growing to more than 100 tons, blue whales are considered to be the largest creatures to have ever roamed the Earth. Seeking to understand why baleen (filter-feeding) whales and toothed whales are so different in body size — and what factors limit their growth — a new study is looking into how much energy the two groups spend when feeding.

Large-scale nomming

“Blue whales and sperm whales are not just kind of big,” said Nicholas Pyenson, curator of fossil marine mammals at the Smithsonian’s National Museum of Natural History and the corresponding author of this study. “They are among the biggest animals ever to have evolved.”

“They rival and, in some cases, exceed the heaviest dinosaurs. That’s pretty spectacular. But why aren’t they bigger?”

Since whales spend most of their time deep underwater, it’s very hard for researchers to actually monitor what they’re doing. The current study, written by an international team of researchers led by Pyenson and Stanford University biologist Jeremy Goldbogen stuck multi-sensor arrays onto the backs of whales, porpoises, and dolphins of various sizes using suction cups.

The sensor arrays were used to track the animals’ underwater activities using accelerometers, pressure sensors, cameras, and hydrophones. Sonar sweeps in the surrounding waters, alongside older records of prey in whale stomachs, were used to estimate the density of prey in the vicinity of each tagged animal. Over 10,000 distinct feeding events were analyzed as part of the study, with the team calculating the energy cost and payoff for each.

“Energy is a key currency for all life, and we wanted to know how energy gain compares to energy use in foraging whales with different body sizes and feeding strategies,” Goldbogen said.

“The ratio of energy gain relative to energy use reveals a whale’s foraging efficiency and that provides clues as to why different whales are big and why they aren’t bigger.”

The net energy return on feeding, they found, largely depended on how each whale fed — i.e. by hunting individual prey (toothed whales) or by filter-feeding (baleen whales). Body size in all whales is limited by this net return of energy.

Less bang for your kill, more bang for your krill

Killer whale.
Image credits Ed Schipul / Flickr.

Filter-feeding whales are the only ones to have evolved a feeding strategy that favors larger body sizes — which turned them into the largest animals to have ever evolved on Earth. Size, they explain, plays right into the feeding style of baleen whales. They feed by straining patches of krill from ocean water — which aren’t particularly smart or able to move out of the way, so catching them is like shooting fish in a barrel. Having a larger body (and mouth) therefore means the whales can generate more calories for the same effort. In other words, their net energy return from feeding increases with the size of the whale.

Toothed whales, in contrast, need to hunt down individual prey, like a cheetah would on land. The whales use echolocation to spot a target and then hunt it down — so they are, in essence, limited to feeding on one target at a time. This hunting process requires a lot of energy, as the prey obviously would rather not be eaten, and does its best to escape. The larger a toothed whale becomes, the more energy it needs to give chase and catch prey. In other words, their feeding strategy favors smaller body sizes.

In some cases, the team reports, larger toothed specimens like sperm whales actually lost energy on some food dives; it simply takes more calories to dive and eat than they get out of what they ate. In effect, energy expenditure related to hunting acts as the hard cap for the size of toothed-whales. There aren’t enough large animals in the ocean for them to grow larger.

“They literally can’t eat enough to achieve a higher energetic payoff before they have to return to the surface and breathe,” Pyenson said.

“Being a sperm whale today is really pushing a serious biological limit.”

Filter-feeding whales, on the other hand, only seek out the densest patches of krill and almost always, the data showed, consume significantly more calories than they expend. The largest whales in this category saw the best energy return rates in the whole study. But they’re also limited in size by prey availability.

Krill population numbers explode but only for short periods of time every year in specific areas of the globe. This seasonal variation is what keeps baleen whale size in check.

Are whales the limit?

“The largest baleen whale species must reap the energy gains of krill patches in only a few of the most productive summer months at high latitudes,” Goldbogen said.

“Highly efficient filter-feeding strategies mean that these whales can build up fat stores that can then power their migrations across ocean basins to breeding grounds at lower latitudes that are leaner and provide much less food.”

While it may seem that filter-feeding whales have it nailed down, Pyenson explains that they’re actually playing a dangerous game here. They’re superbly adapted to take advantage of one resource. It’s an abundant resource, for sure, but if anything were to happen to the supplies of krill, filter-feeding whales have nothing to fall back on. Given their huge bulk, it’s hard to imagine any source of food in the ocean that could serve as a fall-back.

“You have to wonder just how perilous it is for whales living on an energetic knife’s edge,” Pyenson said, noting that climate change, overfishing, and other threats to the oceans are rapidly impacting their food suply.

“If you’re a blue whale and your only prey item is krill, and something causes krill populations to go into decline, then you are at an evolutionary dead end because you would not be able to eat enough to sustain yourself,” he said. “It’s a good reason for us to try to better understand these predator-prey relationships.”

This study identifies food sources as a limiting factor in whale size, but previous research has suggested that it may be biologically impossible for blue whales to grow any larger anyway due to mechanical constraints on their cardiocirculatory systems.

Beyond the immediate value of the findings, the team says it helps us better understand the dynamics between other huge beasts — particularly dinosaurs — and their food source, and how this limited their growth. “We don’t know how badly a herd of sauropod dinosaurs would chomp down on a forest in the Cretaceous period,” Pyenson adds, “but they probably did a number on it.”

The paper “Why whales are big but not bigger: Physiological drivers and ecological limits in the age of ocean giants” has been published in the journal Science.

Mushroom.

Your brain pays more attention to objects it knows are small — no matter how large they seem

The size of an object determines how much attention our brain is willing to allocate to it. However, it’s not how the perceived size of an object that counts, rather how large our brains know them to be from experience.

Mushroom.

Image via Pixabay.

Researchers from the George Washington University (GWU) say that object size is a key factor our brain takes into account when doling out attention. The findings, they say, could pave the way for special training to enable people to better notice certain objects — such as tumors on a radiology plate or hidden items in luggage.

Size does matter

“Since a person can only pay attention to a limited amount of information at a time, our brain uses object size to determine how much attention to allocate to that object,” says Sarah Shomstein, a professor of cognitive neuroscience at the GW Columbian College of Arts and Sciences and paper co-author.

“However, the way our eyes perceive an object can be different from its actual size, such as a car appearing large when it is close and small when it is far. Our study has shown for the first time that the brain adjusts attention based on our knowledge of an object’s size, not how our eyes view it.”

For the study, Dr. Shomstein and her team showed participants images of several everyday items (of various sizes in real life). These items, however — ranging from domino blocks to whole billiards tables — were shown at the same fixed size in all photographs. The team also added ‘probe targets’ in each image. What they wanted to see was how long it took participants to find these targets within each image.

Smaller real-world objects elicited a quicker response than larger ones across the board — even though they occupied the same amount of space in the participants’ eyes. Dr. Shomstein says that this happens because the participants’ previous knowledge of object size overruled their perceived size. Because of this, their brains automatically adjusted how much attention each item received (which, in turn, made it easier or more difficult to spot the targets).

“If objects are of identical size on your eye, but you know that one of them is smaller — such as a domino nearby versus a pool table far away — you allocate more attention to the smaller item,” she explains

The team then showed participants images of everyday items and asked them to rate their size on a scale of one to six (“one” being very small and “six” being very large). The results here showed there’s a direct correlation between how participants rated the size of each object and the time it took them to respond to target stimuli within the image.

“Your own personal ratings determine how efficient you are going to be at attending to that object,” says co-author Andrew J. Collegio. “If you think the pool table is really large, then your attention is going to be less focused.”

The team hopes these findings will help us better understand how people process particular objects as they pay attention to the world around them. In the long run, they add, these findings may ever point the way to new training avenues that would improve people’s ability to pay attention to certain items in different contexts.

The paper “Attention scales according to inferred real-world object size” has been published in the journal Nature Human Behaviour.

Whale and calf.

A single ‘letter’ difference in their DNA made some whales huge, others sleek and predatory

A single-letter difference in their genetic code dictates whether a whale species will be sleek and slim — or big and fat.

Whale and calf.

No disrespect intended.
Image credits David Mark.

Genes dictate everything about how our bodies look, behave, and live. Changes in a genome, then, will have an effect on what the animal encoded by those genes ends up being. Even the minutest of changes can have a really big impact — case in point, cetaceans.

Huge results

A paper led by Liyuan Zhao, a marine biologist from Ocean University in China and co-authored by Roger Cone, an obesity researcher at and director of the University of Michigan Life Sciences Institute, reports that the variation of a single amino-acid in whales led some species to evolve muscular bodies and prey on fish and seals, while other species grew to be the biggest mammals alive today, filter-feeding on immense volumes of krill.

Cone has spent the better part of his career studying the melanocortin system. This is a collection of central nervous circuits dictates how much energy a body stores as fat. In humans, mutations affecting this system are one of the most common genetic causes of early-onset obesity, and it functions similarly in all mammals and fish. While at the lab for a visiting fellowship, Zhao picked up on the idea; given its central role in maintaining energy balance, she wanted to see if variations in genes affecting the melanocortin system could explain the evolution of such different feeding behaviors and body sizes in the two main whale suborders.

Odontoceti, such as dolphins and killer whales, hunt their meals and the smallest members of the suborder usually grow to around 1.5 m (5 ft.) in length. In stark contrast, Mysticeti, such as humpback or blue whales, are filter-feeders which can grow well over 30.5 m (100 ft.).

Tiny causes

Working together with co-author Antonis Rokas, a Professor at the Vanderbilt University of Nashville’s Department of Biological Sciences, the two obtained DNA samples of 20 whale species from an existing repository at NOAA’s Southwest Fisheries Science Center in La Jolla, California. They were looking for the genes encoding the MCR4 neuropeptide receptor (a key receptor in the melanocortin system) and found one single difference, which perfectly correlated with one of the two groups:

Odontoceti (toothed whales) have the amino-acid arginine (A) in position 156 of their genetic code. Mysticeti (baleen whales) have glutamine in the same position of the genome. The team tied glutamine in this position to an increased sensitivity of the MCR4 receptor to the transmitter molecule that activates it.

“Our data suggest that the melanocortin system is more highly regulated in whales that hunt — and, conversely, that the giant filter feeders may receive reduced satiety signals from this system,” Cone explains.

“This difference could well have played some role in the divergence of these two major types of cetaceans — and may help explain the differences in feeding behavior and amazing range of body sizes among whales, which is far greater than in any other type of mammal.”

The team’s main interest is to take the results from “bench to bedside” and apply them to human health. Cone also joked that the research could go from “bench to barnside,” as the U.S. Department of Agriculture and the United States-Israel Binational Agricultural Research and Development Fund have funded the lab to apply their insight into feeding and growth of different species to improve feed efficiency in fish farms.

The paper “Functional variants of the melanocortin-4 receptor associated with the Odontoceti and Mysticeti suborders of cetaceans” has been published in the journal Scientific Reports.

Blue Whale.

Ancient climate change turned whales into Earth’s largest organisms ever , study reports

Ancient shifts in climate may have powered the baleen whale’s growth to such “ginormous” sizes, a new paper reports.

Blue Whale.

“Ginormous” seems rather fitting.

With some individuals growing to be the length of an average basketball court and weighing upwards of 200,000 kilograms (441,000 pounds), the blue whale is big fry indeed — in fact, they’re believed to be one of the largest animals that have ever lived on Earth. Which naturally begs the question of what led them, and their kin, to grow to such proportions.

[Turns out that a long time ago, a larger-than-whales dinosaur roamed the Earth. Why not read about it?]

Up to now, biologists have had (and debated over) two main theories in regards to why. The first one is that whales simply grew because they could, as water provides a lot of buoyancy for their bodies. So although they’d weigh a lot on dry land, way too much to be able to even move, they’re pretty nippy underwater and can still catch prey quite easily. The other theory is that the whales grew out of necessity, as their monumental size made them virtually immune to the attacks of giant sharks or other mega-predators.

Another point of interest is when they got so large. One paper published in 2010 under the lead of Graham Slater, an evolutionary biologist currently at the University of Chicago in Illinois, estimates that cetaceans (the whale’s extended family) split into size groups around 30 million years ago. It’s a lineage that still holds today, the paper argues — so the baleen whales trace their ancestry to the giant group, predatory whales (such as the beaked whale) hail from the middle-sized group, and dolphins from the runts of the litter, becoming the smallest of cetaceans.

Chubby cheeks

A new paper however could address both questions in one single swoop. Penned by Slater alongside Nicholas Pyenson, a whale expert at the Smithsonian Institution’s National Museum of Natural History in Washington D.C., and Jeremy Goldbogen at Stanford University in Palo Alto, California, the paper proposes that the whales’ size is a product of environmental stresses associated with global cooling in the Neogene some 4,5 million years ago.

The paper started taking shape a few years ago when Pyenson and Slater started working with the museum’s cetacean fossil collection to see if the diverging lineages theory holds water. Previously, Pyenson studied living whale populations to determine that a whale’s total size correlated well with the width of its cheekbones. So the duo gathered this numbers for 63 extinct whale species and 13 contemporary ones and plotted these values over the family’s timeline.

The trend showed that there weren’t any big whales early on, contradicting Slater’s earlier theory. There wasn’t any gradual increase in size over time, either — instead, what the team saw was that whales became moderately large and stayed so up until about 4.5 million years ago. After this, baleen whales suddenly grew “from relatively big to ginormous,” Slater says.

In case you’re not familiar with the ginormity scale, whales 4.5 million years ago clocked in at around 10 meters (about 32.5 feet) long — whereas today’s blue whales grow to around 30 meters (98.5 feet). So evolutionary speaking, the whales’ size is a pretty recent development.

Long road, big fins

Whale Fins.

“Laters haters!”

The next step was to look at the going-ons of the time to see what caused this very dramatic, 300% increase in size. The team found that the growth coincided with the beginning of the first ice ages. They explain that the colder climate lead to an increase in glacier cover which would melt during the warmer months of spring and summer, sending cold sediment (and nutrient) rich runoff into coastal waters which supported plankton and zooplankton (who like cold waters) blooms — which the whales were more than happy to dine on.

The problem was that until then, this krill was evenly distributed in the oceans and relatively plentiful, so the whales could go anywhere they pleased and dinner would be waiting for them. But climate change killed off most of the ocean biosphere at the time (ironic isn’t it) and severely weakened existing ecosystems, drastically lowering primary and secondary productivity (the rate at which plants turn sunlight into organic compounds, and the rate at which animals turn plant matter into their own biomass respectively).

Combined, this changed the pattern of food availability from “decent food pretty much anywhere” to “truckloads of food in far-apart areas at certain times during the year,” and the whales had to adapt. Goldbogen, who studies whale eating and diving behavior, helped explain the link between food availability and size. The more concentrated food becomes, larger whales with really big mouths gain a huge boost to feeding efficiency, he says. Moreover, larger whales could travel between feeding areas faster and with less effort than smaller ones.

Overall, these two factors put huge selective pressures on growing larger frames, so the bigger species thrived while smaller whales went extinct.

The paper, while not being the first to show how food and feeding habits shaped whale evolution, does offer a simple and pretty elegant explanation for the whales’ size. It also goes to show that evolution is powered by an interplay of factors, from climate to the way other species adapt to present conditions. And finally, it shows that a species’ adaptation to one particular constraint — in the whales’ case, food availability — can inadvertently address some of its other needs — such as safety from predators — or provide an unexpected boon to ecosystems.

The full paper “Independent evolution of baleen whale gigantism linked to Plio-Pleistocene ocean dynamics” has been published in the journal Proceedings of the Royal Society B.

Lion-tail Macaque eating a fruit.

Eating fruit may have given primates their big brains, paving the way for social structures

A New York University doctoral student has found evidence that would support an alternate explanation of why primates evolved such big brains — the secret isn’t social interaction, but a diet rich in fruit, she says.

Lion-tail Macaque eating a fruit.

Lion-tail Macaque eating a fruit.
Image credits Tambako The Jaguar / Flickr.

Primates don’t have big fangs, menacing claws, or imposing horns to fight off predators or take on pray. They rely on a more subtle, but much more powerful, ace up their sleeve. Namely, primates distinguish themselves from almost all other animals through their big brains. They keep primates safe, let them do all sorts of smart things like use tools or go to the Moon, and help them navigate the ever-busy social life with grace.

Why the long brain?

But it’s not clear why they have evolved these big brains in the first place. The prevailing theory today is the social brain hypothesis, according to which primates evolved a bigger brain because they don’t have claws or horns — they needed to bunch up and work together for safety, which required more processing power to understand all the subtleties of social interaction.

A new study comes to offer another, and somewhat unexpected, explanation to the big brain: fruit. Alex DeCasien, a doctoral student in biological anthropology at the New York University initially set out to see how the brains of monogamous primates stacked up to those of more promiscuous species. She and her team collected data on the social life and diets of over 140 species throughout all four primate groups — monkeys, apes, lorises, and lemurs — and crunched all the data to see which features were most likely to associate with bigger brains.

Her results show that it’s neither monogamy nor promiscuity that best predicts the size of a primate’s brain. Neither did other measures of social complexity, such as average group size. The best indicator of brain size, the team explains, is diet — specifically whether a specie’s diet is fruit or leaf-based.

Squirrel monkey eating fruit.

Rumor has it that eating fruit also correlates well with maniacal cackles and evil in small packages. Like this guy.
Image credits Tambako The Jaguar / Flickr.

But the paper has come under some fire from one of the original authors of the social brain theory. Robin Dunbar, an evolutionary psychologist at the University of Oxford in the United Kingdom, said that while more nutrients do allow for a bigger brain, they can’t become a selective evolutionary pressure by themselves — meaning that better food makes bigger brains possible but not necessary, so they aren’t needed.

He agrees that primates who primarily consume fruits can get a whole lot more energy from their diets than leaf eaters. The nutrients in leaves are locked behind thick cell walls that are hard to break down, so leaf-eating primates have to lie around for hours redirecting all their energy towards digestion. Fruits, on the other hand, are much more nutritious and easily digested, meaning they release more energy for less investment and a lot of that extra energy gets gobbled up by the brain.

“In order to have a bigger brain, you have to have a change in diet,” Dunbar said.

But comparing diet with social life is like “comparing apples and oranges,” he argued. His main gripe with DeCasien’s theory is that it doesn’t offer a reason for which all that energy goes towards a bigger brain and not bigger muscles, for example. The social theory does link those two together, he further explains. Living together helps keep the primates safe and gives easy access to mates, two very powerful selective pressures. But it also requires a bigger, more capable brain to navigate and keep track of increasingly complex social structures — so there’s also an evolutionary incentive to invest energy in bigger brains.

“[Diet and sociality] are not alternative explanations,” Dunbar concludes. “They are complementary explanations.”

DeCasien in turn argues that fruit-eaters need more cognitive oomph to get dinner than their counterparts. If you’re a prima.. If you’re a wild leaf-eating primate chances are there will be leaves all around you, so getting dinner should be as easy as stretching out a hand to get them. Fruit-eaters on the other hand have to remember where the best fruits can be found at different times of the year, so their brain needs to be able to handle a map and a calendar app at the same time. They also need good problem solving skills, or even the capacity to use tools, since fruits can be hard to reach or protected by wooden shells or spines.

Floss silk tree bark.

Go on. Take my fruit. I dare you.
Image credits Brisbane City Council / Flickr.

So it makes sense to assume that smarter primates (those with bigger brains) could get more fruit — creating an evolutionary pressure for bigger brains. This explanation could even go so far as to making social life irrelevant to brain size, even make it a by-product of fruit diets — with bigger brains, primates could now handle social interaction so they bunched up.

Still, DeCasien admits that the answer likely isn’t as clear cut. Diets may have initially promoted brain-growth, enabling social groups — and those groups in turn then drove further evolution.

“It’s definitely impossible to tease apart at some point,” she says.

Problem is, you can’t really pick apart evolutionary pressures and beneficial physiological changes in a study such as this — because again, correlation doesn’t imply causation. But seeing the huge impact cooked food has made for humanity, it’s obvious that diet has a role to play in evolution. Whether it’s the hand that points the way or one that simply shapes in the details will have to be answered by future research.

The paper “Primate brain size is predicted by diet but not sociality” has been published in the journal Nature Ecology & Evolution.