Tag Archives: Krill

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.

How blue whales use their memory to find food

Blue whales eat krill — a lot of it, as it’s not easy to power a basketball-court-sized animal weighing as much as 25 elephants. It is estimated to take 2,200 pounds (1 metric ton) of food to fill a blue whale’s stomach, and during peak consumption periods, they can devour up to 8,000 pounds (3.6 tons) in a single day.

Now scientists have found how they know where to get it.

Blue whale in the Arctic waters — probably looking for some krill to eat. Image credits: AWeith.

Government and university researchers have been examining records of both whale migration and oceanic conditions in the California Current Ecosystem — a coastal upwelling biome, one of the richest and most productive marine ecosystems. Researchers found that blue whales almost perfectly match the timing of their migration to the historical average timing of krill production, rather than matching the waves of krill availability in any given year. The study, which was published in the Proceedings of the National Academy of Sciences (PNAS), found that blue whales rely heavily on their memory in making these movement decisions.

The team used a decade’s worth of tagging data from the Marine Mammal Institute at Oregon State University to determine daily blue whale movements of 60 individual whales. They then compared that with satellite-based measurements of ocean productivity.

There are an estimated 10,000 blue whales in the world, and a quarter of them spend time in the waters off the west coast of the Americas. They travel from the Gulf of Alaska all the way down to an area near the equator known as the Costa Rica Dome. This range makes them quite vulnerable in the face of commercial ships, and it’s unclear exactly just how they came to use these migration routes.

“We think that blue whales have evolved to use historical migration routes and timing that put them in proximity to the most predictably high production feeding areas and then make minor adjustments based on local conditions,” said Daniel Palacios, a principal investigator with Oregon State’s Marine Mammal Institute and a co-author on the study.

“There are various time scales of events that could change the timing of phytoplankton blooms – and thus the availability of the whales’s preferred prey, krill”, he noted, “including La Nina and El Nino events and the Pacific Decadal Oscillation. But the whales’ strategy of relying on memory and historic timing at least gets them into the ‘Goldilocks zone'”.

The study is one of the first to determine how, precisely, ocean life tracks their food. Like whales, countless land species might migrate long distances for their meals. Unlike whales, those species adjust their courses to find the food. Whales, it seems, just go back to the same grocery they went to before.

“We know that many species that migrate on land, from caribou in the Arctic to wildebeests in the Serengeti, enhance their survival by carefully adjusting the pace and timing of their migrations to find food as it becomes seasonally available along the way, rather than just migrating to get from point A to point B,” said Briana Abrahms, a research ecologist with the NOAA Southwest Fisheries Science Center in Monterey, CA, and lead author on the study.

The blue whale is the largest known creature to ever inhabit the Earth but up until very recently, almost nothing was known about its mating and migration routes. Oregon State’s Bruce Mate changed all that, leading a series of tracking studies, but our understanding of blue whale ecology is still quite low. An extra incentive to study them now is climate change. Whales might find it difficult to adapt to ecosystem changes that result from it, said Abrahms.

“With climate change, we’re seeing deviations from those averages that are far outside of the normal ranges of variability,” she said. “It raises concern that the magnitude of change is happening far more quickly than what whales or other animals ever had to adapt to before. We still have a lot to learn about how large animals navigate in the ocean, how they find good habitat and how they are affected by human activities and environmental changes.”

Understanding how blue whales make movement decisions give scientists insight into how whales may, or may not, be able to cope with changing ocean conditions in the future.

Journal Reference: Abrams et al. Memory and resource tracking drive blue whale migrations. Proceedings of the National Academy of Sciences, 2019; 201819031 DOI: 10.1073/pnas.1819031116

Leopard seal.

Drones to offer faster, cheaper monitoring of Antarctica’s ecosystems

The health of Antarctic ecosystems could be monitored much cheaper and faster than today — we only need to replace everybody with drones.

Leopard seal.

Image credits nomis-simon / Flickr.

Researchers working on monitoring the condition of leopard seals have demonstrated a cheaper, faster way to do their jobs. Instead of spending hours to pursue, catch, immobilize, and then measure the animals, researchers can now recover all the necessary data from drone photography. A pleasing prospect, considering they ply their trade in the hellishly cold backdrop of the Antarctic Peninsula.

Peek-a-seal

Leopard seals (Hydrurga leptonyx), which can grow to nearly half a ton, are an important reference species for the overall health of the Antarctic ecosystem. They’re at the top of the local food chain, preying on penguins and Antarctic fur seals. In turn, these rely overwhelmingly on Antarctic krill — small, shrimp-like crustaceans. Krill, however, is an important resource for us human as well, as they’re a key ingredient in nutritional supplements, aquaculture feed, and other industries. Because of this, leopard seals, penguins, and fur seals are used as indicators, helping researchers gauge the health of Antarctic krill populations — estimations that are then used to establish how much krill fishing ships can take out of the area.

Scientists from NOAA Fisheries’ Southwest Fisheries Science Center usually have to go out and brave the cold to estimate the body conditions of these species. Now, however, with help from Aerial Imaging Solutions, they might be able to wait for the data enjoying the comfort of a warm room and a hot cup of cocoa.

To test how accurate the drones’ measurements are, researchers from the AERD sent the drones out, then caught and measured the same animals. It took five people over four hours to capture the 15 leopard seals photographed by the drones (which only needed about 20 minutes and a crew of two people). Still, the effort paid off — the team found that length measurements were accurate to within about 2%, and the weight measurements within about 4%.

Even better, the seals don’t seem to mind the drones at all. They showed no reaction to the robot as long as it stayed above 23 meters (75 feet).

“We’re certainly excited because we can get that much more work done, in less time, and at lower costs than ever before,” said Douglas Krause, a research scientist in the Southwest Fisheries Science Center’s Antarctic Ecosystem Research Division (AERD) and lead author of the paper demonstrating the new research method.

“Catching a single seal can take hours, but the drone can photograph every seal on a beach in a few minutes.”

It’s that last element, in particular, that could make the drones better suited to tracking the seals than humans. The photographs they provide could actually improve the accuracy and depth of data over the longer term because the drones can survey far more leopard seals in the same amount of time than human teams. Researchers will feed a single photograph of each animal through the computer software that calculates their weight and size, to minimize errors.

The drones’ first assignment will be to track changes in the leopard seals’ weight throughout summer. The data will show how much food the animals are getting, and will be used to break the population down by age intervals to better understand the overall health of the population.

Seal-watch drone.

A seal-watcher drone and station.
Image credits Douglas Krause et al., 2017, PLOS ONE.

“We’re always looking for more efficient ways to collect data that informs decisions on how to manage these important resources,” said George Watters, director of the AERD. “The better we understand the ecosystem, the better we can ensure it’s protected for the long term.”

The paper “An accurate and adaptable photogrammetric approach for estimating the mass and body condition of pinnipeds using an unmanned aerial system” has been published in the journal PLOS ONE.

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.