Propeller-free robots may soon be swimming with style. A new model of how fish and other aquatic species are able to propel themselves forward without expending much energy may help create energy-efficient underwater robots that swim just like the real thing.

By closely studying and monitoring how fish, dolphins and other sea creatures swim, at Harvard University and his colleagues found that many of them have remarkably similar styles that can be described with a simple model, depending on how fast and far the tail whips back and forth and the length of the animal.

��è����Ƶs previously homed in on just one parameter that relates the beat of an oscillating fish’s tail and how far it oscillates to the animal’s forward motion. But it turned out to be slightly more complicated.

Saadat and his colleagues identified a second important factor: how far the tail goes to and fro relative to the fish’s length. They found that almost all fish – and many other sea creatures – swim in a narrow optimum range of this parameter to generate thrust, with the length of each tailbeat between one and three-tenths the length of the animal.

One for all

“This gives you the right ballpark. Even for different fish shapes and gaits, if you’re in there, you are cruising at a minimum power input,” says at the University of Illinois at Urbana-Champaign.

The researchers developed the model’s parameters by carefully monitoring trout cruising in a tank in their lab. In the tank, water flows in a circle to allow the animal to remain in one place while swimming upstream, like a fish treadmill. The team also tested a robotic system emulating a fish tail to make measurements of how much power it used as it swam.

In addition, the researchers used previously published data on a variety of other sea life, including mackerel, blacktip sharks, dolphins and killer whales, observing that they consistently swim in line with the parameters they found in trout in the lab.

“It’s amazing that so many differently shaped and different kinds of aquatic organisms fit the same general rules,” says at Harvard University.

Saadat believes these rules could be used to aid the design of power-efficient aquatic robots. Such robots could, for example, explore the sea floor or map coral reefs without disturbing wildlife, and the US Navy is interested in stealthy robots that move without a propeller.

Read more: Dolphins swim so fast it hurts

How close is the next big one?

Devil in detail

Little galaxies can pack a mighty punch. Astronomers detected and carefully weighed the black holes residing at the centres of a pair of extremely compact galaxies, finding to their surprise that they make up a large fraction of the mass of the host galaxies themselves.

These “ultra-compact dwarf galaxies” are now the second and third ones known to house such gargantuan black holes, confirming that the first one — discovered by the same group in 2014 — was not an anomaly.

“We’re three for three so far,” says at the University of Utah in Salt Lake City. He and his colleagues argue that these uniquely dense galaxies were once much larger galaxies, but have been tidally stretched and disrupted by neighbouring galaxies in their crowded environment.

The two newly studied galaxies reside in the Virgo cluster, which is a dangerous neighborhood. Thousands of other galaxies live there too, which means there are many opportunities for them to whiz close by and strip off each other’s stars.

No ordinary dwarfs

Since big black holes tend to reside at the cores of big galaxies, the huge masses of these two compact galaxies’ black holes — about 4 to 6 million times as massive as our sun — are the strongest indication that the dwarf galaxies are not traditional dwarfs and the black holes are not overweight.

Instead, these objects are what’s left of those galactic cores, only a few per cent of the galaxies’ original mass. That’s why the black holes account for one sixth of the dwarf galaxies’ mass, rather than a more typical one thousandth.

Seth and his team estimated the masses with a technique known as adaptive optics, which offsets the blurring effects of the atmosphere and lets them make precise measurements of the motions of the galaxies’ stars. Modelling those motions then lets them infer the central black holes’ masses.

“They are the first to provide compelling observational evidence for these objects being the remnants of bigger galaxies,” says at the University of Michigan in Ann Arbor. If all such compact dwarf galaxies are stripped, she argues, then their black holes may also be unexpectedly massive – meaning these behemoths are much more abundant in the universe than previously thought.

Stellar DNA

Galactic tree

Bottlenose dolphins that engage in synchronised swimming with their peers tend to see the glass as being half full.

Some of these dolphins frequently swim in tight-knit groups, and they’re the ones who appear the most optimistic, according to a study of eight captive animals.

In the experiment, individual dolphins were trained to swim towards one of two targets. They were taught that when they reach the left one, they receive applause and eye contact, while the one on the right delivers herring – the jackpot – and dolphins swim faster towards it.

When presented with a new and ambiguous middle target, some dolphins still swim rather fast, presumably hoping they’ll receive another tasty herring, although it’s only a 50/50 chance.

Those were dubbed the “optimistic” dolphins, and the analysis found that they were the same animals who had participated in the most synchronised swimming recently: moving closely alongside their fellow dolphins and matching their movements.

Social swimming

Dolphins continue to make these optimistic judgements up to two months after frequent synchronised swimming with their friends, but the boost fades after that.

Swimming together is an important social activity for dolphins that increases bonding between them, and the researchers argue that it could be linked to positive emotions.

“I think it’s the social behaviour that drives the dolphins’ optimistic decisions,” says , a zoologist at the University of Paris-North and lead author of the study. Social interactions are thought to be rewarding and associated with positive views, and she believes her study of synchronised swimming confirms that.

“We know that dolphins in the wild and in captivity tend to use synchronous swimming as a form of bonding, such as between pairs of adult dolphins or mothers and calves,” says , a psychologist at the University of Hawaii at Hilo. “People interpret these behaviours as reinforcing close relationships.”
The dolphins’ optimistic behaviour resembles “cognitive bias”: how humans judge situations differently depending on their social environment. People’s social activity affects their outlook on the world, and something similar may happen among some animals, too.

Kinder zoos

Cognitive bias has been studied with laboratory rats, for example, by comparing those residing in enriched and barren cages. The dolphin study says it is the first test of cognitive bias in a marine mammal or a zoo-housed species.

“This study could be used as a tool to probe aspects of animal welfare in captive environments,” says , a behavioural biologist at Emory University in Atlanta, Georgia. Zoo animals, for instance, don’t choose many aspects of their lives, but they can benefit from opportunities for social activities with their peers, she says.

Clegg agrees. Zookeepers and aquarists could use this to monitor how many dolphins often swim together, and manage their practices accordingly. “In better welfare situations, animals judge [things] more optimistically,” she says.

Read more: Two dolphin species band together to form unprecedented alliance; Don’t worry, bee happy: Bees found to have emotions and moods

Few survive

OUR galaxy is past middle age and could be gradually dying. A team of stellar archaeologists have found the first evidence that the Milky Way suddenly stopped giving birth to stars after it formed a thick saucer-like disc around 8 billion years ago, suggesting such “quenching” can happen even before a galaxy runs out of gas.

Galactic life cycles were thought to be driven largely by how much gas a galaxy has to build new stars with. But it was unclear whether most galaxies lose their raw material abruptly when it is ejected by supernovae or a central black hole, burn through their reserves slowly, or stop growing for some other reason.

at the Paris Observatory and his colleagues decided that studying our own galaxy is the best way to address this question. They probed the chemical signatures of tens of thousands of stars – a clue to their ages – recorded by a high-resolution spectrograph at the telescope in New Mexico. With the data, they were able to reconstruct a record of the Milky Way’s past.

They found that our galaxy’s star formation rate dropped by an order of magnitude between 10 billion and 8 billion years ago. It resumed forming stars after this sudden die-off, but at a much slower rate.

This epoch was also when our galaxy formed its bulging disc and bar-like concentration of stars at its centre. “There seems to be a connection between quenching in the Milky Way and its thick disc,” Haywood says.

That means a galaxy can stop growing even while it has reservoirs of gas, as the Milky Way does. Haywood argues that the disc and bar structures could disrupt growth by stirring up the gas, making it too hot to form new stars (). Other spiral galaxies too distant to be probed by these methods could be ageing in a similar fashion.

“It was unclear whether most galaxies lose their fuel abruptly or burn through reserves slowly“

“Star formation boils down to a battle between gravity and other things, like turbulence,” says at the Carnegie Observatories in Pasadena, California. Our galaxy’s disc and bar cause the gas to become turbulent, injecting energy that prevents it from collapsing and initiating star formation, she says. “Their results with the Milky Way give us subtle clues about what we should be looking for in other evolving spiral galaxies, too.”

This article appeared in print under the headline “Milky Way retired early from star-making”