Sharks show us the way in travel dilemma

Today’s jet aircraft are 80% more fuel-efficient than those of the 1960’s, but aviation still accounts for 3.5% of human-induced climate change, writes Richard Collins

With demand for air travel steadily increasing, this figure may reach 5% by 2050. Cars can be powered by electricity generated from renewable sources, but not aircraft. It seems that we can’t travel by air without damaging the environment.

According to a paper just published, a similar problem faced the ancestors of sharks long ago. They had energy resource issues not unlike those facing air travellers today. The ancient fish, however, managed to evolve a two-tier solution to their problems which we might imitate today.

Physicist Jean Potvin, of Saint Louis University, joined a team of biologists investigating how sharks control their buoyancy. Examining the bodies of 32 species, the researchers found that some sharks have small livers while others have large fatty ones, accounting for up to a quarter of their total body-weight.

Sharks with small livers are built for speed; agile and streamlined like fighter planes, they hunt fast-moving fish and squid. With low buoyancy, these predators must keep moving constantly or they will sink to the bottom of the sea. Like aircraft wings, fins produce lift. Such sharks must catch enough prey to pay the energy bill for their expensive lifestyle.

“The evolution of marine fish from largely bottom-dwelling organisms to those capable of swimming at a range of depths represents one of the key evolutionary transitions,” say the researchers.

Liver tissue is relatively light; sharks with big livers have body densities close to that of the sea. With neutral buoyancy, they use little energy when swimming and don’t risk sinking. But there’s a downside to this laid-back approach. Sluggish creatures with big livers can’t catch fast- moving prey, nor are they well-equipped to escape from predators. These sharks, therefore, opted to frequent the cold dark depths of the sea, rather than pursue fish closer to the surface. There is little food where they live but, since the sharks’ energy demands are modest, this isn’t such a problem. Adrian Gleiss, lead-author of the paper, calls the big-livered species “the zeppelins of the shark world, cruising nearly effortlessly at low speeds to save energy”.

Ferdinand von Zeppelin’s early airships were filled with hydrogen, a highly inflammable gas. The high-profile demise of the Hindenburg, and the crash of the R101 on its maiden flight overseas, got his pioneering technology a bad name.

Now, almost 90 years later, things have moved on. With radar GPS and accurate weather forecasting, safety is no longer an issue. An airship captain can tell, well in advance, what may be ahead, plan his journey for maximum efficiency and avoid running into storms. Filled with helium, and using electric motors powered from renewable sources, airships can exploit the winds, the way migrating birds do. Loads could be carried safely with virtually no greenhouse-gas emissions, avoiding the need to run environmentally destructive roads and railways through remote regions. Not needing vast runways, dirigibles can be moored almost anywhere, even in cities.

There is at least a cargo-carrying role for huge ‘dirigibles’, delivering goods directly to their final destinations, with no need for surface transport. While time-challenged business passengers will continue using fixed-wing aircraft, paying through the nose for the environmental damage they inflict, more laid-back travellers, such as tourists, might travel leisurely in carbon-neutral airships. Is a two-tier approach, like the one pioneered by sharks long ago, the way forward for aviation?

• Adrian Gleiss et al. ‘Physical trade-offs shape the evolution of buoyancy control in sharks. Proceedings of the Royal Society B’. Nov, 2017.