48. How can creatures without brains remember where things are?

You might well suppose that the ability to keep track of where things are depends on having a brain of some kind - or at least a nervous system

But you would be wrong. Not only can single-celled organisms find their way towards things they need (like food) and away from things they don’t like (such as too much heat), but some of them can also remember locations. This is of course a key navigational task.

One fascinating class of single-called critters - with the unappetising name of slime moulds - has long been known for its ability to build highly efficient transport networks. As I mentioned in Incredible Journeys/Supernavigators, these giant, unicellular organisms can even solve a problem that has taxed the brains of human engineers: how to create the most efficient rail network connecting a number of cities and towns of different sizes.

Until now it has not been entirely clear how slime moulds achieve these feats. But a new study reveals how these apparently simple organisms can ‘remember’ the things that most matter to them.

Slime moulds are in the business of building tubes for moving nutrients around (just as railroads are used for moving people or goods). It seems that they respond to the presence of food by expanding the diameter of the tubes they build. The richer the source, the bigger the tube leading to it. Information about nutrient locations is therefore encoded directly in the physical structure of the tube network. It is a kind of associative memory.

As the researchers put it, ‘nutrient location is stored in and retrieved from the networks’ tube diameter hierarchy’. Their findings help to explain ‘how network-forming organisms like slime molds and fungi thrive in complex environments’. Since slime moulds and fungi are now known to perform a wealth of vital tasks, this is an important discovery.
What’s more, it may prove helpful to human network designers.

44. Where the Wild Things Go: New Yorker article

The latest edition of The New Yorker magazine (5 April 2021) carries an excellent article by Kathryn Schultz. It’s a review of the science of animal navigation based on her careful reading of several recent books on the subject, including my own Supernavigators. As you might expect from a Pulitzer Prize-winning writer, it’s lucid and elegant - and well worth a read.

34. How cells can successfully navigate a microscopic Hampton Court maze... →

It’s long been known that cells can navigate towards chemicals that attract them by following concentration gradients. This process is known as chemotaxis.

But this simple form of navigation isn’t equal to the task of guiding cells over long distances (more than 1 mm) or along a tortuous, branched paths - which is exactly what we see, for example, when an embryo is developing, during an immune response or when a tumour is metastasising.

So how is any of that possible?

New research suggests that it depends on a process called ‘self-generated chemotaxis’.

The idea is that, rather than passively responding to the presence of a chemical attractant, some cells actively alter their environment by breaking the attractant down as they move along. The local depletion of the attractant results in diffusion of fresh attractant towards the cells from its source. This dynamic flux, rather than the absolute amount of attractant around the cells, seems to be the key to the whole process. In effect, the cells create their own local chemical gradients and use these as a basis for choosing their direction of travel.

Luke Tweedy and his colleagues set a tough pathfinding challenge for cells of an organism called Dictyostelium discoideum (known for its long-range navigational ability), as well as metastatic pancreatic cancer cells. They had to find their way through a variety of mazes, some of which were quite complex. One of them was actually based on the famous maze at Hampton Court Place.

Remarkably enough, the cells could efficiently find optimal routes, detect dead ends before they reached them, and could sense an approaching junction, even when it was around a corner!

Significantly, these experimental results were accurately predicted by a computer model - which means that they probably apply to any system in which an attractant is degraded by cells that are drawn towards it.

The authors believe that self-generated chemo-attractant gradients ‘could offer answers to many unexplained physiological behaviors’.