Penguin colonies resemble liquids, physicists find

5 Apr 2018

Penguins and physicists do not often cross paths, but a new physics study has found king penguin breeding colonies organise and move in a similar way to liquids.

“In this study, we aimed to uncover the structural order in a king penguin colony during the early stage of the breeding cycle, when most of the penguins are already incubating an egg”, said senior author Daniel P. Zitterbart from the Woods Hole Oceanographic Institution.

Lead author Richard Gerum, from the University of Erlangen-Nuernberg, said: “Structural order in animal colonies is well documented – for example flocks of starlings, schooling fish, or groups of insects. It helps with communication and navigation, protects against predators, or helps to hunt prey.

“Some groups of animals also display structural order in static groups. Bird colonies during breeding are particularly interesting among animal colonies that display structural order.

“King penguin colonies were of special interest because only they and emperor penguins do not build nests, and no-one has previously examined the effect this has on their colonies’ structural order.”

Using aerial photographs, the researchers recorded the spatial positions of several thousand individual animals and breeding pairs. They then analysed them with the radial distribution function – which helps describe the average atomic structure of molecular systems such as solids, liquids or gases – and ran a computer simulation to test their findings.

Their results, published today in the Journal of Physics D: Applied Physics, show that the structural order in king penguin colonies resembles a 2D liquid of particles with what is known as a Lennard-Jones type potential, where neutral molecules feel both attractive and repulsive forces based on their relative proximity and polarizability.

Mr Gerum explained: “This liquid order arises from an interplay between repulsive and attractive interactions between neighbouring breeding penguins. The interactions are a territorial pecking radius that leads to a repulsive potential, while space constraints and the need to protect against predators lead to an attractive potential, keeping the colony together.”

The researchers show that a colony with a liquid state is a compromise between density and flexibility. A gas-like state would offer a high degree of flexibility, meaning penguins could walk freely through the colony thanks to a low level of density, but this requires considerably more breeding space for the colony. By contrast, a solid state would give only a marginal gain in density compared to a liquid state, while making it virtually impossible to mend vacancies and local disturbances.

“Our data confirm that the observed colony structure provides sufficient flexibility to adapt to internal and external changes,” said Mr Gerum. “For example, a pair losing or abandoning their egg leaves a vacancy, but we never see vacant spots in our aerial images. We also frequently see elephant seals that pass through the colony and force the penguins to move, but these local disturbances appear to be healed quickly. This is possible because king penguins carry their egg on their feet and do not build nests.”