The need for marine animals to navigate dark waters has given rise to biological sensing mechanisms that go well beyond the sense of sight. Marine creatures have evolved an array of such mechanisms, using complex fluid mechanics to track prey and to avoid becoming a meal themselves.

Perhaps the most famous example of this is echolocation used by dolphins, which is based on the emission and reception of sound waves to map their surroundings and has inspired sound navigation ranging (SONAR) systems.

Less well-known and less well-understood than echolocation — though no less impressive — is the system of intricate whiskers used by seals to sense trails left by fish. Research in the past has shown that using their whiskers, some seals can detect prey as far away as 180 meters, a distance almost twice as long as a football field. 

Bead-like structures lend whiskers their capabilities

A new paper published in the journal Advanced Science by Ajay Giri Prakash Kottapalli, Amar Kamat, and Ming Cao from the University of Groningen, as well as Xingwen Zheng from the University of Tokyo, aims to understand how seals are able to track fish with such remarkable accuracy, and what role their intricately-shaped whiskers play in this. In particular, the team wanted to look at the structure of whiskers unique to certain species of seals. 

“Most mammals, including rats, cats, otters, and pinnipeds  —  seals, sea lions, and walruses  —  have arrays of whiskers on their muzzle that serve as mechanosensors, known to be capable of sensing and interpreting flow or tactile information, thus creating situational awareness of the surrounding environment,” said Kottapalli in an email. “Whiskers of some Phocid seal species, such as grey seals and harbor seals, feature unique, undulating surface structures on these whiskers that resemble beads on a string.”

Kottapalli describes these beaded whiskers as possessing regular, repeating wavy patterns across their length — undulations that are not seen on the whiskers of all seal species. 

“Because of these wavy, undulating structures, the seal whiskers do not vibrate significantly as the seal swims forward during its hunting behavior, thus enhancing the whiskers’ sensitivity to minute flow disturbances, such as those generated by an escaping prey,” said Kottapalli.

“In this manner, the undulating whiskers attain a high signal-to-noise ratio and remain sensitive to biologically relevant signals, such as the vortex wake of an escaping fish,” he added.

Building models

To test the seals’ whiskers, Kamat built digital models of harbor and grey seal whiskers using 3D scanning, while Zheng developed a mathematical framework to accurately recreate their undulating geometry. 

This allowed the scientists to conduct fluid–structure interaction studies and experimental investigations. Thus, the authors were able to explain the vibration characteristics of the whisker array and the interaction between neighboring whiskers in steady flows and fish-wake-like vortices for the first time.

“Results reveal that the downstream vortices’ intensity and resulting vortex-induced vibrations are consistently lower for grey than harbor seal whiskers,” Kottapalli explained. “In addition, neighboring whiskers in an array influence one another by resulting in greater flow vorticity fluctuation and distribution area, thus causing increased vibrations than produced by an isolated whisker, which indicates the possibility of a signal-strengthening effect in whisker arrays.”

What can we learn?

As well as granting insight into how these fascinating creatures hunt, the team’s results suggest designs based on the geometry of the grey seal whiskers could be an ideal template for the construction of vortex-induced, vibration-resistant underwater structures. The increased interest in worldwide deep-water petroleum production has, for example, created the need for slender vortex-induced vibration-resistant marine structures, like risers, making this a hot research area in ocean engineering.

“This raises interesting questions not only from a technological point of view, such as the designing and fabricating of biologically inspired sensors, but also from the point of view of fundamental science related to sensing principles, material properties, fluid mechanics, and execution strategies such as schooling behavior that allow an animal to realize 3D hydrodynamic vision with consistency and high accuracy,” Kottapalli said.

The researcher added that future research into the whiskers will use the device built by the team to investigate how the spatial distribution of the whiskers on the seal muzzles allows multi-point flow measurements while locating prey.

In the future, the team aims to demonstrate biomimetic whisker sensor aided control of underwater robots developed by Cao. “The end goal,” Kottapalli concluded, “is to develop biomimetic flow sensors that can aid underwater robot navigation.”

Reference: X. Zheng, Ajay Giri Prakash Kottapalli, et al., Wavy Whiskers in Wakes: Explaining the Trail-Tracking Capabilities of Whisker Arrays on Seal Muzzles, Advanced Science, (2023). DOI: 10.1002/advs.202203062