We finally know why only some dolphins use sponges to hunt

When a sponge disturbs a biological sonar

Every time a Shark Bay dolphin picks up a sponge from the seabed, it alters, temporarily but profoundly, the acoustic signature of its own sonar. The sponge, fitted over the rostrum like a probing glove, is more than a simple foraging accessory. It acts as an acoustic filter, altering the emission and the reception of echolocation clicks, in particular at the lower jaw, the main area of sound reception in odontocetes.

The sponge: a prospecting tool and a source of interference

The species Echinodictyum mesenterinum, whose conical shape extends the rostrum, alters the properties of the sonar beam in a relatively linear way. At 0.8 metre from the point of emission, directivity is slightly sharpened, the side lobes are attenuated and the acoustic pressure level (RMS) is raised compared with foraging without a tool.

The effect on echo reception is modest but consistent: longer click duration, higher centre frequency, and a higher received level.

Ircinia sp., by contrast, flatter and geometrically less structuring, scatters the beam more. The RMS level drops by nearly 9 dB, the signal lengthens, and the centre frequency is significantly disturbed. The beam becomes irregular, with a marked increase in the side lobes, particularly where symbiotic barnacles are present.

FEM modelling: a window onto a distorted perception

Finite element modelling (COMSOL, vertical 2D) shows that the clicks pass twice through the material of the sponge: on emission, then on the return. The team used tomographic sections of sponges collected in situ and flash-frozen.

The key parameters measured are:

• The -10 dB duration of the click
• The received RMS level
• The centre frequency

For Echinodictyum, the beam stays coherent, even on the return path. For Ircinia, reception at the mandible is weakened, which may complicate the fine discrimination of a reflected signal.

Sensory costs, a long acquisition

The effect of these distortions on neural processing calls for prolonged learning. Unlike simpler feeding behaviours, sponging requires several years of practice to reach a performance comparable to that of expert individuals (at around 20 years of age). The process is transmitted vertically only, from mother to offspring, within specific matrilineal lineages.

The analogy is explicit: every new sponge, even one of the same species, acts as a new acoustic lens. The dolphin has to adjust its sensory processing continuously. Learning is therefore not merely the transfer of a motor technique, but a fine acclimatisation to a fluctuating acoustic environment.

Why do only a few dolphins take it up?

The Shark Bay population shows a unique coexistence: spongers and non-spongers use the same hunting areas. Yet despite this contact, no individual from outside a sponger matriline takes up the practice for good.

The causes are several:

• Sensory complexity increased by the attenuation and the distortion of the signals
• A long learning time incompatible with other behavioural priorities, in particular for males
• Exclusively maternal transmission, favoured by thousands of hours of passive exposure to the altered signals

Sponges and sensory ecology: selective choices

The study finds that nearly 70% of the sponges used are Echinodictyum, against only 20% Ircinia. This preference could be explained not only by abundance, but by the smaller acoustic impact of the former on echolocation.

The conical orientation of the tissue acts as a passive waveguide, concentrating sound energy instead of dispersing it. The barnacles, although present in the tissue, alter the signal only marginally, because their size is below the wavelength (1.5 cm at 100 kHz).

A tool that constrains the phenotype rather than extending it

Unlike other cases of animal tool use where the tool extends natural capacities, here the sponge deliberately limits a critical sense. The dolphin trades part of its sonar precision for greater efficiency in flushing out buried prey that lack a swim bladder, invisible to conventional sonar.

This trade-off gives access to a more lipid-rich trophic niche, but only on condition of extensive sensory training.

A foraging technique turned into a sensory inheritance

The rarity of sponging does not stem from a lack of interest in innovation, but from a high cognitive and sensory barrier. The individual has not only to handle a complex tool, but to reinterpret an altered signal continuously, with variable deviations on every dive.

Foraging here becomes a specialised multisensory skill, dependent on a long learning phase, on thousands of hours of exposure, and on an exclusive social bond between mother and calf. An acoustic know-how passed on like a cultural dialect, through the altered clicks of a sonar that whispers through the sponge.

Source of the study: https://doi.org/10.1098/rsos.241900