Dolphin Head anatomy - why do spinner dolphins have such a long rostrum?
All animals that use echolocation to hunt will face the same problem: how do they track their prey just before they capture it. They send out a sound, generally a click-sound, that will travel to the target and bounce back as an echo that will tell the animal something about the target. Bats time their outgoing clicks to coincide with the return of the echo from the previous click. This way, they can compare the two sounds and determine what type of target the initial click hit. As a result, since we know the speed of sound in air (340 meters/sec), you can tell how far away the bat is looking, since it will be half the distance the sound traveled between clicks (distance traveled by the outgoing click to the target and the echo coming back after bouncing on the target).
Spinner dolphin off Kona, Hawaiʻi. Notice the long rostrum, or beak. Photo credit: Kula Naiʻa Foundation.
Bats, just like dolphins, will start echolocation searches with clicks fairly far apart, apparently searching for prey fairly far away. As they find a target and start to get closer, the click rate increases. Just before they catch the prey, the clicks are so close together that we humans hear it like a buzzing sound, where we cannot separate the individual clicks. This is a way for the bat to focus on its prey as it gets closer. One bat species, the insect eating soprano pipistrelle (Pipistrellus pygmaeus), produces clicks at rates of up to 200 clicks/second, corresponding to a distance to the target of 0.85 m (2.8 ft). This means that, to not lose track of the prey during the last, cruzial part of the approach, the bat will have to rely on some other sense for the last part of the approach, or be able to predict where the insect will be in the next few seconds.
Illustration showing how dolphins echolocate. They produce clicks with their phonic lips (like a Bronx Cheer). These sounds are focused by the melon into a beam, like a beam of light from a flashlight. The outgoing click then travels through the water to the target. Some of the sound is reflected back as an echo, which the dolphin picks up in its jaws, where a certain type of fat leads the sound to its inner ear. Illustration by Uko Gorter.
If dolphins use echolocation in the same way as bats, timing the outgoing clicks to coincide with receiving the echo from the previous click, they have a more extreme challenge than bats. The speed of sound is approximately 4.4 times faster in ocean water than in air (1500 m/s). So if dolphins behave like bats, they would need to have a 4.4 times faster click rate than bats, or about 1000 clicks/sec. This is not completely far-fetched, as harbor porpoises have been recorded producing click rates of up to 670 clicks/sec, corresponding to a target distance of 1.1 m. Bottlenose dolphins have been recorded to produce echolocation click rates of up to some 330 clicks/sec, corresponding to a prey item about 2 m away.
So how do they track their prey the last few meters before they catch it? For the species hunting during the day, we assume that they use their eyes for the last part. This was recently shown to be the case for bottlenose dolphins (see link below). However, what would animals like the spinner dolphin, hunting at night, do? Or, what about the 5 species of river dolphins (not part of the oceanic dolphin family), living in the turbid waters of the Ganges, Indus, Amazon, La Plata and Yangtze rivers. These dolphins have so little use for their eyes, other than to be able to detect light, that they have lost the lens and therefore the ability to form a picture and see their prey. So they would certainly not be able to use their eyes to catch their prey.
The main prey item for spinner dolphins, the lantern fish, got their name because they produce their own light in lots of little spots (photophores) on their bodies, also known as bioluminescence. They use this light as a sort of camouflage during the day, to make their silhouette less visible from below, but they may also use it for communication purposes at night. We don’t know whether spinner dolphins can detect and use this light to catch the fish, or if the fish can turn it off when dolphins are around. This would not be unprecedented, as there are several species of moths that have developed counter-measures to elude bats.
Lantern fish. Notice the many small photophores, emitting a bluish light. Photo from Tag my Fish.
Another possibility is that the long rostrum for which spinner dolphins got its scientific name (Stenella longirostris), is a way for the spinners to extend their mouth closer to the closest point that they can focus on with their echolocation. This is how the very long rostrum on the various river dolphins has been explained.
A spinner dolphin doing a ventral (belly) slap. Notice the long rostrum that gave this species its scientific name (Stenella longirostris). Photo credit: Kula Naiʻa Foundation.
The Yangtze River dolphin. A species of river dolphin that is now presumed to be extinct. As all river dolphin species it had a very long rostrum. Illustration courtesy of Uko Gorter.
So the shape and size of the various dolphin species is the result of millions of years of evolution. A large part of this can be explained by dolphins becoming more streamlined due to hydrodynamics. However, the remaining large variation in shape and size within the Oceanic Dolphin Family must be explained by other factors that affect these species differently. For example, the head shape of Pacific white-sided dolphins are very different from that of spinner dolphins, but they are also highly streamlined and well adapted to the marine environment.
Pacific white sided dolphins have a shorter rostrum than spinner dolphins and most of it is covered by the melon, giving them a very different silhouette than that of the spinner dolphin. Illustration courtesy of NOAA Fisheries.
The shape of the forehead and length of the rostrum is likely influenced by prey type, various feeding strategies and other factors associated with feeding. It is possible that species that cannot use their eyes during the last moments before they catch their prey developed a longer rostrum than those that can. This would put their mouth closer to the point where they can focus on the prey with echolocation. This could be the explanation for the spinner dolphin’s very long rostrum.