A coiled horn, up to three meters long – what is this bizarre hallmark of the narwhal? There has been much speculation about its function. Now a study confirms that the strange structure of the male animals was created through sexual selection. It therefore serves to impress people in the search for a partner: those with the longest horn prevail over competitors and win most females.
In the Middle Ages, people actually believed that the horns traded in Europe came from mysterious horse creatures. However, the “real unicorns” have little in common with these mythical animals: the horn, the male narwhal (Monodon monoceros), it is a tortuous tooth that breaks through the upper lip of the marine mammals and grows far beyond them: the bizarre tusk can make up over a third of the total length of a narwhal. One might think that such an outstanding structure also has an obvious function. But that is not the case. Biologists have long been wondering about the purpose of the horn. The problem is that narwhals are almost as mysterious as their mythical counterparts: Because they live hidden beneath the surface of the sea in the lonely expanses of the Arctic seas, little is known about the behavior of these whimsical representatives of the toothed whales.
Since, with a few exceptions, only males wear a horn, a function in connection with the search for a partner or the competition for females was obvious. However, it also seemed possible that the tooth could play a role in specific food behavior or strategies that may differ between genders. It could serve, for example, to break through the ice sheet, to rummage through the seabed or as a sensitive instrument for orientation – according to some of the previous speculations.
A mysterious entity
A team of researchers led by evolutionary biologist Zackary Graham from Arizona State University has once again devoted a study to the importance of the tusk. They went on the assumption that the tusk has a similar meaning for the whales as the antlers of the deer or the plumage of the peacock. These are characteristics that are subject to sexual selection: Because, for example, a particularly magnificent peacock wheel indicates the biological fitness of its wearer, female peafowl prefer partners with lavish feather headdresses – and this in turn promotes the passing on of this characteristic to the next generation. Charles Darwin already defined this “gender selection” as a possible background for the development of gender-specific traits in living beings. “Sexual selection is responsible for creating some of the craziest traits in biology,” says Graham.
As part of the study, he and his colleagues chose an unusual-looking approach to verify that the narwhal’s quirky tusk was one of those. Earlier studies and Graham’s own studies show that sexual selection is reflected in a characteristic context: when comparing peers, sexually selected characteristics show disproportionate growth. This means that they are often larger than expected on average for a certain body size. Against this background, the scientists have now evaluated the dimensions of 245 adult male narwhals.
Signature of sexual selection
They chose the tail fins of the animals as the comparative body part, which is hardly subject to sexual selection. “With sexual selection of the narwhal tusk, a greater variation in the tusk length compared to the variation in the fluken width can be expected,” explains Graham. And that’s exactly what the researchers found in their data analysis: Only in the tusk length is there an enormous range of variation in the males of the same height. In the specimens examined, it was between 0.5 and 2.5 meters. In contrast, the width of the caudal fin differed only comparatively little in animals of the same size. As the researchers explain, in the case of the tusk, this clearly shows the pattern of a characteristic that is subject to sexual selection.
This result fits with further information, which they report on in their publication. There are isolated observations of narwhal male who seem to “measure themselves”. In doing so, they cross their tusks and rub them against each other, which suggests that they have a role in clarifying the order. The narwhal man with the more impressive tusk may prevail. This characteristic influences their reproductive success and is therefore selected positively, the explanation. The researchers also report scarring in male narwhals. They suggest that these interactions sometimes take more brutal forms.
The narwhal remains mysterious
Despite the convincing evidence of the function in the context of reproductive behavior, questions remain open, the researchers concede. It remains unclear to what extent the females are impressed by the size of the tusk when choosing a partner. It is also possible that the narwhal male can also use the structure for other purposes – for example when looking for food. Graham and his colleagues now hope that closer observations of the animals can clarify in future what functions the hallmark of the unicorns of the sea has.
Source: Arizona State University, technical article: Royal Society Biology Letters, doi: 10.1098 / rsbl.2019.0950