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The long tooth of the narwhal: Size matters when competing for females Published 19.03.2020

A new study in the scientific journal Biology Letters shows that the narwhal with its long, spiralled tusk must be added to the list of animals that use sumptuous means when competing for females. The study has been carried out in a collaboration among Greenland Institute of Natural Resources, Arizona State University and Federal University of São Paulo.

The flamboyant feathers of the peacock, the huge deer antlers, and the powerful crayfish claws are a few examples of extraordinary characteristics in animals that are used in the competition for or seeking the attention of a partner, a process called sexual selection.

Zackary Graham, Arizona State University, is the main author behind the study. He is interested in understanding these traits by studying the morphology, i.e. the size and shape, of them. And here the narwhal and its long tusk is an obvious object of study. Mads Peter Heide-Jørgensen and Eva Garde from Greenland Institute of Natural Resources have supplied the data behind the study. “For more than three decades Greenland Institute of Natural Resources has collected data from the Greenlandic catch of narwhal in terms of samples and information on the whales, including body length, tooth, and tail flukes. “The amount of data and information from 245 narwhals used in this international study is unusually large”, says Eva Garde from Greenland Institute of Natural Resources.

“The long tusk of the narwhal grows out through the left side of the upper lip and can become almost 3 metres long. It is primarily males that develop the tusk, while females only very rarely have this tooth”, she continues. Through time there has been much speculation about the function of such an impractical tooth, and there have been many suggestions: a weapon in battles between males, a theory build from the many scars that male narwhals often have; a tool to catch fish; to stir up the sea floor; or to perforate the ice. Another theory has been that the tooth is used as a temperature and climate sensor. However, the most likely theory through all times has been that the tusk has been used for determining the hierarchy between males in their competition for females and/or for attracting females. Among other things, this theory is based on observations of a certain behaviour in narwhals “tusking”, where two narwhals cross and rub their teeth. This behaviour supports the theory that the tusk is used as a form of communication both among males and between male and female. And now this theory has been substantiated with the use of data from Greenlandic narwhals.

When comparing individuals of the same age, the traits that are sexually selected (selected as a gender-specific trait) often demonstrate an unusual growth. This means that for a certain body size, the sexually selected traits are often larger than expected in the largest individuals.  In this study, the growth (or the scale) of the tusk is compared with the scale between body size and a characteristic which is not, as far as we know, of any sexual function – namely the flukes.

“If the long tusk of the narwhal is sexually selected, then we would expect a greater variation in tooth length when compared with the tail flukes”, says Graham. This is because many sexual traits are related to body condition, so that only the largest and strongest individuals can afford spending a lot of energy on producing these enormous characteristics.

And this was exactly what Graham and his colleagues discovered; that the size of the tusk has a greater variation in relation to body size compared with the tail flukes, which precisely proves that the tooth is sexually selected. “When combining these results with already known properties of the tooth.” (e.g. how much force the tooth can withstand before breaking), we find that the tusk is a signal, which is sexually selected for the use in “tusking” fights among males”, says Graham. “It is very simple what the tooth tells the opponent, namely that: ‘I am bigger than you’.”

And if it is only males of the highest quality that produce the largest tusks, then the tooth in all likelihood sends an honest signal of the male’s quality to other males and females.

“By including valuable information from the Greenlandic catch of narwhal, we now know the function of the narwhal’s special tooth. In the coming years, we hope to learn even more about the narwhal’s behaviour, e.g. diving and foraging behaviour, as well as their social structures, using state-of-the-art technology in electronic tags and drones”, says Eva Garde.

 

For further information, please contact: Eva Garde. Email:  evga@mail.ghsdk.dk

 Phone: +45 6166 0460