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. 2020 Mar;16(3):20190950.
doi: 10.1098/rsbl.2019.0950. Epub 2020 Mar 18.

The longer the better: evidence that narwhal tusks are sexually selected

Zackary A Graham  1 , Eva Garde  2 , Mads Peter Heide-Jørgensen  2 , Alexandre V Palaoro  3
Affiliations

The longer the better: evidence that narwhal tusks are sexually selected

Zackary A Graham et al. Biol Lett. 2020 Mar.

Abstract

Once thought to be the magical horn of a unicorn, narwhal tusks are one of the most charismatic structures in biology. Despite years of speculation, little is known about the tusk's function, because narwhals spend most of their lives hidden underneath the Arctic ice. Some hypotheses propose that the tusk has sexual functions as a weapon or as a signal. By contrast, other hypotheses propose that the tusk functions as an environmental sensor. Since assessing the tusks function in nature is difficult, we can use the morphological relationships of tusk size with body size to understand this mysterious trait. To do so, we collected morphology data on 245 adult male narwhals over the course of 35 years. Based on the disproportional growth and large variation in tusk length we found, we provide the best evidence to date that narwhal tusks are indeed sexually selected. By combining our results on tusk scaling with known material properties of the tusk, we suggest that the narwhal tusk is a sexually selected signal that is used during male-male contests.

Keywords: allometry; animal signals; animal weapons; exaggerated trait; sexual selection.

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Conflict of interest statement

We declare we have no competing interests.

Figures

Figure 1.
Figure 1.
This narwhal (Monodon monoceros) behaviour, called tusking, has been recorded between two males with a female present, and with two or more males with no female present. Visual observations and photographs of this behaviour were used to recreate this image.
Figure 2.
Figure 2.
(a) The scaling relationship between narwhal (Monodon monoceros) body size and tusk length (solid red) demonstrates the steep scaling of the tusk (y = −2036.5 + 9.06 × x − 0.009 × x2). When we removed three outliers that likely drive the downward slope of our data, an asymptote of tusk allocation appears (dashed red; y = −1640 + 7.26 × x − 0.007 × x2). Fluke width (grey), on the other hand, demonstrates the shallow scaling of a non-sexually selected trait (y = 3.689 + 0.23 × x). (b,c) Coefficients of variation and residual variation for tusk length (red) and fluke width (grey) demonstrates considerable variation in tusk length, but not fluke width, consistent with the hypothesis that the tusk is sexually selected.
See this image and copyright information in PMC

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