Sensory ability in the narwhal tooth organ system

Nweeia, Martin; Eichmiller, Frederick C.; Hauschka, Peter V.; Donahue, Gretchen A.; Orr, Jack; Ferguson, Steven H.; Watt, Cortney A.; Mead, James G.; Potter, Charles W.; Dietz, Runé; Giuseppetti, Anthony A.; Black, Sandie R.; Trachtenberg, Alexander J.; Kuo, Winston Patrick

doi:10.1002/ar.22886

Cited by 39 publications

(22 citation statements)

References 116 publications

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“…Additionally, the same claws play a role in chemical communication and prey capture [55,56]. In narwhals, the tusk might also possess other functions related to sensing the environment and in rare cases, catching prey [1]. Thus, although we have supported the hypothesis that the tusk is sexually selected, large tusks may benefit narwhals in non-sexual acts that we are currently unaware of.…”

Section: Discussionmentioning

confidence: 56%

“…Despite immense speculation, the function of the tusk remains unclear. Based on the tusk's anatomy, a recent study proposed that narwhal tusks sense chemical changes in their environment [1]. By contrast, reports of head scarring, broken tusks and tusks impaled in the sides of male narwhals suggest that males use their tusks as weapons during aggression [2][3][4].…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2020

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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.

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Section: Discussionmentioning

confidence: 56%

Section: Introductionmentioning

confidence: 99%

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

et al. 2020

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“…For instance, male narwhals present flukes that are different in shape to those of the females or to those of other toothed whales, and are thought to compensate for the increased drag imposed by the elongated tusk [Fontanella et al, , but see Ralls & Mesnick, for other cetacean dimorphic flukes], and females seem to be able to swim faster than males, which is important for their foraging dives (Laidre et al, ). We consider the narwhal's tusk to be a unique ISW (Gerson & Hickie, ; but see Nweeia et al, ) because this species has only vestigial teeth besides the males' enlarged canines. Comparative studies are needed to understand and quantify the costs (or lack thereof e.g.…”

Section: Resultsmentioning

confidence: 99%

Intrasexually selected weapons

2018

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We propose a practical concept that distinguishes the particular kind of weaponry that has evolved to be used in combat between individuals of the same species and sex, which we term intrasexually selected weapons (ISWs). We present a treatise of ISWs in nature, aiming to understand their distinction and evolution from other secondary sex traits, including from 'sexually selected weapons', and from sexually dimorphic and monomorphic weaponry. We focus on the subset of secondary sex traits that are the result of same-sex combat, defined here as ISWs, provide not previously reported evolutionary patterns, and offer hypotheses to answer questions such as: why have only some species evolved weapons to fight for the opposite sex or breeding resources? We examined traits that seem to have evolved as ISWs in the entire animal phylogeny, restricting the classification of ISW to traits that are only present or enlarged in adults of one of the sexes, and are used as weapons during intrasexual fights. Because of the absence of behavioural data and, in many cases, lack of sexually discriminated series from juveniles to adults, we exclude the fossil record from this review. We merge morphological, ontogenetic, and behavioural information, and for the first time thoroughly review the tree of life to identify separate evolution of ISWs. We found that ISWs are only found in bilateral animals, appearing independently in nematodes, various groups of arthropods, and vertebrates. Our review sets a reference point to explore other taxa that we identify with potential ISWs for which behavioural or morphological studies are warranted. We establish that most ISWs come in pairs, are located in or near the head, are endo- or exoskeletal modifications, are overdeveloped structures compared with those found in females, are modified feeding structures and/or locomotor appendages, are most common in terrestrial taxa, are frequently used to guard females, territories, or both, and are also used in signalling displays to deter rivals and/or attract females. We also found that most taxa lack ISWs, that females of only a few species possess better-developed weapons than males, that the cases of independent evolution of ISWs are not evenly distributed across the phylogeny, and that animals possessing the most developed ISWs have non-hunting habits (e.g. herbivores) or are faunivores that prey on very small prey relative to their body size (e.g. insectivores). Bringing together perspectives from studies on a variety of taxa, we conceptualize that there are five ways in which a sexually dimorphic trait, apart from the primary sex traits, can be fixed: sexual selection, fecundity selection, parental role division, differential niche occupation between the sexes, and interference competition. We discuss these trends and the factors involved in the evolution of intrasexually selected weaponry in nature.

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“…The tusk of M. monoceros, although clearly a secondary sexual characteristic, has been found to be highly sensitive to changes in salinity and water temperature, and may therefore be used as an additional sensory apparatus (Nweeia et al. ). Dissections showed a direct connection with the maxillary division of the trigeminal nerve (CNV) (Nweeia et al.…”

Section: Discussionmentioning

confidence: 99%

“…Dissections showed a direct connection with the maxillary division of the trigeminal nerve (CNV) (Nweeia et al. ). Correspondingly, larger trigeminal nuclei or maxillary portion of the trigeminal nerves may be expected on the endocasts of male M. monoceros or other individuals possessing tusks, as has been found in other mammals, such as the duck‐billed platypus and its extinct relatives (Macrini et al.…”

Section: Discussionmentioning

confidence: 99%

Neuroanatomy and inner ear labyrinths of the narwhal, Monodon monoceros, and beluga, Delphinapterus leucas (Cetacea: Monodontidae)

2018

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Narwhals (Monodon monoceros) and belugas (Delphinapterus leucas) are the only extant members of the Monodontidae, and are charismatic Arctic-endemic cetaceans that are at risk from global change. Investigating the anatomy and sensory apparatuses of these animals is essential to understanding their ecology and evolution, and informs efforts for their conservation. Here, we use X-ray CT scans to compare aspects of the endocranial and inner ear labyrinth anatomy of extant monodontids and use the overall morphology to draw larger inferences about the relationship between morphology and ecology. We show that differences in the shape of the brain, vasculature, and neural canals of both species may relate to differences in diving and other behaviors. The cochleae are similar in morphology in the two species, signifying similar hearing ranges and a close evolutionary relationship. Lastly, we compare two different methods for calculating 90var - a calculation independent of body size that is increasingly being used as a proxy for habitat preference. We show that a 'direct' angular measurement method shows significant differences between Arctic and other habitat preferences, but angle measurements based on planes through the semicircular canals do not, emphasizing the need for more detailed study and standardization of this measurement. This work represents the first comparative internal anatomical study of the endocranium and inner ear labyrinths of this small clade of toothed whales.

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Sensory ability in the narwhal tooth organ system

Cited by 39 publications

References 116 publications

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

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

Intrasexually selected weapons

Neuroanatomy and inner ear labyrinths of the narwhal, Monodon monoceros, and beluga, Delphinapterus leucas (Cetacea: Monodontidae)

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