“Nose-metrics” of wild southern elephant seal (Mirounga leonina) males using image analysis and geometric morphometrics

Abstract

The elephant seal (genus Mirounga) proboscis is a textbook example of an exaggerated secondary sexual trait, whose function is debated. The proboscis can be related to sexual status advertising, emission of aggressive vocalizations, and/or female mating choice. The study of the proboscis is complicated, because it is a soft trait that needs to be studied when males vocalize and it is thus expanded. Here, we combined field stimulation experiments, 2D photogrammetry, and geometric morphometrics, to study the proboscis of wild elephant seals in natural conditions. The goal of our study was twofold: (a) to demonstrate that photogrammetry and geometric morphometrics can be effectively applied in the field to wild, large, non‐sedated mammals; (b) to study the proboscis shape development during maturation. We found that it is possible to accurately estimate the proboscis size and shape using photographs of vocalizing males taken in the field. Moreover, we showed that mature and non‐mature males differ not only in proboscis size but also in its shape, a difference that is largely allometric and can have important effects on the frequency structure and individual signature of male vocalizations. These results open new avenues for future research on this enigmatic structure, its function in aggressive displays and potential role in sexual selection, and also exemplify a very promising approach, that could be applied in field studies of other large mammals.

Seeing Distinct Groups Where There are None: Spurious Patterns from Between-Group PCA

“…On the other hand, in datasets where strong correlations among variables are expected, such as is common in GM, where additional covariance is introduced by the Procrustes superimposition itself (Rohlf and Slice, 1990) and many semilandmarks are used (because physically close semilandmarks tend to covary strongly), one might hope to circumvent some of the issues raised in this paper by reducing the number of variables used in the bgPCA. Indeed, in GM studies, it is often the case that distance matrices among specimens assessed using a few landmarks are highly correlated with those derived from the full set of landmarks plus many semilandmarks (Skinner et al, 2009;Ferretti et al 2013;Watanabe, 2018;Galimberti et al 2019). This can be assessed formally, for instance, through matrix correlations where testing whether full (all landmarks and semilandmarks) and reduced (a subset of the full configuration) data matrices are highly correlated.…”

A cross-species framework to identify vocal learning abilities in mammals

“…These species could either be exceptions to our hypothesis, not full-fledged vocal learners or may combine VPL and anatomical adaptations. For instance, in southern elephant seals ( Mirounga leonina) , a potential capacity for VPL [ 39 ] could coexist with anatomical adaptations (their proboscis) which may both be driven by strong sexual selection and push sound frequencies downwards [ 40 ].…”

Comparative examination of pinniped craniofacial musculature and its role in aquatic feeding

“…The function of the northern elephant seals’ iconic nose is still debated. It has been postulated that the nose plays a role in identifying age and breeding status, in male visual displays, and in aiding male vocalizations (Galimberti et al, 2019 ; Sandegren, 1976 ; Sanvito et al, 2007 ), but this requires further investigation. Similar to bearded seals and walruses, northern elephant seals are hypothesized to be suction feeders (Antonelis et al, 1987 ; Kienle & Berta, 2016 ; Naito et al, 2013 ); therefore, the increased size of the facial expression muscles may aid in suction feeding.…”

“…The function of the northern elephant seals' iconic nose is still debated. It has been postulated that the nose plays a role in identifying age and breeding status, in male visual displays, and in aiding male vocalizations (Galimberti et al, 2019;Sandegren, 1976;Sanvito et al, 2007), but this requires further investigation.…”