On Hokusai's great wave off Kanagawa: Localization, linearity and a rogue wave in sub-antarctic waters

International audienceThe Hokusai woodcut entitled The great wave off Kanagawa has been interpreted as an unusually large storm wave, likely to be classed as a rogue wave, and possibly generated from nonlinear wave dynamics (J. H. E. Cartwright and H. Nakamura, Notes Rec. R. Soc. 63, 119-135 (2009)). In this paper, we present a complementary discussion of this hypothesis, discussing in particular how linear and nonlinear mechanisms can both contribute to the emergence of rogue wave events. By making reference to the Great wave's simultaneous transverse and longitudinal localization, we show that the purely linear mechanism of directional focusing also predicts characteristics consistent with those of the Great wave. In addition, we discuss the properties of a particular rogue wave photographed on the open ocean in sub-Antarctic waters, which shows two-dimensional localization and breaking dynamics remarkably similar to Hokusai's depiction in the woodcut

High-frequency Near-field Physeter macrocephalus Monitoring by Stereo-Autoencoder and 3D Model of Sonar Organ

International audiencePassive acoustics allow us to study large animals and obtain information that could not be gathered through other methods. In this paper we study a set of near-field audiovisual recordings of a sperm whale pod, acquired with a ultra high-frequency and small aperture antenna. We propose a novel kind of autoencoder, a Stereo-Autoencoder, and show how it allows to build acoustic manifolds in order to increase our knowledge regarding the characterization of their vocalizations, and possible acoustic individual signature

High-frequency Near-field Physeter macrocephalus Monitoring by Stereo-Autoencoder and 3D Model of Sonar Organ

International audiencePassive acoustics allow us to study large animals and obtain information that could not be gathered through other methods. In this paper we study a set of near-field audiovisual recordings of a sperm whale pod, acquired with a ultra high-frequency and small aperture antenna. We propose a novel kind of autoencoder, a Stereo-Autoencoder, and show how it allows to build acoustic manifolds in order to increase our knowledge regarding the characterization of their vocalizations, and possible acoustic individual signature

Underwater photo-identification of sperm whales (Physeter macrocephalus) off Mauritius

The long-term monitoring of long-lived animals often requires individual identification. For cetaceans, this identification may be based on morphological characters observable from a boat such as shape, spots and cuts of the back, fluke and dorsal fins. However, for some species such as the sperm whales (Physeter macrocephalus), this approach may be challenging as individuals display a rather uniform skin pigmentation. They also do not very often show their fluke, complicating individual identification from a boat. Immature sperm whales that usually have an unharmed fluke may be excluded from photo-identification catalogues. Within the framework of the Maubydick project, focusing on the long-term monitoring of sperm-whales in Mauritius, passive underwater observation and video recording were used to identify long-lasting body markers (e.g. sex, ventral white markings, cut-outs of fins). A catalogue of 38 individuals (six adult males, 18 adult females and 14 immatures) enabled observers to record some nearly-daily, and yearly resightings. Advantages and disadvantages of this method are presented here. Such catalogues represent a robust baseline for conducting behavioural, genetic and acoustic studies in social species. Benefits of such newly acquired knowledge are of primary importance to implement relevant conservation plans in the marine realm.plement relevant conservation plans in the marine realm

Sperm whales ultra high frequency near field multichannel analysis

International audienc

Kin relationships in cultural species of the marine realm: case study of a matrilineal social group of sperm whales off Mauritius island, Indian Ocean

International audienceUnderstanding the organization and dynamics of social groups of marine mammals through the study of kin relationships is particularly challenging. Here, we studied a stable social group of sperm whales off Mauritius, using underwater observations, individual-specific identification, non-invasive sampling and genetic analyses based on mitochondrial sequencing and microsatellite profiling. Twenty-four sperm whales were sampled between 2017 and 2019. All individuals except one adult female shared the same mitochondrial DNA (mtDNA) haplotype—one that is rare in the western Indian Ocean—thus confirming with near certainty the matrilineality of the group. All probable first- and second-degree kin relationships were depicted in the sperm whale social group: 13 first-degree and 27 second-degree relationships were identified. Notably, we highlight the likely case of an unrelated female having been integrated into a social unit, in that she presented a distinct mtDNA haplotype and no close relationships with any members of the group. Investigating the possible matrilineality of sperm whale cultural units (i.e. vocal clans) is the next step in our research programme to elucidate and better apprehend the complex organization of sperm whale social groups

Nursing Behavior in Sperm Whales (Physeter macrocephalus)

In mammals, lactation is the universal behavior of feeding offspring and has a fundamental nutritional and social value with offspring staying near their mothers. In order to obtain milk, terrestrial mammal offspring squeeze the breast of lactating females and suckle the nipples with their tongues. In the specific case of cetacean species, it was reported that lactating females intentionally eject milk from their mammary slit into the calves’ mouths. Nursing behavior in sperm whales has already been broadly described, but the results of our current study, based on 127 underwater videos, recording over 7 years and displaying explicit nursing behavior, bring a higher level of understanding. We first showed that sperm whale calves are proactive in getting milk. We were then able to illustrate and describe with a high level of precision their suckling behavior: firstly, the calf bumps its head onto the female’s genital area to signal the mother its willingness to suckle; secondly, the calf introduces its lower jawbone into the genital slit, this action makes the nipple pop up from its slit; thirdly, the calf squeezes the nipple with its tongue against the hard palate and suckles; fourthly, the calf removes its jawbone from the female and swims off. Moreover, our underwater visual observations provided the first direct evidence for allosuckling in sperm whales, a situation during which a calf obtains milk from an adult female who is not its mother

Long Distance Runners in the Marine Realm: New Insights Into Genetic Diversity, Kin Relationships and Social Fidelity of Indian Ocean Male Sperm Whales

Adult male sperm whales (Physeter macrocephalus) are long distance runners of the marine realm, feeding in high latitudes and mating in tropical and subtropical waters where stable social groups of females and immatures live. Several areas of uncertainty still limit our understanding of their social and breeding behavior, in particular concerning the potential existence of geographical and/or social fidelities. In this study, using underwater observation and sloughed-skin sampling, we looked for male social fidelity to a specific matrilineal sperm whale group near Mauritius. In addition, we captured a wider picture of kin relationships and genetic diversity of male sperm whales in the Indian Ocean thanks to biopsies of eight individuals taken in a feeding ground near the Kerguelen and Crozet Archipelagos (Southern Indian Ocean). Twenty-six adult male sperm whales were identified when socializing with adult females and immatures off Mauritius. Sloughed-skin samples were taken from thirteen of them for genetic analysis. Long-term underwater observation recorded several noteworthy social interactions between adult males and adult females and/or immatures. We identified seven possible male recaptures over different years (three by direct observation, and four at the gametic level), which supports a certain level of male social fidelity. Two probable first- and thirty second-degree kin relationships were highlighted between members of the social unit and adult males, confirming that some of the adult males observed in Mauritian waters are reproductive. Male social philopatry to their natal group can be excluded, as none of the males sampled shared the haplotype characteristic of the matrilineal social group. Mitochondrial DNA control region haplotype and nucleotide diversities calculated over the 21 total male sperm whales sampled were similar to values found by others in the Indian Ocean. Our study strongly supports the existence of some levels of male sperm whale social fidelity, not directed to their social group of birth, in the Indian Ocean. Males sampled in breeding and feeding grounds are linked by kin relationships. Our results support a model of male mediated gene flow occurring at the level of the whole Indian Ocean, likely interconnected with large-scale geographical fidelity to ocean basin, and a small-scale social fidelity to matrilineal social groups