Whispering and avoiding predators!

  • Une baleine noire est accompagnée de son veau.
    Cette baleine noire de 22 ans est accompagnée de son troisième veau connu, âgé de deux semaines. Il s'agit du premier nouveau-né observé pendant la saison de reproduction 2018-2019. // This 22-year-old right whale is accompanied by her third known calf, which is just two weeks old. This is the first observed newborn of the 2018-19 breeding season. © Florida Fish and Wildlife Conservation Commission, taken under NOAA permit 20556-01
    28 / 11 / 2019 Par Camille Proust

    A number of studies show that in different species of baleen whales, mothers and their offspring communicate in a very special way: they whisper…

    Compared to when she communicates with other adults, a mother communicating with her calf will use lower amplitude sounds, i.e. sounds that are less loud in terms of decibels. The calf also uses lower amplitude sounds. This appears to be the case for humpback whales and southern right whales, and the findings of a recent study suggest that whispering might also be used by North Atlantic right whales.

    What might be the benefit of whispering for mothers and their calves?

    Researchers working on these studies hypothesize that whispering might be a predatory avoidance behaviour to protect calves.

    Many animals use avoidance behaviours in order to protect their young against predators. For example, if a fox approaches, the female killdeer feigns a broken wing and calls repeatedly as she moves as far away as possible from her chicks. This behaviour helps keep the predator away from the young, which are still unable to fly. The vole’s sharp sense of smell allows it to detect the presence of predators and thus avoid mating and rearing its young in a hostile territory.

    In baleen whales, although calves are usually already large at birth (about 2 m for the smallest species), their inexperience, slow speed in the water and their size compared to adult killer whales (6-9 metres) and some large sharks can make them ideal prey. Whales mainly use sound to communicate. Potential predators of baleen whales can detect them by hearing the sounds they produce. Whispering is therefore likely a perfectly suited behaviour to the way their predators locate them and which would still allow them to continue to communicate.

    How did researchers discover these whispers?

    Studies on this kind of behaviour do not necessarily use the same protocols, but are based on the same process: a comparison of sound production between nursing females (and sometimes their calves, if possible) and non-nursing individuals.

    With regard to the most recent North Atlantic right whale survey, the protocol consist of placing a sound-recording tag on different individuals, including lactating females and non-lactating individuals. The tags are retrieved and the recordings are studied.

    This study reports that the rate of low-amplitude sounds produced by lactating females is much greater than those produced by non-lactating individuals. The humpback study reports a significant amount of low-amplitude sound production between mothers and their calves. The study on southern right whales also indicates that lactating females produce few sounds, and when they do, they are mostly of low amplitude.

    Proportion of high- versus low-amplitude signal production by lactating females and non-lactating whales on the calving grounds for North Atlantic right whale © Royal Society https://doi.org/10.1098/rsbl.2019.0485

    The results of these studies are very interesting for a number of reasons. Firstly, they could play an important role in decision-making for the shipping industry and the protection a number of whale species. The fact that mother-calf pairs whisper to communicate leads researchers to wonder whether shipping-related noise pollution could increase the risk of mother-calf separation. Indeed, boat engines can sometimes produce high-amplitude sounds that could mask the soft sounds emitted by mothers and their calves. Additionally, shipping vessels often emit sounds of the same frequencies as those produced by baleen whales, further aggravating the risk of interrupting communication between a mother and her newborn. This could have a significant impact on whale species that use whispering behaviour.

    These results also raise other questions: Might there be other selective benefits associated with whispering between mothers and their calves in baleen whales? What other avoidance behaviours might exist in whales?