The apparent precocity of whales raises questions




The first whales were observed at the end of April in the north of French Polynesia.  Credit: MMPoole 2015
The first whales were observed at the end of April in the north of French Polynesia.  Credit: MMPoole 2015

The first whales were observed at the end of April in the north of French Polynesia. Credit: MMPoole 2015

Tahiti, May 24, 2022 – The first humpback whales were observed north of the Australs at the end of April, indicates the Mata Tohora association. The apparent precocity of these mammals in our waters may raise questions but should not mask the complex relationships of these cetaceans with their increasingly threatened environment.

Humpback whales are back in Polynesian waters. The Mata Tohora association, in a message published on May 21 on Facebook, reports that the first jumps of the year were observed on April 25. The association does not specify why it withheld this information for a month, but we can assume that it is for the same reason that it does not wish to reveal its exact location: to protect the animals from curious people. “It’s not in the Australs, but further north”, evades Agnès Benet, president of the association. It would be for the moment a few individuals, two or three adults per island, on four different islands, which “are well shown, near the ribs”, according to this biologist. She sees the signs of the first wave of humpback migration towards the warmer waters of the Society Islands and the northern Tuamotus.

Humpback whales make a journey of more than 6,000 km each year from their feeding grounds near Antarctica to the warmer waters that serve as their breeding and calving grounds. This migration from South to North takes place at the end of the austral summer. In general, we can start to observe them from mid-May in the Austral Islands, then in June in Tahiti. The greatest population density is found off the Society Islands between August and October. At the end of the austral winter, the whales begin their descent towards polar waters, but some have been observed much later, during the holidays.

Ever earlier migrations?

Therefore, the presence of humpback whales so early in the season raises questions: are the very early or very late sightings of these cetaceans off the Society Islands a sign of a recent change in their migratory habits? For the president of Mata Tohora, no doubt, the “whale season” starts earlier and ends later. She says that for 5 years, the association’s observers and its network of specially trained fishermen have shown that the stays of cetaceans in the waters of northern French Polynesia are increasingly long and no longer completely comply with the periodicity “normal”.

Agnès Benet is formal: “At first, we thought it was random, but this earliness has become very regular. We’ve been observing them for 20 years. Our observers, our network of fishermen, are trained and attentive and all of them tell us about changes in the migratory behavior over the last 5 years”. For the president of Mata Tohora, these visible changes are due to the pressures that global warming and human activity put on both the feeding grounds and the breeding grounds of the whales.


The observation of an early migration of a few whales is not necessarily significant on the scale of the whole population, according to biologist Michael Poole.  Credit: MMPoole 2016
The observation of an early migration of a few whales is not necessarily significant on the scale of the whole population, according to biologist Michael Poole.  Credit: MMPoole 2016

The observation of an early migration of a few whales is not necessarily significant on the scale of the whole population, according to biologist Michael Poole. Credit: MMPoole 2016

Observational bias at work

“Let us be sure of the fact, before worrying about the cause”, recalled Fontenelle. This is essentially the posture of Michael Poole, a researcher specializing in marine mammals in Polynesia for 35 years and owner of a company offering “eco-tours” to observe these mammals. He wishes to weigh the scope of the observations made by the association: “It is not because we observe a few individuals that we can say that the migration period has really begun”. At the level of individuals, there are always behaviors that are in the margins, but they are not necessarily significant of a change in behavior of the entire population of humpback whales that frequent Polynesian waters, “unless hundreds of individuals are observed in May”, he insists.

The researcher then speaks of a “observation bias” : There are more whales, since the ban on its hunting and, above all, more observers. Early migrations of small numbers of individuals have always existed, he says, but they were less observed. “Just because the absolute number of sightings is increasing doesn’t mean the frequency of the phenomenon is also increasing.” Scientific data, according to him, do not allow for the moment to confirm this extension of the migratory period of humpback whales. “As a scientist, for the moment, I am not worried about the subject of an extension of the migratory period of humpback whales”, he summarizes.

Ocean warming threatens our tohorā

This does not mean, however, that the warming of the oceans will have no effect on the behavior of our tohorā. If the waters of northern French Polynesia warm up by a few degrees during the winter, it is likely that humpback whales will gradually take up residence around the archipelagos further south, in the Gambiers or the Australs, whose waters are cooler, believes Michael Poole. These cetaceans prefer temperatures that are neither too cool nor too hot for resting or giving birth (see our box). According to the biologist, such a change is not yet observable, but the hypothesis seems probable to him. The other crucial point concerns the impact of overfishing and ocean warming on the availability of krill populations. These tiny crustaceans living in schools in cold waters are the favorite food of humpback whales. A scarcity of their main food resource, or even a displacement of schools of krill in search of a more favorable climate, would constitute a major threat to the survival of humpback whales.


Of love, islands and cool waters

A marine biology study, published in 2019, compiling data from 19 years of sightings across the South Pacific, looked at the breeding sites chosen by humpback whales. It shows that this choice is directly correlated to two main criteria: shallow waters (coasts, seamounts, lagoons, etc.) and an aquatic temperature between 22.3 and 27.8°C. To reproduce, whales therefore prefer the proximity of islands and water that is not too warm. However, most of the sites studied will exceed 28°C before the end of the century, suggesting a gradual shift of breeding sites to islands and archipelagos further south. However, in this region of the globe, the further south you go, the rarer the islands. How the preferences of humpback whales will change in the face of global warming is still an enigma.

Source:
Derville S., Torres LG, Albertson R., Andrews O., Baker CS, Carzon P., Constantine R., Donoghue M., Dutheil C., Gannier A., ​​Oremus M., Poole MM, Robbins J., Garrigue Claire. (2019). Whales in warming water: assessing breeding habitat diversity and adaptability in Oceania’s changing climate. Global Change Biology, 25 (4), p. 1466-1481. ISSN 1354-1013. https://doi.org/10.1111/gcb.14563


The apparent precocity of whales raises questions
The apparent precocity of whales raises questions
The hymns of the whales

The famous song of humpback whales is both a sexual expression and a cultural marker specific to each population. Songs can be transmitted from group to group across an entire ocean.

The song of humpback whales is a fascinating natural means of expression. Only the males sing. The animal weighing the equivalent of 6 or 8 African elephants positions itself head down, generally motionless, then emits sounds so powerful that nearby swimmers can feel them as much as hear them. A song can last about thirty minutes and be repeated several times, for hours or even days. These songs are very structured. The researchers managed to cut these songs into themes, sentences, sub-sentences, etc.

It is not yet known if this song is intended to charm females, to establish a hierarchy between males, or both. Even more surprising, “Each population of humpback whales around the world has its own song, like a national anthem”, writes Michael Poole in 2015. All males of the same population therefore sing globally in the same way. The population that crosses in Hawaii will therefore have a distinct song from that of the West Indies or Polynesia, for example. The sub-phrases, the modulations, evolve despite everything over the season, little by little.

True cultural manifestations, these songs are transmitted within a population, but they can also sometimes spread through several very distinct groups. In 2002, in French Polynesia, Michael Poole and his colleagues recorded a song whose several themes and phrases had previously been spotted in Eastern Australia two years earlier. However, these populations were not in direct contact. A 2011 study, conducted over a period of 11 years, shows that these songs can sometimes spread in two years from west to east, from Australia to Tahiti, gradually taken up by all groups of humpbacks in the South Pacific.

Source:
Garland EC, Goldizen AW, Rekdahl ML, Constantine R., Garrigue C., Hauser ND, Poole MM, Robbins J., Noad MJ (2011). Dynamic horizontal cultural transmission of humpback whale song at the ocean basin scale in Current Biology 21, p. 687–691, https://doi.org/10.1016/j.cub.2011.03.019


Written by Antoine Launey on Tuesday, May 24, 2022 at 7:11 p.m. | Read 1421 times


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