Mother and baby humpback whales whisper to one another
Humpback whales are notorious for their loud, majestic songs in the breeding season. However, new research, published in the journal Functional Biology, has revealed that newborn humpback whales and their mothers whisper to each other. Mother and calf pairs may have also developed a silent method to initiate suckling in their effort to go undetected.
Researchers attached multi-sensor Dtags to eight calves and two humpback whale mothers to record the whales while they swam in their breeding grounds of Exmouth Gulf in Western Australia. The aim of the study was to better understand neonate suckling behaviour.
Humpback whales are long distance migrators that travel between their wintering breeding grounds in warm, low latitude, tropical waters and their feeding grounds in cooler, higher latitude polar waters. There is a critical time period during which neonatal calves must acquire sufficient energy via suckling to undertake the migration to their feeding grounds.
The Dtags revealed the vocalisations between mother and calf, which included very weak tonal and grunting sounds, could only be heard 100 metres away. In comparison, singling male vocalisations were 40 decibels higher and could be detected over kilometres. The quiet communication between mother and calf likely reduces the risk of exposure to predators and male humpback whale escorts that interrupt critical periods of nursing and resting in an attempt to mate with the nursing mother, which ultimately reduces calf fitness. It is likely that the calls are used to keep the mother and calf together when water visibility is poor which is true of the Exmouth Gulf.
Furthermore, vocalisations between the pairs were produced more frequently during active dives than suckling ones, suggesting that instead of acoustic cues, mechanical stimuli were used to initiate suckling.
This type of behaviour is very sensitive to increases in noise-producing human activities including whale watching, shipping and fishing, which may mask the quiet calls between mother and calf and therefore increase the risk of separation. This could have negative consequences on calf fitness by reducing the time spent suckling or increasing the use of loud calls between mother and calf thereby increasing the likelihood of detection. The discovery of this unique communication will therefore be important in targeting conservation efforts.
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