A new study into the behaviour of the marine mammals has discovered that the mothers’ habitat choices are strongly influenced by the size and age of their calves.
Monitoring of 72 pairs over a series of December-April breeding seasons showed that the mothers swam to progressively greater depths as their calves matured.
The final sightings of the mother-calf pairs also showed a preference for waters with rugged seabed terrain.
Humpback mothers on the breeding grounds like areas with relatively few males, and researchers believe this type of terrain makes it easier for them to rest and also means mother-calf communications are less likely to be detected by eavesdropping males.
The research was carried out by Edinburgh Napier University marine mammal scientist Dr Alison Craig and colleagues Prof Louis Herman and Prof Adam Pack from the University of Hawaii and The Dolphin Institute over an 11-year period, and their findings have now been published in the scientific journal Animal Behaviour.
After spotting the whales from small outboard boats in Hawaii, the researchers snorkelled alongside the whales to film them so that they could use a ‘videogrammetry’ technique to measure the length of each calf. They also took photographs of the whales’ tail flukes so that mothers and calves could be identified individually using the unique shape and pigmentation of their tails.
Dr Craig said: “When mother-calf pairs were in deeper waters they had a preference for rugged seabed terrain, and we suspect that this terrain provides more ‘acoustic camouflage’ than flat terrain. This might help the females to avoid males by making it less likely that they are overheard communicating with their calves.”
Dr Pack added: “In essence, we found that the depth of water that a female and calf occupy is related to the age and size of the calf, so individual females tend to be found in deeper waters as their calves mature.”
One feature of areas with rugged seabeds is that the noise from snapping shrimps is louder than it is over sandy bottoms.
This can prove useful to mothers as the breeding season progresses and male humpbacks increasingly turn their attentions to females with calves as they are the last females to leave the breeding grounds.
Humpback whale calves remain with their mothers for around 12 months and researchers suspect that moving to increasingly deeper waters as the calf grows prepares them for leaving the breeding grounds at the end of the season.
The full paper can be read here