Southern right whale mothers rest upside-down to conserve energy while nursing
New research reveals that southern right whale mothers rest upside-down at the ocean surface not because they're sick, but as a strategy to manage energy and regulate milk feeding during breeding season.
Southern right whales migrating to Australia's southern waters each breeding season have developed an unusual resting behaviour that puzzled observers for years: floating upside-down at or just below the surface. According to a new study published in the journal Mammalian Biology, this remarkable habit is not a sign of distress, but rather a sophisticated energy-management strategy.
Research conducted along the Great Australian Bight used drone footage to document the behaviour of breeding whales over hundreds of flights. Scientists observed the upside-down resting in 25% of the mothers they tracked, suggesting a direct link to the extreme physical demands of reproduction.
A migration like no other
Southern right whales (Eubalaena australis) undertake journeys exceeding 6,000 kilometres from the Southern Ocean and Antarctica to Australian breeding grounds — a distance equivalent to travelling from Perth to Sydney and back. Crucially, they do not feed during this entire period on the calving grounds.
These whales are "capital breeders," meaning they feed intensively for half the year before migrating to rely entirely on stored energy in their blubber.
Mothers nursing newborn calves face the most intense energy demands of all. They must produce enough milk to sustain a rapidly growing calf while conserving enough energy reserves to survive the return journey to feeding grounds. This creates a precarious balance.
A behaviour unique to nursing mothers
The drone observations revealed that only mothers with newborn calves and one mother in late pregnancy adopted the upside-down resting posture. The behaviour was not observed in other adults, juveniles, or calves — a pattern that reinforces its connection to reproductive stress.
When mothers did rest this way, they spent roughly 19% of their observed time in the inverted position. According to the research, one key advantage of this posture is that it prevents calves from accessing their mother's mammary slits. By resting upside-down, mothers can regulate milk feeding, allowing them to rest without continuously nursing — a critical way to manage depleting energy stores during one of the most energetically demanding periods of their lives.
To date, no other large whale species has been recorded resting in this manner, making it a behaviour unique to southern right whales facing the particular stresses of long-distance migration and nursing at sea.