Whale Calves Are Born On “Humpback Highway”, Changing What We Knew About Migration

Jane McPhee-Frew had already started a PhD at the University of New South Wales on whales when she saw something that surprised her and her fellow marine biologists. “I was working part-time as a skipper on a whale-watching boat in Newcastle when I first spotted a calf in the area,” McPhee-Frew said in a statement. “It seemed out of place,” she added. “The calf was tiny, obviously brand new. What were they doing here?”

This went against the accepted understanding that humpback whales feed at high latitudes in the summer, and then migrate to the tropics to breed and calve in shallow, protected bays. “But none of my tourism colleagues seemed surprised,” McPhee-Frew said. They’d seen young calves making their way up the coast before.

The feeding/breeding separation was already starting to look somewhat simplistic, following evidence humpbacks also sometimes feed during the migration. Moreover, McPhee-Frew told IFLScience, tracking data on one pregnant humpback in the North Atlantic revealed she gave birth at temperate latitudes and far from shore.

McPhee-Frew and colleagues reached out as widely as they could in search of observations either of humpback whales in Australian or New Zealand waters calving further south than expected, or of very young whales at unexpected latitudes.

McPhee-Frew told IFLScience they received great assistance from whale-watching organizations and parks authorities. This led the team to collate more than 200 reports, thought to represent at least 169 calves. Archival records also revealed that whalers sometimes reported the presence of young calves well to the south before they drove humpbacks to near extinction.

Recently, a humpback birth was witnessed off Kaikoura, New Zealand, at a latitude of 42 degrees south, 14 degrees or 1,500 kilometers (932 miles) further south than most previous reports. A young calf was sighted even further south, off Port Arthur, Tasmania. It’s possible it was born further north and the mother had started heading the wrong way. However, McPhee-Frew told IFLScience all reports that indicated a direction of swimming suggested the young mothers and calves were following the so-called “humpback highway” north.

Reports from Western Australia reveal pregnant mothers stopping for a few days in Flinders Bay before resuming the journey north with calf in tow. Although the lack of follow-up observations means that, in most cases, we can’t tell how long the mothers stop before resuming their migration, we know it isn’t long. 

Co-author Professor Tracey Rogers said, “Mums with newborns swim much more slowly. Newborns are like Great Dane puppies. They have those long, enormous fins that they need to grow into, and they’re not very strong swimmers. So they rest a lot of the time on their mum’s back.” 

For the largest humpback population, the northern part of their migration cycle means swimming against the powerful East Australian current, which seems a particularly unfair burden to place on a newborn. McPhee-Frew told IFLScience that whales can dodge the current by swimming further from shore, but based on observations, plenty don’t.

McPhee-Frew thinks most humpback births occur in tropical waters, but calving outside that zone may still be quite common. Scientists were unaware of it in part because low humpback populations numbers made such events rare until recently, and also because non-scientist observations were not being communicated.

“With more eyes than ever before looking at the water with technology like drones and with activities like whale watching,” things are improving, McPhee-Frew said, noting most reports were from 2023 and 2024.  However, she also told IFLScience that her experience of discovering her fellow whale-watching crew knew things scientists didn’t highlights the opportunity for better collaborations.

“I think non-scientists often underestimate how much they have to contribute from their own observations,” she said.

The standard explanation for tropical calving has been that warmer waters mean the calf can devote more of the energy it obtains to growth. Starting life with a big swim out of colder water seems very undesirable. Tropical waters also mean a reduced orca threat when the calves are most vulnerable. In contrast, it’s hard to think of an advantage to giving birth while migrating. 

Much as the authors would like to know why these births occur, they acknowledge that “ethical and logistical considerations preclude manipulative experiments” that might test some explanations. You try telling a pregnant humpback where she can give birth. However, radio tags on pregnant whales would reveal when they slowed down to allow a young calf to keep up, and if any time was taken from the journey. So far, shallow bays appear to be favored as nursery sites, tropical or not, but tags would reveal if this is just where humans are most likely to be watching.

Besides the effort demanded of a young calf, the journey is hazardous, taking them through busy shipping lanes. McPhee-Frew’s own observation of a mother and calf occurred in a busy shipping lane off the world’s largest coal export port. Shipping strikes are now thought to be a major cause of whale mortality (wind farms are not).

For that reason, McPhee-Frew said, learning more about these mid-migration births is crucial, both to improve public awareness of the dangers and potentially to change policy on whale protection.

The study is published in Frontiers in Marine Science.