Humpback whales give birth mid-migration, far further south than was thought

The textbook says that humpback whales feed at high latitudes and give birth in the tropics, in warm, sheltered waters. A study published in Frontiers in Marine Science shows that this division is less clear-cut than was believed: females give birth along the way, in temperate waters, then continue their migration with the newborn.

Two hundred and nine newborns

Newborns recorded	209
of which sightings of live animals	168, reported opportunistically
of which strandings	41
Area covered	Australia and New Zealand, all jurisdictions

The use of opportunistic reports deserves to be underlined: these are sightings reported by whale-watching operators, recreational boaters and coastal residents, then compiled and checked. No research programme would have the means to cover such a seaboard continuously. It is an example of what citizen science produces, and our page on how to take action lists the schemes.

How a newborn is recognised

The robustness of the result rests on a point of method: one has to be sure that the animal observed has just been born, and is not a juvenile from the previous year.

Several criteria make this possible, and they are clear enough to be applied by trained observers. The most distinctive is the presence of foetal folds, pale vertical marks left by the folded position in the uterus, which fade within a few weeks. To these are added a dorsal fin that is still soft, having not finished stiffening, a pale coloration, and a length of around four metres, far below that of a yearling.

Behaviour provides a final clue: a newborn swims pressed against its mother’s flank, comes up to breathe far more often than she does, and its swimming is visibly less assured.

These criteria are not infallible on a photograph taken from a distance, and a margin of error remains in any opportunistic census. They are nevertheless discriminating enough that the occurrence of births in temperate waters does not rest on a confusion of age.

What this changes

The idea of calving confined to warm waters rested on a thermal logic: a newborn has a thin layer of blubber and would lose too much heat in cool water. The observation of births further south does not cancel that argument, but shows that the constraint is less absolute than was assumed.

The practical consequence is regulatory. Protection measures for breeding grounds, navigation restrictions and rules governing approach are set on tropical sectors defined as the places of birth. If calves are born along the route, the migratory corridor itself becomes a sensitive area, along stretches of coast that are often far busier with boats.

The thermal constraint, stated precisely

The classic argument deserves to be set out properly, because it is not wrong, it is a matter of degree.

A newborn loses heat through its surface and produces it through its volume. Since surface grows more slowly than volume, a small animal cools more readily than a large one. A four-metre calf is already far from small, but its blubber layer is thin at birth and thickens rapidly thanks to very rich milk.

The constraint is therefore not a threshold beyond which birth becomes impossible, but a rising cost as the water gets colder. In temperate waters that cost is bearable, provided the calf receives enough milk and the mother has sufficient reserves. Our video on the blubber layer of cetaceans returns to this mechanism.

A mother and a newborn on the move

The second result is behavioural: the females do not stop. They continue their migration with a calf a few days old that is able to keep up. This implies sustained swimming in an animal that has only just been born, and lactation on the move.

Humpback whale milk is extremely rich, which allows rapid growth and offsets the cost of travel: our page on lactation in cetaceans gives the orders of magnitude, and the one on life cycles places the period of dependence in context.

New behaviour, or new attention?

This is the question raised by any observation of this kind, and it calls for three possible answers.

1. The phenomenon has always existed and was not documented. The most economical hypothesis. Births outside the known areas have never been the subject of organised collection, and nothing ensured that an isolated report would reach a researcher.

2. There are simply more observers. The number of whale-watching boats, drones and phones on these coasts has soared over twenty years, and reporting channels have become organised. The same underlying frequency now produces far more reported cases than in 2005.

3. There are more whales. The eastern Australian population has grown several dozen fold since whaling stopped. Even at a constant rate, more pregnant females means more births along the way.

These three explanations are not mutually exclusive, and the study does not make it possible to decide between them. It establishes a fact, not a trend. The distinction matters: management founded on the idea of a new behaviour is not justified in the same way as management founded on a long-standing behaviour that has finally been measured.

What this changes for boats

The most immediate practical consequence concerns the risk of collision. A mother and newborn pair has three unfavourable characteristics: it swims slowly, it stays near the surface because the calf has to breathe often, and it keeps close to the coast, where recreational and commercial traffic is densest.

Yet the migratory corridor runs along urbanised coasts for hundreds of kilometres. If births occur there, the periods and sectors to watch are no longer only the tropical breeding bays, but the route itself. Our pages on sighting hotspots and the legal framework show how hard it is to regulate a corridor rather than an area.

The limits of this kind of data

A census based on opportunistic reports says nothing about the proportion of births involved: it establishes that the phenomenon exists and is not anecdotal, without making it possible to say whether it concerns one per cent or a quarter of calvings. Survey effort is not uniform, and a busy coast mechanically produces more reports than a deserted one.

The share of strandings, 41 out of 209, is a further reminder that a birth during migration is not without risk, although this rate cannot be compared with that of tropical births, for want of a denominator.

Source: “Humpback whales continue migration after giving birth in temperate waters in Australia and New Zealand”, Frontiers in Marine Science.