Humpback Whales: The brawl to mate
In a violent humpback whale mating strategy, lusty males fight for access to females, sometimes even to the death.
A SPEEDING HUMPBACK slices through the swell. The vapour from her blow rises and dissipates into the tropical air as she heads towards the open ocean from Hunga, the westernmost island in the Kingdom of Tonga’s Vava’u group. Her pace is understandable: three huge, 13m males are in pursuit, trying to displace her current escort from his enviable position beside her.
Her escort isn’t about to swim away gracefully. Lunging through the water, he inflates the ventral pleats extending from his chin to his belly button to make himself look bigger than his sizeable 40-tonne bulk. Occasionally, he dives down deep to blow bubble curtains. He may even try to strike his challengers with his pectoral fins and tail flukes, or ram them with his head.
I’m watching the action from the stern of the 7m yacht Dream Catcher. Skipper Ali Takau is aiming to be about 40m in front of the female or beside her – the best place to drop off swimmers, he says.
Humpback whales migrate each year from high-latitude summer feeding grounds in the Arctic and Antarctic to winter mating and calving areas in the tropics, such as around Tonga. For tourists, the region is unlike other humpback wintering areas. Here, as long as you’re with a licensed operator, you’re not restricted to boat-based vantage points. You can join them in the water and observe from a distance determined safe by guides.
Bucket list for whale watching
Whale tourists tend to have a wish list of observations: a calf and mother; a singing whale (always male and often alone, hanging upside down and sending out his complex songs to attract a female); and, finally, the frenetic, multi-whale mating battle.
In Tonga, this battle is often called a ‘heat run’. But this is misleading because there’s little hard evidence that females are in oestrus, says Associate Professor Michael Noad, of the University of Queensland’s Cetacean Ecology and Acoustics Laboratory. Some experts believe, however, that lone females are in heat and are attracting males, or that they have already mated and their escorts are guarding them to prevent further matings, explains Dr Adam Pack, an expert on marine mammal behaviour at the University of Hawai’i at Hilo.
Despite all the effort put into the underwater battling, it is unknown whether any of these feisty males even mate with the female during these clashes. However, she is the centre of attention: she tends to stay near the front of the group and isn’t aggressive, but she may produce trumpeting sounds and slap her 5m-long pectoral fins on the water’s surface, Adam says.
‘Secondary escorts’ are often seen trailing the principal and may jostle with each other. Sometimes an escort becomes a challenger and attempts to displace the principal escort. The size of these groups can range from three to more than 20 whales. On the east coast of Australia they often include three or four whales, Michael says, but the Hawaiian breeding ground groups tend to be larger.
Studying humpback whales
Although group size varies, the behaviour of the whales does not, says Professor Scott Baker of the Marine Mammal Institute, at Oregon State University. A founding member of the South Pacific Whale Research Consortium, Scott has studied humpbacks for more than 30 years, and has helped to chart typical aggressive behaviours of males in competitive groups – some of which can result in serious injury or death.
Humpbacks spend about 90 per cent of their time under water. To better understand them, Adam and his team attached small cameras to the backs of male escorts in Hawaii. They found the whales would dive to depths of up to 200m, even down to the sea floor, where they would battle with one another. And, rather than behaving passively, the female in these cases appeared to “solicit male attention by stroking her pectoral fin very close to the principal escort and riding in the male’s slipstream,” Adam says.
Research by Adam’s team has shown that size matters. “Mature-sized females [12m or larger] prefer being escorted by the largest mature-sized males [11.5m or larger], and large females attract greater numbers of escorts than do smaller ones,” he says.
If he maintains his position, the principal escort typically doesn’t remain with a female longer than a day. During this time, the competitive group often whittles down to just the female and principal. Exactly when they part is unknown, but researchers believe that, once impregnated, the female departs the breeding grounds, where she has been fasting, to return to the feeding grounds. The male remains behind to seek opportunities to mate as long as his fat reserves allow.
Humpback whale behaviour still a mystery
Despite this research, much about humpback mating behaviour remains a mystery. As for swimmers entering the water when male humpbacks are competing, neither Scott nor Adam is convinced it’s safe. However, I’m willing risk to it, and several days into my trip I get the chance.
I slide into the water and kick towards the only whales I can see. Whether it’s a female and her principal escort, or challengers, I don’t know, but they don’t appear aggressive. They are diving in concert through rays of sunlight, and my imagination follows them long after our swim is over.
The full story can be found in Australian Geographic #120.