Sarah Bedolfe
Normally-peaceful leviathans further their species in a massive fight for mating rights – find out why!
Imagine being on a boat in the clear waters of Hawaii. The outlines of the green mountainous islands are not far away but you’re in deep water. Even though it’s clear as glass, you can’t see the bottom. You could hardly even dream up a more beautiful and peaceful scene. Splashes on the horizon start to get bigger when you realize, not one humpback whale, not two, but a whole group of them, are moving your way, and fast! As they pass your boat, you notice that the usually-serene whales are suddenly producing lots of bubbles. Sometimes they even aggressively lunge at one another, with their throat pleats inflated wide, making them look even more mind-blowingly massive. This is no play fight – they might even draw blood. Before you realize what’s happening, this incredible combat is already past you, disappearing rapidly into the distance.
The impressive clash of marine titans you just saw is called a heat run. If you’re lucky enough to witness a heat run, you’re seeing one female humpback being chased by many males, all competing for her attention. The female lures them along on a chase that can last for hours. The males follow, fighting one another aggressively all the while.
As we described in a previous blog, humpback whales (Megaptera novaeangliae) travel massive distances in their annual migrations. Polar waters are rich in food so whales spend summers there feeding. In the tropics, there is not a lot of food, but the warm water is ideal for birthing and raising calves. Winter in the tropics is also the season for humpback breeding!
A female whale spends huge amounts of energy birthing and raising her calf, so females spend more time than males at the polar feeding grounds. It seems females also don’t mate every year. Males on the other hand, spend more time at the tropical areas looking for chances to mate. As a result, at any given time, there are far more males than females in the mating grounds.
Humpback whales do not form long-term couples. Instead they mate with various different partners. But they do seem to be very picky in who they want to mate with each time. This is a dynamic called “mate choice” and it seems to be a very important factor for humpback reproduction.
By spending lots of time feeding in summer, a female humpback has already taken a step towards reproductive success: she has to ensure that she has lots of energy for motherhood. When breeding season comes, available females are outnumbered by males so she will have lots of males chasing her. To get the strongest offspring, she should choose the strongest mate.
For a male humpback to have the most success with reproducing, he needs to mate with as many females as possible. At the same time, however, males spend so much energy fighting each other to win one of the few females. If they have to fight to pass on their sperm, then they should fight for the fittest mother. That will give them the best chance at healthy offspring.
This is why heat runs occur: the normally-peaceful leviathans are furthering their species by duking it out with one another for mating rights – resulting in a conflict of epic proportions. Seeing a heat run has been likened to witnessing a dinosaur battle. While most of us will never get to see one live, the MacGillivray Freeman film crew were lucky enough to get to come across a heat run – and the good news is that they caught it on camera! For a life-size, close-up humpback whale heat run experience, check out the giant screen film Humpback Whales. There is more to these gentle giants than meets the eye.
Sources:
Craig, A, LM Herman, AA Pack. 2002. Male mate choice and male-male competition coexist in the humpback whale (Megaptera novaeangliae). Can. J. Zool. 80: 745–755
Craig, A, LM Herman, CM Gabriele, AA Pack. 2003. Migratory Timing of Humpback Whales (Megaptera novaeangliae) in the Central North Pacific Varies with Age, Sex and Reproductive Status. Behavior 140(8):981-1001.
