Ecology: Physical competition more successful than singing in recovering humpback whale populations

Post-whaling increases in eastern Australian humpback whale numbers may have led to males shifting their mating tactics from singing to physically competing with other males. The findings, published in Communications Biology, highlight how humpback whales have adapted their social behaviours as their populations have recovered.

Eastern Australian humpback whales were hunted almost to extinction in the 1960s but the population has since recovered to pre-whaling numbers. However, it has not been clear what effect these population density increases have had on mating tactics.

Rebecca Dunlop and Celine Frere studied the mating tactics of humpback whales off the coast of Peregian Beach, Australia, by analysing data collected over 123 days in 1997, 2003 to 2004, 2008 and 2014 to 2015. During this period the population increased from approximately 3,700 to 27,000 whales.

As the population recovered, the authors observed a decrease in the use of singing as a mating strategy, with the proportion of singing whales decreasing from two in ten males in 2003 to 2004 to one in ten by 2014 to 2015. Males in 2003 to 2004 were also less likely to sing when the density of males in their social circle was higher, with singing males having three or fewer other non-singing males in their social circle and non-singing males having four or more other non-singing males in their social circle. Additionally, the authors identified a shift in how successful singing versus non-singing mating strategies — such as physically competing with other males — were at enabling males to access females. In 1997 a singing male was 1.8 times more likely to be sighted temporarily joining a group in an attempt to breed with a female than a non-singing male. However, by 2014 to 2015, non-singing males were 4.8 times more likely to be sighted joining a group than singing males.

The authors speculate that male eastern Australian humpback whales may be less likely to use singing as a mating tactic when the population size is larger in order to avoid attracting other males to their potential mate. As whaling primarily targeted mature whales, post-whaling changes in the age composition of humpback populations may also have influenced the shift in mating tactics, they add.