Humpback whales distribution, reproduction and behaviour
Humpback whales can be found in most oceans and are well known for their extensive migrations which are some of the longest documented for any mammal species (over 8000 km in one direction). They migrate from their feeding grounds in polar waters to their breeding grounds in tropical waters. Why do they undertake these long migrations? Scientists are still debating why, but it is most likely to take advantage of the high productivity in the Arctic/Antarctic waters and the energy-conserving properties of warm tropical waters.
There are approximately 15 recognized breeding populations of humpback whales around the world, however they are still classified as one species. Recent genetic studies have shown that many of these populations are genetically distinct and should be, at least, classified as subspecies. The northern and southern hemisphere populations never occur together, are isolated geographically and therefore cannot interbreed and should therefore be classified as different species but further work is needed to confirm this.
Humpback whales become reproductively active between 8-10 years of age, have an 11-12 month gestation period and give birth to a single calf that measures about 4m in length and weighs approximately 700 kg. The calf will suckle milk from its mother for 6-12 months and will be weaned at 2-3 years of age. Humpback whales are one of fastest growing whale species and the calf can reach 8-10m in its first year. Most females have a new calf every 2-3 years, but about 8% of the females will have a new calf on consecutive years. One female was recorded giving birth 4 years in a row, indicating that there was enough food for her, the calf and her foetus to survive. The maximum life span of humpback whales is unknown, but it is believed they can live to 80 years or older.
During the breeding season male humpback whales can get extremely aggressive and compete with each other for the females by singing or physically fighting. The males produce an elaborate song that is repeated for several hours. These songs are made up of 3 – 9 themes that are sung in a particular order. Each theme is made up of repetitive phrases that last about 15 seconds and a full song may last 10 – 15 minutes before being repeated. Their songs are unique to each individual although the same basic song can be found among males that breed in the same location.
Humpback whales display several types of feeding behaviour. The most notable is ‘bubble netting’, whereby a group of whales work co-operatively to produce a screen of bubbles around a shoal of fish and then take it in turn to lunge up through the entrapped fish. They consume a variety of prey but their main food source is krill and small schooling fish such as herring, mackerel, sardines and anchovies. Humpback whales consume up to 1½ tons of food per day during the feeding season and they then survive on their fat reserves during their long migrations and breeding season.
Humpback whales are extremely charismatic and acrobatic, performing full body breaches, sailing (lifting their tails out of the water for long periods) and flipper slapping. It is unclear why they show these behaviours, but the most common thought is that they are a form of communication because the noise created from slapping on the water can travel long distances.
For general information on Humpback Whales please see last week’s post.
– Whales dolphins and other marine mammals of the world (2006) by Hadoram Shirihai and Brett Jarrett. Prinston field guides 2006
– Smithers’ Mammals of Southern Africa a field Guide by Peter Apps Struik Nature edition 2012
– Jefferson T. Webber M. Pitman R. 2007 Marine mammals of the world: A comprehensive guide to their identification 1st edition
– Best, Peter 2007. Whales and dolphins of the southern African subregion. Cambridge University Press.