Interestingly, the average size of many whales has decreased over the last few centuries. This may seem like a mystery but has a logical explanation. Whaling has led to the demise of many species and the threat of the survival of many more. Whalers usually sought the biggest whale in order to get the most meat, blubber and skin from one kill. This usually meant killing the largest males. As a result, those that survived this human onslaught were the smaller, weaker males that did not appeal to the ruthless hunters. This was especially true of Sperm Whales.
The whale bulls may differ slightly from the females in terms of their colours and patterning too. However, the exact differences vary from species to species.
The juvenile bull will break away from its mother’s pod when it is weaned (which can be anywhere between 10 months and 26 months of age). It is likely to join other young males in a bachelor pod. These bulls are not yet ready to mate, but will begin to live a life of independence, away from the matriarchal society of their parental pod. The male will likely only reach sexual maturity (and, thus be ready to mate) at between seven and 10 years of age.
Once sexual maturity is reached, the bull will begin to search for available cows with which to mate. Whales do not mate for life, and are not loyal to one reproductive partner. Therefore, the bull will mate with as many cows as are willing to accept him. He may try to impress the cows with his vocalisations, surfacing behaviour (such as breaching, lobtailing or spyhopping), or by physically battling it out with another male.
The penis, testis and vas deferens of the whale bull is concealed within a genital slit on its abdomen. When mating (or preparing to mate), this penis is easily protruded out of the slit by the use of muscles so that the bull can mate with the receptive female. Litres of sperm are ejaculated. In fact, the Blue Whale produces an average of just less than 20 litres of sperm per ejaculation. This allows for some of the ejaculation entering into the water, thereby killing the sperm cells, so that the chances of conception are increased.
Once mating has been achieved, the bull will leave the female in search of more potential mates. He will not participate in the birth, training or weaning of the calf. Rather, the cow and her midwives (who make up her pod) will be responsible for this.