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Far bigger than any dinosaur, the blue whale is the largest known animal to have ever lived.
An adult blue whale can grow to a massive 30m long and weigh more than 180,000kg - that’s about the same as 40 elephants, 30 Tyrannosaurus Rex or 2,670 average-sized men. But this giant among giants started as something far smaller.
Like all whales, the blue whale (Balaenoptera musculus) evolved from a four-legged mammal that lived on land some 48 million years ago. This ancient ancestor, Pakicetus, was only 1.8m long. It foraged in streams and some of its descendants became adapted to living in water. This eventually led to a completely aquatic creature called Dorudon ("Spear-Tooth"), which lived 37 million years ago and grew 4.5m long.
In Dorudon, we see the beginnings of what makes whales so special. Its nostrils moved back from its snout to the top of its head, its forelimbs became stiff flippers, its body elongated, its hind limbs became virtually non-existent and its tail evolved into two rubbery flukes, which propelled the animal forward through the water with an up and down motion, rather than the side-to-side movement of a fish tail.
The evolution from Dorudon to blue whale involved a number of changes to cope with its enormous size. Here are nine you need to know about:
Blue whales can dive for up to an hour at a time, going to a depth of 100m, so they need highly efficient lungs to survive. Two enormous blowholes, big enough for a small child to crawl into, allow the fast and efficient exchange of oxygen. Blue whales exchange between 80 and 90 per cent of oxygen in their lungs each time they breathe, compared to just 10 or 15 per cent in humans.
Oxygen is pumped around its enormous body by an equally massive, four-chambered heart. Weighing some 900kg – and the size of a Mini car – the blue whale’s heart beats once every 10 seconds, pumping 220 litres of blood through its body, and beats so loudly it can be heard from 3km away through sonar equipment.
A blue whale’s skin markings are unique, much like fingerprints. The pale bluish-grey colour gives the species its name, although the skin can also look silvery grey or tan, depending on the light. A blue whale has between 80 and 100 long grooves running along the length of its throat and chest.
Blue whales have relatively small eyes for their body size – each about the size of a grapefruit – and their eyesight is thought to be weak. They have no tear glands or eyelashes.
Despite having no external ears, blue whales are believed to have excellent hearing, using air sinuses and bones to detect sound. They communicate using low-frequency whistles or rumbling noises which can travel hundreds of kilometers and reach 188 decibels – louder than a passenger jet.
Their gigantic mouths – big enough to house 100 people – can capture enormous quantities of prey with each gulp of water, filtering the nutritious krill from the expelled water with stiff bristles that grow from the roof of the mouth. During the summer months, they eat up to 6,000kg of krill a day.
Blue whales reach sexual maturity between five and 10 years of age. They seek warmer equatorial waters before embarking on an elaborate mating ritual that involves the male and female rolling over one another, diving in a deep dive, then suddenly swimming to the surface for copulating. The males have the biggest penis in the animal kingdom, about 30cm in diameter when erect and 3m in length.
Blue whales are placental mammals and the foetus develops in the uterus of the mother. The developing foetus grows quickly and after seven months, it is about 3.5m long. The calf is born tail first at 12 months and weighs about 2,700kg, swimming immediately to the surface for air. It suckles on its mother’s two nipples, feeding on up to 180 litres of fat-rich milk a day, allowing it to grow at a daily rate of 90kg. Weaning occurs at around seven or nine months, when the calf is some 15m long.
It is thought that there were once more than 250,000 blue whales. Now it is estimated there are between 10,000 and 25,000 left in the world. After decades of being hunted for their meat, oil, and other valuable body parts, they are now classified as an endangered species on the IUCN’s Red List.