Can a blue whale have twins?

There is something vertiginous about the idea of a blue whale pregnant with twins. The largest animal of all time bringing two newborns into the world at once, five tonnes of calf to carry, to suckle and to protect. The biological reality is more sober: twin births do occur in blue whales, but they are extraordinarily rare and, in almost every case, bound to fail. The reason lies in an unforgiving energy budget.

A single calf, a colossal investment

To understand why twins are so exceptional in the blue whale, one first has to grasp the energy investment a single calf requires. At birth a blue whale calf already measures about 7 metres and weighs 2.5 to 3 tonnes: the equivalent of an adult elephant. Its growth is then extremely fast: it puts on about 90 kilos a day during the lactation phase, doubling its body mass in a few months.

To sustain that growth, the calf takes between 380 and 570 litres of milk a day. And this milk is no ordinary liquid: it contains 30 to 50% fat, ten times more than human milk. Its consistency is close to a thick cream. Producing such milk, in such quantity, and keeping it rich in lipids demands an enormous energy expenditure from the mother, all the more so because she barely feeds at all during this period (birth takes place in tropical waters poor in krill).

Why not two?

Doubling that investment would reach proportions that are physiologically difficult to sustain. A blue whale pregnant with twins would have to carry a combined foetal mass of 5 to 6 tonnes at the end of gestation, the equivalent of one and a half to two calves. Then suckle two young each requiring 400-500 litres of milk a day. For a mother who does not eat during this period, the cost is close to prohibitive.

Natural selection has therefore strongly favoured single births. Resources are concentrated on one calf, which receives all the mother’s attention and all her milk production. Statistically, it is better to raise one calf well to independence than to raise two twins badly that will probably die before weaning.

Documented cases of twins

That said, twins do occur in blue whales, as in most mammals. The examination of carcasses of blue whales taken during the twentieth century revealed a few cases of twin foetuses in the uterus of females. The observed rate of twin pregnancies was around 0.5 to 1% of pregnancies, a figure comparable to other large whales such as the fin whale.

But these twin pregnancies rarely end in live births. In most cases the foetuses are resorbed or lost in the course of gestation by the mother. If birth does occur, the twins are generally smaller than average and show a very high neonatal mortality rate. No confirmed observation of twins surviving for any length of time in the blue whale has been reported in the recent scientific literature.

A slow and precious reproductive cycle

This “one calf at a time” strategy belongs to a reproductive cycle that is slow and precious throughout. Sexual maturity comes late: between 5 and 15 years depending on the individual. Gestation lasts 10 to 12 months. Lactation continues for about 7 to 8 months. And after a birth the female waits 2 to 3 years before breeding again, the time needed to rebuild her energy reserves, recover her body condition and prepare a new pregnancy.

All in all, a female blue whale produces only one calf every 2 to 3 years, at most some twenty young over her whole reproductive life. That is extremely little to ensure the renewal of a population. This “K” strategy (few young, heavy parental investment) is typical of long-lived, slow species, the opposite of “r” species, which produce many young with little investment in each.

A vulnerability in the face of pressures

This slow reproduction explains why blue whale populations, devastated during the twentieth century (from several hundred thousand individuals to fewer than 5,000 in the 1970s), are rebuilding very slowly despite the moratorium on whaling in force since 1986. Every female counts. Every calf that dies before maturity is a significant loss to the dynamics of the population.

It is also what makes twins counterproductive in evolutionary terms: producing two young that both die of maternal exhaustion is worse than producing one that survives. Nature has been making that calculation for millions of years. And evolution has settled it: for the largest animal of all time, one calf, magnificently fed, is the better course.

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