Ask the Experts: Why are octuplet births so rare in humans?

By Brendan Borrell

A litter of one: Evolutionary Biologist Robert Sikes explains why humans are not designed to have a gaggle of newborns.

Late last month, a team of 46 doctors and nurses in the Los Angeles suburb Bellflower pulled eight babies from the belly of a 33-year-old woman. The octuplets—the second known set to have been born in the U.S.—were nine weeks premature and were delivered in just five minutes via caesarean section.

The feat shifted from a source of biological wonderment to opprobrium as it was revealed that the mother, Nadya Suleman, an unemployed, graduate student who lives with her parents, already had six other children, ages two to seven. Many people expressed outrage that taxpayers had been footing the bill for her other children (through Social Security disability payments and food stamps) when the Associated Press estimated her medical costs could be $1.3 million in addition to the $2.7 million estimated to raise all 14 children to the age of 17. Police are even investigating death threats against her, and the once-celebrated mom of many has reportedly gone into hiding.

Although humans typically give birth to a single child per pregnancy, twins, triplets and larger "litters" have become more common with the rise of in vitro fertilization, the technique Suleman undertook whereby eggs are fertilized by sperm in a laboratory and later implanted into the uterus. Because of the cost of each implantation and the likelihood that one or more embryo will not survive, doctors typically implant three fertilized eggs into a woman's womb to up the chances of a viable birth. Some believe that Suleman's fertility doctor, Michael Kamrava, implanted more than that, violating professional ethical guidelines, although not breaking any U.S. laws. Kamrava, a general practitioner who lacks board certification for obstetrics and gynecology, has one of the worst records for successful in vitro implantations in the country, according to Forbes.

Economic and ethical considerations aside, humans are not biologically equipped to handle so many offspring per birth. Suleman's babies ranged in weight from 1.5 to three pounds (0.7 to 1.4 kilograms), and low-birth weight correlates with lower intelligence and a higher risk of developing type 2 diabetes, according to BBC reports. Premature infants, common in multiple births, face a greater risk of breathing problems, brain and organ damage and, later in life, developmental problems and cerebral palsy.

Meanwhile, other mammals, like dogs and rodents, are perfectly capable of having more than 10 babies in a litter. What makes them different from primates? To find out more about the evolution of litter size, we spoke to Robert Sikes, an evolutionary biologist at the University of Arkansas in Little Rock, who has studied the northern grasshopper mouse, an animal that has between one and six offspring at a time.

[An edited transcript of the interview follows]

How many offspring do most mammals have at a time? What about other animals?
Lots and lots of mammal species have singletons (a litter of one), and a few have large litters. The lab rat has litters of 10 or more, although part of that may have been a result of breeding in the lab. Many dogs will have litters in the mid-teens.

If you look at animals like salmon, they are going to produce hundreds of thousands of eggs. Many invertebrates are going to produce lots of eggs, as well.

How has variation in litter size evolved?
Organisms that have large litters or a large number of offspring tend to live in an unstable, boom-or-bust environment. They produce many offspring, and many don't make it. If you look at salmon, that's a great example: maybe a couple of eggs are going to survive. If the population is constant over time, each individual is replacing itself in the next generation. Whereas a female may produce 1,000 eggs, only two survive.

On the other hand, organisms that inhabit stable environments have fewer offspring and they allocate more energy to those offspring. Mammals certainly do this. Even within mammals, species that have large litters tend to inhabit more variable environments than those that inhabit more stable environments.

Those mammals that inhabit stable environments tend to have very altricial young: they are born naked with the eyes closed and they are helpless without the mom. Precocial young are well-furred at birth, eyes open, and don't need a lot of attention from mom.

What are some examples of this?
Cotton rats are small rodents common throughout the U.S. They are kind of a "weedy" species. Their litter size is highly variable. They have lots of young when resources are plentiful and smaller litters when resources are scarce.

On the other hand, wood rats have a much smaller and very stable litter sizes, but they allocate a heck of a lot more energy to each individual offspring. Cotton rats don't nurse as long and their pups are weaned at much smaller sizes and the size at which they are weaned is quite variable. Wood rats nurse for a longer period of time and are weaned at a larger size.

Humans are altricial and fit in pretty far toward the 'stable environment' side of the spectrum. We usually have a single offspring. That offspring is helpless at birth. It requires a long period of complete maternal dependence.

The evolutionary lineage we came from—the great apes—are similar in that respect.

So we are not built to have octuplets?
Humans are ill equipped to handle large litters. Evolution has simply not set us up to do that well.

Typically, litter size in nature is matched by the number of mammary glands we have. We have two. How do you nurse the additional offspring?

If you look at the energetic cost of offspring, as the litter size increases, the cost of nursing those young increases dramatically. In the grasshopper mice I work with, a mother rearing a litter of one is going to increase her food intake by 50 percent. A mouse mother rearing the maximum number of offspring, six, is going to triple her food intake. It takes a lot of energy to produce this milk. Do the mice compensate? Yes, but they cannot completely meet those needs. The mouse mother is eating three times as much and eating so much, she can't process any more food. These mice are at the upper end of their digestive efficiency during lactation, and the intestine lengthens to help extract as many nutrients as they can. Young from a litter of two are generally larger than animals from a litter of three and those are typically larger than a litter of six; the larger the young, the better the chance of survival. If the mother mouse is nursing that much longer, then she has to be foraging longer, and when mothers are out foraging they are subject to predation.

What happens in humans? If we were subject to natural selection, octuplets would be a tough load to bear. Obviously, with our technological abilities, we can step outside of those physical constraints. Human mothers rearing two, four, six and eight offspring are almost certainly not just nursing enough to meet the energetic demands of those young. [They will likely be using infant formula.]

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