Twinning has been around longer than we thought, according to new research led by Western Washington University.
Jack H. McBride & Tesla A. Monson performed a comprehensive investigation of primate litter size using life history data for 155 primate species, and litter size data for an additional 791 mammals. Image credit: Jason Brougham.
“Almost all primates give birth to single offspring,” said Western Washington University’s Professor Tesla Monson and Yale University Ph.D. student Jack McBride.
“However, some genera, including marmosets, tamarins, lemurs, lorises, and galagos, regularly give birth to twins and even triplets.”
“While humans almost always give birth to singletons, twin pregnancies do occur spontaneously at a global rate of around 1.1-1.5%.”
“With advances in assisted reproductive technologies, twinning has increased in some regions to a rate of about 3% over the last 50 years.”
“It has become imperative to understand the effects of twinning on pregnancies, mothers, and newborns.”
In the study, the authors collected reproductive and body size data for nearly 1,000 different mammalian species to investigate the evolutionary history of twinning in primates.
The traits they analyzed include litter size (number of offspring), gestation length, body size, and life span.
Contrary to previous assumptions, the analysis demonstrates that the earliest primates likely gave birth to twins.
The researchers also found that litter size is tightly linked to gestation length (length of the pregnancy).
“Animals that have larger litters on average, tend to be pregnant for a shorter amount of time,” Professor Monson said.
“This also applies to humans — here in the US, 38 weeks, rather than 40, is considered full-term for twins, and many twins are born even earlier.”
“This may be related to restrictions on maternal energy.”
“The next step is to investigate litter size more broadly across mammals, and to see which other reproductive, brain, and body size traits may be linked to twinning.”
“We are particularly interested in understanding the link between twinning and tooth morphology.”
“It’s always about the teeth for me.”
The study appears in the journal Humans.
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Jack H. McBride & Tesla A. Monson. 2024. The evolution of primate litter size. Humans 4 (3): 223-238; doi: 10.3390/humans4030014
This article is based on a press-release from Western Washington University.