Scientists have long noted that marmo-sets are unusual among primates for their doting fathers. Chimeras may be the source of their attention. It's possible, Ross suggests, that chimeras give off a wider range of odors that signify that they are related to a father, increasing his attention.
The results of the marmoset study appeared last week in The Proceedings of the National Academy of Sciences.
"It's potentially a really interesting finding," said David Haig, an evolutionary biologist at Harvard. Haig is particularly intrigued by the genetic relatedness of chimeric twins. On average, two normal fraternal twins would share about half their genes. Marmoset twins are more closely related because they have more genes in common.
That common bond might drive the evolution of unusual kinds of behavior. Haig notes that in a band of marmosets, only the oldest female can reproduce, while the younger individuals help raise her offspring.
Ross, who now works at the South Texas Centers for Biology in Medicine, is continuing to study the marmosets to understand the effects of chimerism. The questions she hopes to answer are not just scientific, but philosophical.
"This changes how we think of marmosets as individuals, but it also changes how we think of the term at all," she said. A male mates with a female, who gives birth to his brother's offspring. "But most of his body also has his brother's genes. So what is he as an individual?"
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