Differing Thyroid Levels May Determine Behavioral differences in Bonobos and Chimpanzees

For both humans and chimpanzees, levels of the thyroid hormones in the body decline after reaching puberty. But new research reveals that the bonobo, while sharing a number of biological similarities with chimps and humans, does not experience a similar drop in thyroid levels after the onset of puberty. Instead, bonobos retain elevated thyroid concentrations well into their adult years, which researchers suspect may play a role in the primate's natural behavioral patterns as well as its mental capacity.

Researchers from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and the Royal Zoological Society of Antwerp in Belgium report that while chimpanzees and bonobos exhibit only minor behavioral differences during their first years of life, the fact that their behavioral patterns differ so remarkably in adulthood is probably linked to this difference in thyroid levels.

Compared to mature male chimpanzees, mature male bonobos are less aggressive, can carry out lasting friendships with females and receive life-long support from their mothers.

While in the womb, thyroid is responsible for brain and somatic growth as well as maturation. Later in life, thyroid hormones, specifically Triiodthyronin (T3) and Thyroxin (T4), influence somatic growth and the emergence of specific developmental stages such as puberty and adulthood.

The latest research suggests that the difference in aggression between chimps and bonobos has to do with bonobos retaining their juvenile behaviors until adulthood, which leads them to not develop the aggressive behavioral suite needed by chimpanzees living in aggressive, hierarchical social groups.

"Our study showed that male bonobos, who are known for their low levels for aggressive behavior, had higher thyroid hormone levels than females," said Verena Behringer of the Max Planck Institute for Evolutionary Anthropology. "High thyroid hormone levels likely reduce aggression in male apes."

Additionally, cognitive development is delayed in bonobos compared to chimpanzees. By the time a chimpanzee is 10 years old, its brain is already completely developed, the researchers said. But the delayed thyroid reduction observed in bonobos may be an clue to why bonobos attain their full cognitive development later in life.

"These are challenging results and we want to find out next, what the biological relevance of the high thyroid levels in bonobos is in detail," Behringer said. "As the thyroid hormone decline in humans is between the one of bonobos and chimpanzees it remains to be seen which of the two species represents the original thyroid rhythm, that is whether the chimpanzee is early ripe or the bonobo a late bloomer."