Bonobos Stay Younger for Longer: Thyroid Hormones Trump Chimps

While bonobos and chimpanzees are similar at birth, they very quickly develop different behavioral patterns later in life. Now, scientists may have found out why. They've discovered that bonobos retain an elevated hormone level which changes their behavior for years to come.

The two species show minor differences in their behavior during their first years of life. Yet these differences become more and more apparent once they reach adulthood. Male bonobos, for example, are less aggressive, engage in lasting friendships with females and receive life-long support from their mothers. In contrast, the social structure of chimps consists of a mixture of male-male cooperation and aggressive behavioral strategies in males that aim at maintaining a high social status.

In the past, some researchers related this difference in aggression to the dominance relations between males and females. Others, in contrast, suggested that bonobos retain juvenile behaviors until adulthood and thus do not develop the behavioral suite that characterizes chimps.

In order to better understand these behaviors, the researchers examined thyroid hormones from urine samples of zoo-living chimps and bonobos. They found that bonobos retain elevated thyroid hormone concentrations well into adulthood while in humans and chimps, thyroid hormone concentrations decline after puberty. This late decline of thyroid hormones may be what is responsible for the bonobo behavior.

The thyroid hormones Triiodthyronin (T3) and Thyroxin (T4) influence the ontogenetic development in both humans and animals. Prenatally, these hormones are responsible for brain and somatic growth as well as maturation. Later, they influence somatic growth, puberty and adulthood. While the thyroid hormone pattern of western modern humans and chimps was very similar, samples of bonobos differed.

"Our study showed that male bonobos who are known for their low levels of aggressive behavior, had higher thyroid hormone levels than females," said Verena Behringer, one of the researches, in a news release. "High thyroid hormone levels likely reduce aggression in male apes."

The findings reveal exactly how much of an influence hormones have on behavior. In addition, it reveals that the cognitive development is delayed in bonobos as compared to chimpanzees; in other words, bonobos stay younger for longer. Currently, researchers hope to investigate what the "original" thyroid rhythm was and find out whether bonobos are late bloomers or chimpanzees mature early.

The findings are published in the Journal of Human Evolution.