High levels of a key thyroid hormone may be the reason why bonobos are peaceful creatures, scientists have found.
Despite the fact that chimpanzees and bonobos share similar starting conditions at birth they develop different behavioural patterns later in life.
Male bonobos are less aggressive, engage in lasting friendships with females and receive life-long support from their mothers.
In contrast, the social network of male chimpanzees consists of a mixture of male-male cooperation and aggressive behavioural strategies in males that aim on gaining and maintaining high social status.
Scientists from the Max Planck Institute for Evolutionary Anthropology have now found that bonobos retain elevated thyroid hormone concentrations well into adulthood, whereas in humans and chimpanzees thyroid hormone concentrations decline after puberty.
The late decline of thyroid hormones in bonobos might have consequences on their behaviour and might also indicate a delayed development of their mental capacities.
Triiodothyronine (T3) is a hormone in the thyroid gland which is crucial for development in all vertebrates.
For their study, researchers took urine samples from around 200 zoo-living individuals ranging between one and 56 years of age.
Hormone measures showed that the thyroid hormone pattern of western modern humans and chimpanzees was very similar, with high levels before puberty and a decrease of this hormone during and after puberty.
The samples of bonobos differed: The concentration of thyroid hormone T3 remained high well until adulthood.
In other words, compared to chimpanzees, bonobos retain the elevated levels of thyroid hormone which are a characteristic trait for young individuals for a longer time and experience decline of thyroid hormones late in life.
"Our study showed that male bonobos, who are known for their low levels for aggressive behaviour, 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," Behringer said.
Moreover, psychological studies indicate that the cognitive development is delayed in bonobos as compared to chimpanzees.
It is known that by the age of ten years the growth of a chimpanzee's brain volume and bones has already been finished. The late decline of thyroid hormones in the urine of bonobos might thus show that these animals' mental capacities start developing later in life, researchers said.
The study was published in the Journal of Human Evolution.
