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Key points
- After a young Bonobo was born, the older siblings made more body contact with their mothers.
- Levels of urinary cortisol in older siblings also increased an average of five times their normal level.
- Getting less attention from the mother, and other changes, may be responsible for the increased stress levels.
Data from studies of captive great apes abound, but information from research on wild great apes, while ever-growing, often doesn't offer the detailed information that can be gathered by studies of individuals who live highly controlled lives in all sorts of cages where movement is limited, group size and composition don't reflect the ways in which their wild relatives live, and humans interfere in their lives. An essay summarizing research on wild bonobos called "Adjusting to a New Sibling Is Highly Stressful for Bonobo Infants" caught my attention.1
Bonobos, formerly called pygmy chimpanzees, display a wide array of unique behavior patterns including female-run social groups, frontal sex, and sex for fun and pleasure. Females also give birth to children when there are older siblings still dependent on care from them. This sets up a situation for researchers to learn if older siblings are affected by the presence of younger ones. To do so, Verena Behringer and her colleagues studied a wild population of bonobos living in Salonga National Park in the Democratic Republic of the Congo. The behavior of 20 female and six male youngsters was studied when their mothers gave birth and their urine was sampled to measure levels of the stress hormone cortisol and neopterin, an indicator of immunity.
These data from wild bonobos are highly unique. All in all, the researchers learned that after a young sibling was born, the older children made more body contact with their mothers, possibly to gain their attention or to learn more about this new individual. Levels of urinary cortisol also suddenly and significantly increased an average of five times their normal level and remained high for around seven months before they returned to a normal level. Most of the older siblings had already been weaned, so the increase in cortisol indicating higher stress wasn't because the older siblings were forced to stop nursing.
Behringer and her colleagues write, "Our data from wild bonobos demonstrate that sibling birth induces a physiological stress response in the older offspring that is independent of the older offspring’s age." They also "speculate that the overall change in their environment, such as less attention by the mother, change in their social environment and other new situations, may be responsible for the increased stress levels."
It's not known if high stress had any long-term effect on older sibs but levels of neopterin suddenly decreased when new bonobos were born. Some studies of chimpanzees and baboons showed that older sibs had reduced growth and shorter lifespans when younger siblings were around, perhaps because they were stressed.
How the information from this unique study of bonobos relates to what happens in humans isn't clear because many young humans benefit from care from individuals other than mom. However, not only are the data themselves important, but they also show why it's essential to study wild animals to complement what is observed in captivity where individuals, including moms and their kids, may be subjected to a wide variety of unnatural situations and manipulations.
References
1) Detailed information about methodology and data can be found in "Transition to siblinghood causes substantial and long-lasting physiological stress reactions in wild bonobos." The abstract for this essay reads: "In mammals with a slow ontogeny, the birth of a sibling marks a major developmental transition. Behavioral studies suggest that this event is stressful for the older offspring, but physiological evidence for this is lacking, and it remains unknown whether the birth of a sibling is stressful beyond mere weaning stress. Studying transition to siblinghood in wild bonobos, we investigated physiological changes in urinary cortisol (stress response), neopterin (cell-mediated immunity), and total triiodothyronine (metabolic rate), and related them to behavioral changes in mother-infant relationship and feeding (suckling, riding, proximity, body contact, independent foraging). With sibling’s birth, cortisol levels increased fivefold in the older offspring and remained elevated for seven months, independent of age. This was associated with diminished immunity but not with behavioral or metabolic changes. Our results indicate that transition to siblinghood is stressful beyond nutritional and social weaning and suggest that this effect is evolutionary old."
