New research suggests that miscarriage may help mothers to balance their investment in current and future offspring
A female mandrill nurses her infant while resting, surrounded by lush vegetation.
© Berta Roura-Torres
To the point
- Infant presence and fetal loss: Female mandrills that become pregnant while still caring for dependent offspring are more likely to miscarry.
- Infant tantrums: The risk of fetal loss is particularly high when dependent offspring exhibit more protest behaviours, such as making persistent demands for maternal care.
- Balancing maternal investment: Fetal loss may allow females to adjust their investment in current and future offspring.
- Long-term wild primate data: The study draws on 11 years of monitoring the only habituated population of wild mandrills, including 356 pregnancies in 117 females.
A new study shows that female mandrills are more likely to miscarry when they become pregnant while still caring for a dependent young offspring. Led by scientists from the Institute of Evolutionary Science in Montpellier, the University of Turku and the Max Planck Institute for Evolutionary Anthropology, the research suggests that fetal loss may sometimes be part of a broader reproductive strategy, helping females balance their investment in current and future offspring.
In mammals, mothers face a fundamental challenge: how to divide their limited energy and resources between existing offspring and those yet to be born. This trade-off is particularly demanding when a female becomes pregnant while her previous infant is still dependent on her for milk, protection and care. Such conflicts are difficult to study in primates because they involve the mother's reproductive history, her current physical condition, and the behaviour of her dependent offspring.
Fetal loss occurs in an estimated 10 to 20 per cent of primate pregnancies, including those of humans and non-human primates. Although it is often considered a form of reproductive failure, the causes and consequences of fetal loss in natural populations are not well understood.
Combining long-term reproductive and behavioural data
To investigate the relationship between maternal investment, infant behaviour and pregnancy outcomes, researchers analysed 356 pregnancies in 117 wild female mandrills that had been monitored for 11 years. This study has been based on the only habituated population of wild mandrills, enabling the researchers to observe individuals over many years and record their reproductive outcomes and detailed social behaviour.
The team found that females who conceived while still caring for dependent offspring were more likely to experience miscarriage than females without dependent young. Pregnant females with dependent offspring were also more likely to reject their infants' demands for care. Notably, fetal loss was more likely when the offspring exhibited more protest behaviours.
"We combined over a decade's worth of observations on reproductive histories with non-invasive observations of pregnancies, as well as detailed behavioural observations of mothers and infants," says lead author Nikolaos Smit, a postdoctoral fellow at the University of Turku who was formerly based at the Max Planck Institute for Evolutionary Anthropology. "This unique combination of long-term demographic and behavioural data enabled us to reveal connections between infant behaviour, maternal investment and subsequent reproduction that would otherwise have been challenging to identify."
Reproductive failure or adaptive adjustment?
The study also examined the reproductive consequences of miscarriage. In the medium term, females that lost a fetus experienced better reproductive outcomes than females that gave birth and then lost their newborn. Across several species, when females produce two offspring close together, the second offspring is often at high risk of dying before being weaned. Among mandrills, females that had experienced fetal loss ultimately produced more offspring that survived to adulthood and had shorter intervals between births than females whose infants died after birth.
These findings suggest that fetal loss may be a physiological response to an early pregnancy, when the mother is still heavily investing in a dependent offspring. Rather than merely representing reproductive failure, miscarriage may reduce the costs associated with carrying a pregnancy that is unlikely to result in a successfully weaned young.
"Fetal loss is typically considered a form of reproductive failure," says Marie Charpentier, director of the Mandrillus Project, which provided the data for this study. "Our findings suggest that, when females are already investing heavily in a dependent infant, this may be a way of adjusting investment between current and future offspring."
Mother-offspring conflict in the wild
The results also highlight the importance of infant behaviour. Offspring who protest more may impose higher energetic or social costs on their mothers, particularly when the mother is pregnant. Such protest behaviours may reflect a conflict of interests between mother and offspring: the young continue to demand care, while the mother may benefit from reducing her investment in them in order to focus on future reproduction.
By linking infant protest behaviours to fetal loss, the study sheds new light on the potential connection between behavioural interactions between mothers and offspring, and physiological reproductive outcomes. Fetal loss could, therefore, paradoxically help to limit the consequences of reproducing too rapidly. By ending a pregnancy that conflicts with the investment required for raising a dependent offspring, females may be able to protect their existing young while preventing long delays in their future reproduction.
