Pregnant monkeys more likely to miscarry while also nursing infant

By Stephen Beech

Pregnant monkeys are more likely to miscarry when also nursing an infant, reveals new research.

Infant tantrums also increased the risk of mandrills losing an unborn baby, according to the findings of the 11-year observational study.

But females who experienced fetal loss tended to conceive again sooner and produced more surviving offspring overall, compared with mothers whose babies died soon after birth.

Scientists explained that pregnancy and nursing are “physically demanding” tasks, so female mammals must allocate their resources carefully to maximize their reproductive success.

When a pregnant female is simultaneously nursing an infant, it creates a “trade-off” between the needs of their current offspring and their developing fetus.

To investigate how females adapt to the conflicting physiological demands, an international research team observed the behavior and reproductive lives of a wild population of mandrill monkeys in Gabon.

Mandrills breed seasonally and females usually conceive every other year, but can sometimes become pregnant again while still caring for an infant.

Pregnant female mandrills have visible swellings, enabling the research team to record both successful and failed pregnancies.

During the 11-year study, they observed 356 pregnancies across 117 females, involving 52 instances of fetal loss.

Fetal loss was more common in females that were simultaneously caring for an infant, according to the findigs published in the journal PLOS Biology.

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Females were also more likely to experience fetal loss when their infant threw more “tantrums” — characterized by distinctive distress vocalizations, or when there were more alpha male takeovers in their social group during the pregnancy.

However, females who experienced fetal loss tended to conceive again sooner, and produced more surviving offspring overall, compared with females whose infants died soon after birth.

In the long term, females that lost a pregnancy produced a similar number of surviving offspring as females that had not.

The research team says their findings suggest that fetal loss may be a physiological response to the extra energetic demands of simultaneous pregnancy and nursing, enabling mothers to focus on the survival of their current offspring.

It may also reduce the physical costs to the mother when she is unable to sustain a pregnancy, enabling her to reproduce again sooner once her current offspring has reached independence.

The researchers say the correlation between alpha male changeover and fetal loss could represent an adaptation to the increased risk of infanticide by new alpha males, or may reflect increased stress for females during times of social upheaval.

Study first author Nikolaos Smit said: “We combined 11 years of reproductive histories with non-invasive observations of pregnancy and detailed behavioral observations of mothers and infants.

“This unique combination of long-term demographic and behavioral data allowed us to uncover links between infant behavior, maternal investment, and subsequent reproduction that would otherwise be difficult to detect.”

Smit, from the University of Turku, Finland, added: “Females who conceive while caring for a dependent infant are more likely to experience fetal loss, especially when this infant has been showing more tantrum behavior.”

Senior author Marie Charpentier said: “Fetal loss is usually seen as a reproductive failure.

“Our findings suggest that, when females are already investing heavily in a dependent infant, it may instead be a way of adjusting investment between current and future offspring.”