Mandrills miscarry more often when nursing infant, study finds

Female mandrills are significantly more likely to experience pregnancy loss when they are simultaneously nursing a dependent infant, according to an 11-year observational study published in PLOS Biology. The research, led by Nikolaos Smit of the Max Planck Institute for Evolutionary Anthropology and the University of Turku, suggests that this pattern may reflect a physiological trade-off between the demands of current offspring and future offspring. The study observed a wild population of mandrills (Mandrillus sphinx) in Southern Gabon, providing a long-term view of how these primates manage competing reproductive pressures in their natural environment.

Pregnancy and nursing are both physically demanding processes that require substantial energy allocation. When a female mammal is pregnant while still caring for an infant, she faces a conflict between investing resources in the existing child and the developing fetus. To investigate how females adapt to these conflicting physiological demands, the research team monitored the behavior and reproductive histories of a wild mandrill population over an extended period.

Mandrills breed seasonally, and females typically conceive every other year. However, they can sometimes become pregnant again while still caring for a previous infant. The researchers were able to identify pregnant females by visible physical swellings, which enabled them to record the outcomes of each pregnancy. Over the course of the 11-year study, the team observed 356 pregnancies across 117 individual females. During this period, they documented 52 instances of fetal loss, providing a substantial dataset to analyze the factors influencing pregnancy outcomes.

The analysis revealed that fetal loss was more common in females that were simultaneously caring for an infant. This association suggests that the energetic costs of nursing may contribute to the termination of a subsequent pregnancy. The findings indicate that the body may prioritize the survival of the current offspring when resources are limited, effectively adjusting maternal investment to maximize the chances of the existing infant reaching independence. This physiological response may serve as a mechanism to reduce the physical costs to the mother when she is unable to sustain a new pregnancy.

Infant behavior also played a role in the likelihood of fetal loss. Females were more likely to experience pregnancy loss when their infant threw more tantrums, which were characterized by distinctive distress vocalizations.

Social dynamics within the mandrill group also appeared to impact pregnancy outcomes. The researchers found that females were more likely to experience fetal loss when there were more alpha male takeovers in their social group during the pregnancy. 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, the authors say.

Interestingly, females who experienced fetal loss tended to conceive again sooner than those whose infants died soon after birth. Despite the initial loss, these females produced more surviving offspring overall compared to females who lost an infant postnatally. In the long term, females that lost a pregnancy produced a similar number of surviving offspring as females that had not. This suggests that while fetal loss may be a costly event, it does not necessarily reduce the lifetime reproductive success of the mother. Instead, it may allow her to reproduce again sooner once her current offspring has reached independence, thereby maintaining her overall fitness.

The study’s findings challenge the traditional view of fetal loss as a simple reproductive failure. Instead, the authors propose that it may be a strategic adjustment in maternal investment, enabling mothers to focus on the survival of their current offspring when resources are scarce. The research provides valuable insights into the physiological and social factors that influence pregnancy outcomes in wild primates, with potential implications for understanding reproductive trade-offs in other mammalian species.

The observational design of the study allowed for the collection of detailed behavioral and demographic data over a long period, which is essential for uncovering subtle links between infant behavior, maternal investment, and subsequent reproduction. However, as an observational study, it cannot establish causation. The associations found between nursing, infant tantrums, alpha male takeovers, and fetal loss may be influenced by other unmeasured variables.

Subject of Research: Physiological trade-offs in mandrill reproduction involving simultaneous nursing and pregnancy

Article Title: Pregnant monkeys are more likely to miscarry when also nursing an infant

Article References: Pregnant monkeys are more likely to miscarry when also nursing an infant. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: mandrill, fetal loss, pregnancy, nursing, reproductive trade-off, wild primates, Southern Gabon, observational study, infant behavior, alpha male takeover, PLOS Biology, Max Planck Institute, University of Turku, maternal investment, reproductive success, physiological demand, social dynamics, life history theory, distress vocalizations, seasonal breeding