The Bruce Effect

Infanticide and feticide are two unusual occurrences that are observable in many mammalian species (Roberts et al., 2012; Zipple, 2020). The killing of infants or fetuses is usually a tool nonsire males use to bring a female back to fertility faster to mate with them instead of allowing their male competitor’s offspring to wean from their mothers first (Roberts et al., 2012; Zipple, 2020). This evolutionary tool is countered with numerous strategies females employ to combat the sexual and reproductive conflict (Roberts et al., 2012; Zipple, 2020). One of these tools is the Bruce effect. 

The Bruce effect is where a female will terminate her current fetus after contact with a nonsire male (Roberts et al., 2012). This phenomenon has been observed in laboratory experiments but has only recently been observed in wild primates, specifically wild geladas (Roberts et al., 2012). In gelada society, males fight for the mating rights to large harems of females, and researchers found that the dominant male is replaced every 2.37 years on average (Roberts et al., 2012). This replacement is followed by infanticide of all infants sired by the previous reigning male (Roberts et al., 2012). So, if the Bruce effect is also observed following the replacement of a dominant male, it begs the question: why would the female ever abort her own offspring if reproduction is a key component of the female’s overall fitness?

The evolutionary advantages of the Bruce effect for a female become clear when thinking about physiology in a larger context. Reproduction is a huge physiological investment, especially for females (Roberts et al., 2012; Zipple, 2020). Hormonal influxes cause numerous energy-expensive effects throughout the female’s body: blood volume and cardiac output increase also increasing resting heart rates, glomerular functions increase, all endocrine glands functions increase, etc. (Soma-Pillay et al., 2016). Essentially much of the female’s metabolism becomes directed towards the developing fetus. Effectively, aborting a current fetus saves the female energy and resources rather than carrying the offspring to term only for it to be killed by the nonsire male (Roberts et al., 2012; Zipple, 2020). This also allows her to return to fertility faster to mate with the new male and so a higher chance of developing, nursing, and weaning new offspring before the dominant male is also replaced (Roberts et al., 2012; Zipple, 2020). 

The physiological mechanism of the Bruce effect is still unknown (Roberts et al., 2012; Zipple, 2020). In laboratory experiments with rats, gonadotropin was released by the females to decrease prolactin secretion (Roberts et al., 2012). Prolactin is a hormone that is involved in stimulating lactation and is also involved in other pregnancy-related mechanisms (Soma-Pillay et al., 2016). In wild geladas, the proximate mechanism is not known and there is some speculation that the usually stressful shift in power from one male to another could trigger the Bruce effect to occur (Roberts et al., 2012). There is a call for further research to understand the physiological triggers and changes that occur in a female undergoing the Bruce effect. I am curious if and/or how stress levels play a role and if other species that are capable of the Bruce effect terminate pregnancies in similar or different ways to the geladas. 

Roberts, E. K., Lu, A., Bergman, T. J., & Beehner, J. C. (2012). A Bruce effect in wild geladas. Science (New York, N.Y.), 335(6073), 1222–1225. https://doi.org/10.1126/science.1213600

Soma-Pillay, P., Nelson-Piercy, C., Tolppanen, H., & Mebazaa, A. (2016). Physiological changes in pregnancy. Cardiovascular journal of Africa, 27(2), 89–94. https://doi.org/10.5830/CVJA-2016-021

Zipple, M. N. (2020). When will the Bruce effect evolve? The roles of infanticide, feticide and maternal death. Animal Behaviour, 160, 135–143. https://doi-org.dml.regis.edu/10.1016/j.anbehav.2019.11.014