The Bruce effect, or pregnancy block, is the tendency for female rodents to terminate their pregnancies following exposure to the scent of an unfamiliar male. The effect was first noted in 1959 by Hilda M. Bruce, and has primarily been studied in laboratory mice (Mus musculus). In mice, pregnancy can only be terminated prior to embryo implantation, but other species will interrupt even a late-term pregnancy.
The Bruce effect is also observed in deer-mice, meadow voles, collared lemmings, geladas, and it has also been proposed, but not confirmed, in other non-rodent species such as the lion.
In an experiment published in 1959, zoologist Hilda Bruce of the National Institute for Medical Research in London housed pregnant mice with male mice that were not the father of the carried embryo. As a result, the rate of miscarriages increased, followed by mating with the new male. No increased rate of miscarriages occurred when pregnant mice were paired with castrated or juvenile male mice. The effect remained when the male mice were kept out of sight or hearing of the females. This suggested that females were distinguishing the males by smell. To test this hypothesis, Bruce and her colleague Alan Parkes recruited perfumers to smell pieces of cloth from the mouse cages. The perfumers could distinguish the smells of different mouse strains.
Mechanisms of action
Detection of pheromones
The vomeronasal system serves as a “vascular pump” that, stimulated by the presence of a novel male, actively draws in substances. Male mouse urine contains MHC class I peptides that bind to receptors in the female’s vomeronasal organ, a mucus-filled structure in the nasal septum. These chemical signals, which are specific to each male, are learned by the female during mating, or shortly after. The hormone vasopressin is crucial in coupling a chemosensory cue with an appropriate physiological response. When the vasopressin 1b receptor gene is knocked out in females, the presence of an unfamiliar male does not trigger pregnancy disruption.
Recognizing familiar males
Exposure to a male’s urinal pheromones will activate a neuroendocrine pathway leading to pregnancy failure. However, if the pheromones correspond with those memorized by the female (usually the male mating partner), a release of noradrenaline will lower the receptivity of the accessory olfactory bulb to these pheromones. The pregnancy disruption will, thus, be averted. This role for noradrenaline has recently been called into question. The hormone oxytocin is also important in this social memory process. Females treated with an oxytocin antagonist are unable to recognize the urinary scent of their mate, and will terminate pregnancy when exposed to any male, known or unknown.
The activation of vomeronasal neuron receptors by male pheromones triggers a complex neuroendocrine pathway. The pheromonal information travels via nerves to the accessory olfactory bulb, and then to the corticomedial amygdala, accessory olfactory tract, and stria terminalis. These areas stimulate the hypothalamus to increase the release of dopamine, which thus prevents the secretion of prolactin from the anterior pituitary. In the absence of prolactin, an essential hormone for maintaining the corpus luteum, luteolysis takes place. As the corpus luteum can no longer release progesterone, the uterus remains unprimed for embryo implantation, and the pregnancy fails.
Role of estrogens
Androgens and estrogens, particularly estradiol (E2), are also crucial chemosignals regulating the Bruce Effect. However, they are believed to act via a separate pathway to that discussed above. Small steroid molecules such as E2 can enter the bloodstream directly via nasal ingestion and travel to the uterus, which has a high density of suitable receptors. Normally, E2 is essential in preparing both the blastocyst and uterus for implantation. However, excessive E2 will prevent implantation from taking place. Castrated males are incapable of terminating female pregnancies, except when castrated males are given testosterone. estradiol, a metabolic product of testosterone, is known to disrupt pregnancy in females, and is present in male urine.
The incidence of the Bruce Effect depends on the timing of pheromone exposure. Post-mating, females experience twice-daily surges of prolactin. Pregnancy is only terminated if exposure to novel male scent coincides with two prolactin surges, one of these occurring in a daylight period.
When given the opportunity, male mice tend to direct their urine in the female’s direction. This allows males to improve their fitness success by “sabotaging” the pregnancy of a male competitor, and more quickly returning the female to estrus. The Bruce Effect can also aid in maintaining social status, with dominant males leaving more urinal scent markings, and so blocking the pregnancies initiated by subordinate males.
Females can control their likelihood of terminating pregnancy by pursuing or avoiding novel male contact during their most susceptible periods. In this way, females can exert a post-copulatory mate choice, reserving their reproductive resources for the highest-quality male. Certainly, females are more likely to seek proximity to dominant males. In many rodent species, males kill unrelated young; pregnancy block may avoid the wasted investment of gestating offspring likely to be killed at birth. The Bruce Effect is most common in polygynous rodent species, for which the risk of infanticide is highest.
- Heske, E. J.; Nelson, RJ (1984). "Pregnancy interruption in Microtus ochrogaster: Laboratory artifact or field phenomenon?". Biology of Reproduction 31 (1): 97–103. doi:10.1095/biolreprod31.1.97. PMID 6380603.
- Hofmann, J. E.; Getz, L. L.; Gavish, L. (1987). "Effect of Multiple Short-Term Exposures of Pregnant Microtus ochrogaster to Strange Males". Journal of Mammalogy 68 (1): 166–169. doi:10.2307/1381067. JSTOR 1381067.
- Becker, Stuart D.; Hurst, Jane L. (2008). "Pregnancy Block from a Female Perspective". Chemical Signals in Vertebrates 11 (3): 141–50. doi:10.1007/978-0-387-73945-8_13. ISBN 978-0-387-73944-1.
- Bruce, Hilda M. (1959). "An Exteroceptive Block to Pregnancy in the Mouse". Nature 184 (4680): 105. Bibcode:1959Natur.184..105B. doi:10.1038/184105a0. PMID 13805128.
- Labov, J. B. (1981). "Pregnancy Blocking in Rodents: Adaptive Advantages for Females". The American Naturalist 118 (3): 361–371. doi:10.1086/283828. JSTOR 2460637.
- Eleftheriou, Basil E.; Bronson, F. H.; Zarrow, M. X. (1962). "Interaction of Olfactory and Other Environmental Stimuli on Implantation in the Deer Mouse". Science 137 (3532): 764. Bibcode:1962Sci...137..764E. doi:10.1126/science.137.3532.764. PMID 13889805.
- Clulow, F. V.; Langford, P. E. (1971). "Pregnancy-Block in the Meadow Vole, Microtus Pennsylvanicus". Reproduction 24 (2): 275–7. doi:10.1530/jrf.0.0240275.
- MALLORY, F. F. (1980). "Infanticide and Pregnancy Failure: Reproductive Strategies in the Female Collared Lemming (Dicrostonyx groenlandicus)". Biology of Reproduction 22 (2): 192–6. doi:10.1095/biolreprod22.2.192.
- Eila K. Roberts, Amy Lu, Thore J. Bergman, Jacinta C. Beehner (2012). "A Bruce Effect in Wild Geladas". Science 335 (6073): 1222–1225. Bibcode:2012Sci...335.1222R. doi:10.1126/science.1213600.
- Packer, C.; Pusey, A. E. (1983). "Adaptations of Female Lions to Infanticide by Incoming Males". The American Naturalist 121 (5): 716–728. doi:10.1086/284097. JSTOR 2460874.
- MRC National Institute for Medical Research (2014). A Century of Science and Health. MRC National Institute for Medical Research. p. 208.
- Guzzo, A. C; Berger, R. G; Decatanzaro, D. (2009). "Excretion and binding of tritium-labelled oestradiol in mice (Mus musculus): Implications for the Bruce effect". Reproduction 139 (1): 255–63. doi:10.1530/REP-09-0382. PMID 19793839.
- Zufall, Frank; Leinders-Zufall, Trese (2007). "Mammalian pheromone sensing". Current Opinion in Neurobiology 17 (4): 483–9. doi:10.1016/j.conb.2007.07.012. PMID 17709238.
- Brennan, Peter A. (2009). "Outstanding issues surrounding vomeronasal mechanisms of pregnancy block and individual recognition in mice". Behavioral Brain Research 200 (2): 287–94. doi:10.1016/j.bbr.2008.10.045. PMID 19071163.
- Brennan, Peter A.; Zufall, Frank (2006). "Pheromonal communication in vertebrates". Nature 444 (7117): 308–15. Bibcode:2006Natur.444..308B. doi:10.1038/nature05404. PMID 17108955.
- Wersinger, S. R.; Temple, J. L.; Caldwell, H. K.; Young, W. S. (2007). "Inactivation of the Oxytocin and the Vasopressin (Avp) 1b Receptor Genes, but Not the Avp 1a Receptor Gene, Differentially Impairs the Bruce Effect in Laboratory Mice (Mus musculus)". Endocrinology 149 (1): 116–21. doi:10.1210/en.2007-1056. PMC 2194605. PMID 17947352.
- Neumann, I. D. (2008). "Brain Oxytocin: A Key Regulator of Emotional and Social Behaviors in Both Females and Males". Journal of Neuroendocrinology 20 (6): 858–65. doi:10.1111/j.1365-2826.2008.01726.x. PMID 18601710.
- Rosser, A. E.; Remfry, C. J.; Keverne, E. B. (1989). "Restricted exposure of mice to primer pheromones coincident with prolactin surges blocks pregnancy by changing hypothalamic dopamine release". Reproduction 87 (2): 553–9. doi:10.1530/jrf.0.0870553.
- Valbuena, Diana; Martin, Julio; De Pablo, Jose Luis; Remohı́, José; Pellicer, Antonio; Simón, Carlos (2001). "Increasing levels of estradiol are deleterious to embryonic implantation because they directly affect the embryo". Fertility and Sterility 76 (5): 962–8. doi:10.1016/S0015-0282(01)02018-0. PMID 11704118.
- Ma, W.-g.; Song, H; Das, SK; Paria, BC; Dey, SK (2003). "Estrogen is a critical determinant that specifies the duration of the window of uterine receptivity for implantation". Proceedings of the National Academy of Sciences 100 (5): 2963–8. Bibcode:2003PNAS..100.2963M. doi:10.1073/pnas.0530162100. PMC 151449. PMID 12601161.
- Bruce, H. M. (1965). "Effect of Castration on the Reproductive Pheromones of Male Mice". Reproduction 10 (1): 141–3. doi:10.1530/jrf.0.0100141. PMID 14337805.
- Decatanzaro, Denys; Khan, Ayesha; Berger, Robert G.; Lewis, Elaine (2009). "Exposure to developing females induces polyuria, polydipsia, and altered urinary levels of creatinine, 17β-estradiol, and testosterone in adult male mice (Mus musculus)". Hormones and Behavior 55 (1): 240–7. doi:10.1016/j.yhbeh.2008.10.013. PMID 19027019.
- Huck, U. W. (1982). "Pregnancy block in laboratory mice as a function of male social status". Reproduction 66 (1): 181–4. doi:10.1530/jrf.0.0660181.
- Desjardins, Claude; Maruniak, J. A.; Bronson, F. H. (1973). "Social Rank in House Mice: Differentiation Revealed by Ultraviolet Visualization of Urinary Marking Patterns". Science 182 (115): 939–41. Bibcode:1973Sci...182..939D. doi:10.1126/science.182.4115.939. PMID 4745598.
- Becker, S. D; Hurst, J. L (2009). "Female behaviour plays a critical role in controlling murine pregnancy block". Proceedings of the Royal Society B: Biological Sciences 276 (1662): 1723–9. doi:10.1098/rspb.2008.1780. PMC 2660991. PMID 19324836.
- Schwagmeyer, P. L. (1979). "The Bruce Effect: An Evaluation of Male/Female Advantages". The American Naturalist 114 (6): 932–938. doi:10.1086/283541. JSTOR 2460564.
- Pillay, Neville A.; Kinahan, Anouska A. (2009). "Mating strategy predicts the occurrence of the Bruce effect in the vlei rat Otomys irroratus". Behaviour 146 (1): 139–51. doi:10.1163/156853908X390968.
- de la Maza, Helen M.; Wolff, Jerry O.; Lindsey, Amber (1999). "Exposure to strange adults does not cause pregnancy disruption or infanticide in the gray-tailed vole". Behavioral Ecology and Sociobiology 45 (2): 107. doi:10.1007/s002650050544. JSTOR 4601583.
- Kenney, A. McM.; Evans, R. L.; Dewsbury, D. A. (1977). "Postimplantation pregnancy disruption in Microtus ochrogaster, M. Pennsylvanicus and Peromyscus maniculatus". Reproduction 49 (2): 365–7. doi:10.1530/jrf.0.0490365. PMID 321775.
- Mahady, Scott; Wolff, Jerry (2002). "A field test of the Bruce effect in the monogamous prairie vole (Microtus ochrogaster)". Behavioral Ecology and Sociobiology 52 (1): 31–7. doi:10.1007/s00265-002-0484-0. JSTOR 4602102.
- Fraser-Smith, A. (1975). "Male-induced pregnancy termination in the prairie vole, Microtus ochrogaster". Science 187 (4182): 1211–3. doi:10.1126/science.1114340. PMID 1114340.
- Stehn, R. A.; Jannett, F. J. (1981). "Male-Induced Abortion in Various Microtine Rodents". Journal of Mammalogy 62 (2): 369–372. doi:10.2307/1380713. JSTOR 1380713.
- Storey, Anne E.; Snow, Dianne T. (1990). "Postimplantation pregnancy disruptions in meadow voles: Relationship to variation in male sexual and aggressive behavior". Physiology & Behavior 47 (1): 19–25. doi:10.1016/0031-9384(90)90037-5. PMID 2183249.
- Storey, Anne E. (2010). "Pre-implantation Pregnancy Disruption in Female Meadow Voles Microtus pennsylvanicus (Rodentia: Muridae): Male Competition or Female Mate Choice?". Ethology 98 (2): 89–100. doi:10.1111/j.1439-0310.1994.tb01060.x.
- Wolff, Jerry O. (2003). "Laboratory Studies with Rodents: Facts or Artifacts?". BioScience 53 (4): 421–7. doi:10.1641/0006-3568(2003)053[0421:LSWRFO]2.0.CO;2. ISSN 0006-3568.