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Pheromones and the “Bruce effect” in wild or domesticated horses and primates

Posted on March 14, 2012 by James Kohl.

Subscription required to read full report from Science: A Bruce Effect in Wild Geladas

Abstract: Female rodents are known to terminate pregnancies after exposure to unfamiliar males (“Bruce effect”). Although laboratory support abounds, direct evidence for a Bruce effect under natural conditions is lacking. Here, we report a strong Bruce effect in a wild primate, the gelada (Theropithecus gelada). Female geladas terminate 80% of pregnancies in the weeks after a dominant male is replaced. Further, data on interbirth intervals suggest that pregnancy termination offers fitness benefits for females whose offspring would otherwise be susceptible to infanticide. Taken together, data support the hypothesis that the Bruce effect can be an adaptive strategy for females.

My comments on the article can be read at the site linked above and here:

(Kotitschke et al, 2009) provided evidence for “…a novel mechanism of rapid nongenomic cross talk between the hypothalamic-pituitary-gonadal and hypothalamic-pituitary-adrenal axes via GnRHR-dependent phosphorylation and activation of the unliganded GR in response to GnRH.”

The epigenetic effect of mammalian pheromones on GnRH neurosecretory neurons (e.g., as indicated by c-fos expression) and thereby on glucocorticoid receptors seems consistent with effects of pheromones on both the neuroendocrine and neuroimmune system. This molecular epigenetic effect on regulation of sexual reproduction also is indicated across an evolutionary continuum beginning in species of yeast that incorporate a form of mammalian GnRH into a self vs. non-self recognition mechanism involving an “alpha mating pheromone” linked to their “sexual orientation”(Diamond et al,1996).

Nothing that I know about molecular biology indicates that epigenetic effects of non-olfactory/pheromonal stimuli directly impact the HPA axis or HPG axis of mammals, or that visual input plays a primary role in the sexual behavior of any species from microbes to man. Is it not clear that mammalian pheromones are responsible for the “Bruce effect” in lab rats and in wild or domesticated horses and primates, and that pheromones also condition the hormone responses associated with visual input and secondary-process learning and memory mechanisms, which sometimes interface with tertiary-process cognitive-thoughtful functions and behavior in some humans?


The “Bruce effect” is commonly attributed to pheromones that cause changes in hormones, presumably via their direct effect on gonadotropin releasing hormone (GnRH) in mammals. I was surprised to read that it was attributed to visual observations of pregnancy and estrus in the study conducted on wild (feral) horses. Is there a model for that? I have not seen evidence for the direct effect of non-olfactory/pheromonal input on the hypothalamic-pituitary-gonadal axis or the hypothalamic-pituitary-adrenal axis in mammalian species in captivity or not in captivity. This suggests there may be no model for the direct effect of visual input on the neuroendocrine mechanisms that underlie the “Bruce effect.”

CORRECTION (to the comment above): I probably misinterpreted a link from visual input and initially thought the authors had proposed cause and effect. I now suspect that the scientists visually observed evidence of   pregnancy and estrus and attributed  earlier onset to killing of offspring that resulted from previous insemination by another stallion.





James Kohl
Retired medical laboratory scientist

James Kohl

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