The Bruce Effect

by Dave Hanks

The Bruce Effect is the result of sexual pheromones in rodents. It is the manipulation of pregnancy by pheromones. For instance, a female rat comes into estrus and mates. She is very familiar with her partner’s scent (pheromones), and will not tolerate a strange male. If another male appears, his odor will cause the female to abort her fetuses.

This pregnancy block effect was first noted in 1959 by Hilda M. Bruce – thus the name. Besides rats, the pheromone was observed in deer mice, meadow voles, and collared lemmings. The blockage has also been suggested, but not confirmed, in lions.

Rodents are the largest group in the mammal category. These species include mice, rats, hamsters, guinea pigs, beavers, muskrats, porcupines, prairiedogs, marmots, chinchillas, voles, and lemmings. Rabbits are sometimes (but not always) included. Over one quarter of all mammals are rodents and are found on all continents except Antarctica. The landmass with the most rodent diversity is South America.

The DESERT WOODRAT (pictured) is identified by its large ears, white feet, dark throat hairs, gray-brown back, and bi-colored tail. It is adept at moving among the spines of cacti without injuring itself. Cactus serves as food, along with yucca pods, Pinyon nuts, bark, berries, and any available green vegetation.


Caught in its act of midnight thievery

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