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Pheromones

Pheromones are chemical substances produced by one animal which provide olfactory stimuli and communication to another animal. Pheromones play important roles in the behavior and reproduction of mice. Mice secrete two types of pheromones, known as signaling and priming pheromones.

Signaling pheromones include a fear signal, male and female sex attractants, and aggression inhibitors. Preputial gland pheromones of male mice provide female attractant stimuli. Urine from dominant male mice contains both aversion and aggression promoting pheromones. Application of urine from dominant males discourages investigation of the area by subordinate animals and incites aggression in other dominant males.

Priming pheromones include an estrus inducer, an estrus inhibitor, and adrenocortical activators. These pheromones can affect the estrous cycle of female mice; therefore, an understanding of these pheromonal effects is crucial for successful management of mouse breeding colonies. Three effects have been well characterized in mice; the Bruce effect, the Lee-Boot effect, and the Whitten effect.

The Bruce effect, or strange-male pregnancy block, occurs when a recently mated female is housed with or near a strange male. Implantation is inhibited in 30% of females and pregnancy is blocked when the strange male is introduced within 24 hours of mating. Affected females return to estrus in 4 - 5 days. The maximum effect occurs when the strange male is of a different strain than the breeding male. Direct contact with the male is not required for the block to occur. Males castrated prior to puberty cannot induce this effect.

The Lee-Boot effect is induced by housing female mice in groups of 4 or more. A higher incidence of pseudopregnancy is observed in these groups of female mice than in singly housed females in the absence of matings. This suggests that female mice produce a pheromone which influences the estrous cycle. Anestrus (cessation of cycling) can occur if female mice are housed in groups of 30 or more.

The Whit ten effect is seen when female mice are paired with male mice for breeding after an extended time of housing with other females only. Most of these females will mate on the third night after being introduced to the male. For comparison, mating is normally evenly distributed over the first 4 nights after pairing females with males when the females were previously caged individually. This effect can be taken advantage of to orchestrate timed pregnancies.