Female mice (and other lower animals) go through the estrous cycle which is analogous to the human menstrual cycle. During this cycle, rodents will display behavioral estrus during a certain phase. Therefore, when a female mouse goes in heat, the behaviors she displays will be collectively referred to as ‘behavioral estrus’.
The estrous cycle has four stages: proestrus, estrus (where behavioral estrus occurs), metestrus, and diestrus.
Behavioral estrus is a phrase used to describe the timeframe in which female rodents are most receptive to sexual behaviors. Therefore, behavioral estrus is a sexual behavior.
Behaviors Observed During Behavioral Estrus
Behavioral estrus is comprised of behaviors in which the female mouse tries to entice the male mouse into mounting her. She may dart towards him and then run (or hop) away. Darting and retreating is an approach-retreat sequence which may be observed multiple times. It is also common to observe the female mouse wiggling her ears at the male mouse.
Also, if the female mouse observes the male interacting with another female, she may run in between the male and the other female, in order to interrupt their encounter and disrupt his attention.
Higher Frequency of Lordosis During Behavioral Estrus
If a male is attracted to these behaviors and begins to mount her, then behavioral estrus will segway into lordosis which is the female mating posture. Lordosis, in rodents, is most often expressed during behavioral estrus.
Lordosis is most likely to occur during the time in which behavioral estrus appears, therefore there is a relationship between these two events.
Heightened Odor Preference During Behavioral Estrus
One group of researchers demonstrated that odor sensing vomeronasal sensory neurons are active and responsive to certain male pheromones during estrus but silent to them during diestrus, indicating that there is a physiological relationship between scent and sexual behavior which is specifically activated during estrus.
Function of Behavioral Estrus
Behavioral estrus is a collection of behaviors in which the female mouse indicates she is in heat and receptive towards sexual encounters with the male mouse. Therefore, behavioral estrus has a sexual function.
By acting and behaving in a certain way, a female mouse can indicate to a male mouse that she is ready for reproduction and thus increase her chances of reproductive success. The overall function of behavioral estrus is to become pregnant and reproduce.
Applications of Behavior
- Estradiol: A female mouse will begin demonstrating behavioral estrus upon reaching maturity. Development from childhood into adulthood is marked by hormonal changes, including cyclical fluctuations of hormones such as estradiol which is a major female steroid hormone involved in the regulation of female sexual and reproductive behaviors.
- External Applications Adopted by Researchers: A recent research trend has involved combining females with behavioral estrus and male mice which are on a disease model, in order to assess how male mice which are not healthy react to behavioral estrus. One study demonstrated that autistic BTBR T+ tf/J male mice are more likely to mate with estrus than diestrus females, a preference also seen in control male C57BL/6 mice. Such results were surprising because the researchers expected to detect abnormality in the autistic mice. This experiment demonstrates that behavioral estrus is also a behavior that can be used to test males’ cognition and behavior using a variety of disease models.
Research Techniques for Studying the Behavior
- Staining and Microscopy: In order to monitor the estrous cycle and thus know what stage the female mice are in, researchers will take vaginal smears daily and analyze the sample through staining and microscopy.
- Behavioral assessments: An array of behavioral assessments are used to study behavioral estrus and how it affects behavior and cognition, depending on the researcher’s hypothesis and what they wish to study.
Behavioral Tests for Assessing Behavioral Estrus
- Y-Maze: A modified Y-Maze which makes use of a fan to blow odors (such as the scent of male urine) is frequently used in research studying behavioral estrus, in order to study the interaction between pheromones and behavior.
- Sexual Behavior Test: This test is commonly performed by placing a male and female mouse (which is on estrus or has been injected with progesterone in order to induce it) and measuring the amount of time it takes for mounting to occur.
- Video Analysis: For observational studies measuring frequencies of behavior, video analysis is frequently used, in order to accurately measure the behaviors which were exhibited during a particular time frame.
Pharmaceutical Studies on Behavioral Estrus
Hormone Injections Induce Behavioral Estrus
The behaviors which comprise behavioral estrus have been experimentally demonstrated as reproducible by means of injecting mice with sex hormones such as progesterone and estrogen. Another hormone which induces behavioral estrus via quickened maturation is leptin, a hormone released from fatty adipose tissues. Leptin signals to the brain’s hypothalamic region and triggers young mice’s reproductive function to have an earlier onset than expected.
Mouse Strains Prone to Modulation Due to Behavioral Estrus
C57BL/6J Mouse Strain is Minimally Affected
Some mice, such as the C57BL/6J strain, show minimal to negligible modulation over the course of the estrous cycle. The female mice of this strain do not change their cognitive abilities while on the stage of the estrous cycle which elicits behavioral estrus.
BALB/cByJ Female Mice Are Affected by the Estrous Cycle
Other strains, such as the BALB/cByJ females, are more likely to be significantly affected by the estrous cycle. For example, BALB/cByJ females, when on the estrus stage of the estrous cycle, spend minimal time in the center part of the Open Field Test, a characteristic that is not found in C57BL/6J mice.
Behavioral Estrus Affects Behavioral Assessment
Behavioral estrus has been established as a factor that can affect performance. This factor should be of utmost importance to researchers conducting behavioral studies. For more information on this, please check out our article on how the estrous cycle affects behavior in mice.
Characteristics of Behavioral Estrus During Motor Tasks
One study, measuring motor activity with the barrier test (wherein a 1 cm high hurdle is placed in the center of the home cage in order to measure motor activity), showed that female mice reach a peak in their jumping abilities during the estrus stage and have a low point between the metestrus and diestrus stages. The non-estrous females jumped on average roughly 10 times in the duration of this experiment while the estrous females jumped about 17 times.
Open Field Performance During Behavioral Estrus
It has also been noted that BALB/cByJ female mice in the estrus stage will perform differently in the Open Field Test than other female mice which are in different stages of the estrous cycle. For example, out of all the stages in the estrous cycle, female BALB/cByJ mice will spend the least percentage of time in the center of the Open Field during the estrus phase, possibly indicating increased levels of anxiety.
Characteristics of Behavioral Estrus During Affective Tasks
Performance in the Tail Suspension Test has also been measured using BALB/cByJ and C57BL/6J female mice in order to determine behavioral differences across estrous stages. In both strains, immobility times (which are associated as behavioral equivalents of desperation) were significantly more frequent during the estrus stage, indicating significantly greater instances of immobility.
- The provoking behaviors that a female mouse exhibits when interacting with a male are collectively known as behavioral estrus and indicates that she is receptive to copulation.
- Behavioral estrus is expressed during a specific phase of the estrous cycle in mice, specifically the estrus stage.
- Lordosis is the most frequently observed when the female mouse is also in behavioral estrus.
- Hormone injections of estrogen, progesterone, or leptin will induce behavioral estrus.
- Females that display behavioral estrous have heightened odor preferences and decreased performance when exercising.
- Behavioral estrus collectively serves to indicate to male mice that the female mouse is available to mate and, thus, functions to perpetuate reproduction within the species.
- Staining and microscopy techniques, together with behavioral assessments, are commonly used to explore this behavioral phenomenon.
- BALB/cByJ females are more likely to have their cognition and behavior significantly affected by behavioral estrus than C57BL/6J female mice.
- Since estrus is associated with significant changes in behavior compared to other stages in the estrous cycle, researchers should proceed with caution when using female mice in their experiments, given the fact that they could be introducing a potential confounding variable.
Behavioral estrus is more than just a set of behaviors that are sexually provocative to male mice. In fact, a growing body of research shows that female mice have significantly different behaviors and abilities in cognitive, motor, and affective tasks when they are experiencing behavioral estrus. However, more research needs to be conducted in order to investigate how behaviors shift across the estrous cycle and how difference mouse strains are affected by the different stages of the cycle.
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