Rats can have consecutive ejaculations, with a short inter-ejaculatory interval, and no mounts or intromissions before the second ejaculation

The Rare Phenomenon of Consecutive Ejaculations in Male Rats. Joanna M. Mainwaring, Angela C. B. Garcia, Elaine M. Hull and Erik Wibowo. Sexes 2021, 2(2), 183-188, May 20 2021. https://doi.org/10.3390/sexes2020016

Abstract: Mounting, intromission and ejaculation are commonly reported sexual behaviours in male rats. In a mating session, they can have several copulatory series with post-ejaculatory intervals in between ejaculations before they reach sexual satiety. Here, we describe a phenomenon where male rats displayed consecutive ejaculations (CE) with a short inter-ejaculatory interval (IEI). Male rats were daily mated with a sexually receptive female rat. Two out of 15 rats displayed CE in one of their mating tests. The first rat had CE at 9.9 and 10.1 min (IEI = 16.3 s) after the start of the test. The second rat showed CE at 28.1 and 28.5 min (IEI = 18.7 s) after the test onset. During the IEI, the rats did not show any mounting or intromission.

Keywords: multiple ejaculations; male rodents; male sexual behaviour; multiple orgasms; consummatory behaviour; refractory period

1. Introduction

Male sexual behaviours in rodents are characterised by three distinct behaviours: mounting, intromission, and ejaculation [1,2]. During an ejaculation, there is a vaginal penetration (the deep forward pelvic thrust), and the male rat freezes on the female for one to three seconds [1]. While the actual semen expulsion is not usually visible, a plug can occasionally be found in the vagina or surrounding area because rat semen coagulates quickly to form a plug. Typically, a male rat could reach an ejaculation after a series of mounts and intromissions. Following ejaculation, the rat enters a refractory period, during which he does not respond to sexual stimuli for several minutes [3] before he resumes another series of mounts and intromissions until the next ejaculation. Male rats can have multiple copulatory series for about 150 min [4], ranging from 5 to 12 copulatory series [5]. After this, the rats will reach sexual satiety or sexual exhaustion, and remain sexually inactive for a prolonged period of time.

Recently, we conducted a study on the impact of chronic sleep deprivation (CSD) on male sexual behaviours in rodents [6]. The study involved sexually experienced male rats, which were subjected to CSD, imposed by keeping them awake for the last four hours of the light phase. Control rats were left undisturbed in their home cage at the same time of day. In that study, two of the rats (one from each group) showed two consecutive ejaculations (CE), separated by <20 s. In reviewing the literature for such behaviour, some studies have reported that rats are capable of having multiple ejaculations in a single mating session, and each pair of ejaculations is separated by a post-ejaculatory interval (PEI), as well as a series of mounts and intromissions [5,7]. However, we did not find any report stating that rats can have CE, with a short inter-ejaculatory interval (IEI), and no mounts or intromissions before the second ejaculation.

As in rodents, humans can also display ejaculation, i.e., expulsion of semen following penile stimulation (be it during solo masturbation or penetrative sex). However, humans can also experience an orgasm, i.e., an intense, pleasurable response to genital or non-genital stimulation [8]. In humans, ejaculation and orgasm may be perceived as a single event, even though they are not the same biological process [9]. Furthermore, there is evidence that some men can have an ejaculation without having an orgasm [10,11], and some men can have an orgasm without an ejaculation [12,13,14,15]. There are also case reports on men who can have multiple ejaculations within a short period, but they required at least a few minutes of sexual stimulation between ejaculations [16,17]. However, to date, we are aware of no published report of men who can have ejaculations with no sexual stimulation (e.g., penile stimulation) before the second ejaculation. Despite this, one of us (EW) has received several anecdotal claims from men who reported having minimal or no refractory periods and are able to have multiple ejaculations with and without orgasms within a narrow time frame. This raises the question of whether the CE behaviour that we observed in our rats could be used as a model for multiple ejaculations and/or multiple orgasms in men.

The CE behaviour in rodents may bear a resemblance to men with minimal or short refractory periods, who can have multiple ejaculations. However, whether the CE behaviour can mimic multiple orgasms in men is difficult to answer, because we cannot assess orgasm in rats. Pfaus et al. [18] recently described how rats can have orgasm-like responses. For example, during ejaculation, male rats have pelvic floor contractions [19], as are also observed in humans [20]. Whether or not a rat experiences orgasm as a human does, in the presence of a receptive partner, a male rat will approach her in a way to maximise the ‘reward’ associated with ejaculation.

The purpose of this article is to describe a rare phenomenon where rats displayed two ejaculations consecutively with a short IEI, and no mounts or intromissions, between them. This behaviour was observed in one strain of inbred laboratory animals. To date, there is no documentation of whether other strains or non-laboratory rats can also show such behaviour.

4. Discussion

For the first time, we report CE in male rats. Currently, it remains unknown what the neurobiological basis is for them to show such behaviour, such as hormonal or neurotransmitter factors that can affect the ability to have CE. In addition, it remains unclear to what extent penile grooming post-ejaculation plays a role in stimulating the second ejaculation during CE. Sensory input from the genitals during genital grooming may be conveyed to the spinal ejaculation generator in the lumbar spinal cord [25]. However, one of the CE rats groomed his genitals in between ejaculations, but the other one did not.

We are aware of no published data on similar behaviour in humans either. As noted above, one of us (EW) received several anecdotal claims from men with no or minimal refractory periods, who reported CE. However, there are published data that some men reported multiple orgasms with ejaculations with intervals of several minutes between them [9]. Undoubtedly, the volume of the ejaculate decreases with subsequent ejaculations [16]. Another observation in a man with such an ability showed that the person did not have orgasm/ejaculation-induced prolactin release [17]. It remains to be determined whether the dampening of prolactin release after ejaculations may play a role in the CE in rats. Another potential mechanism for the CE we observed may involve a change in the serotonergic system. Past studies indicate that the administration of a 5-HT1A receptor agonist reduces the number of intromissions before ejaculation and shortens the ejaculation latency in male rats [26,27]. Considering that the animals in our study were not treated with any serotonergic agents, there is a possibility that they may have individual variation in their serotonergic system.

Future study in this area will be challenging, given the rarity of the phenomenon, and we cannot predict when they would show such a behaviour. One potential future study would be to explore hormonal (e.g., on prolactin) and neurobiological (e.g., on the serotonergic system) differences between rats who are and are not capable of having CE. It would be interesting to investigate whether prolactin receptor knock-out rodents display CE at an elevated frequency compared to wild-type rodents. Another possibility is to test whether some psychostimulants can increase the frequency of displaying CE, because some men have reported having multiple orgasms while having sex under the influence of psychostimulants, although it is unclear if they also had ejaculations [9].