Do Norway rats (Rattus norvegicus) synchronize their estrous cycles?

doi:10.1016/S0031-9384(00)00395-4

Physiology & Behavior

Volume 72, Issues 1–2, January–February 2001, Pages 129-139

https://doi.org/10.1016/S0031-9384(00)00395-4 Get rights and content

Abstract

Estrous synchrony was tested using 10 pairs of sibling female rats (Rattus norvegicus). A Monte Carlo bootstrap simulation was used to construct random control groups to avoid previous statistical errors and to test for significance when there are irregular cycles. The 10 pairs of females did not exhibit estrous synchrony. The effect of cycle irregularity on the limits of synchrony was analyzed using an equation that the related degree of cycle regularity to the degree of synchrony. This equation significantly predicted the limits of synchrony: cycle irregularity limits both the maximum and minimum degree of synchrony that can occur between two females. Finally, simulations of the expected levels of synchrony in groups of five rats were compared to the original study on estrous synchrony. The simulations indicated that the results of the original study were consistent with chance levels of synchrony. It is concluded that there is no evidence that Norway rats synchronize their estrous cycles. Evolutionary implications are discussed.

Section snippets

Subjects

Twenty non-mated Sprague–Dawley female rats (five groups of four sisters) were used; born and bred at Indiana University from stock originally obtained from Taconic (Germantown, NY). Animals were housed in standard polypropylene maternity cages (48×20×26 cm) and provided with food and water ad libitum. Colony rooms were maintained at 24°±2°C and illuminated from 08:00 to 20:00 hours.

The females came from litters that were culled to four females and four males at 3 days of age (day of birth is

Results

The level of synchrony from the first closest pair to the last closest pair remained the same (mean difference in days=0.8, both at the beginning and end, t=0, df=9, P=1, two-tailed; see Fig. 1 for the estrous state matches). This implies that moving the females to another room and separating them into pairs did not affect their level of synchrony over the 48-day observation period. The degree of match between pairs of females also did not differ from chance (mean match=0.247, P>.62). Fig. 4

Discussion

No effect of synchrony was found, and the results were consistent with chance levels of synchrony. Failure to produce synchrony in a particular experiment does not disprove the phenomenon of synchrony. Indeed, one cannot rule out the possibility of a Type II error given the small expected effect size of synchrony previously reported [18], [34]. However, assessing the plausibility of a Type II error in this study requires a more detailed analysis of the phenomenon of synchrony in female rats.

Methods

For a pair of rats, deviations in cycle length away from 4-day cycles should be a predictor of the maximum degree of synchrony attainable by the pair. There are at least two types of deviations away from 4-day cycles that may predict these limits. First, the rat in a pair with the maximum frequency of non-4-day cycles (i.e., an irregularly cycling rat) should be negatively correlated with the maximum degree of match between the pair. Second, the absolute difference in the frequency of 4-day

Methods

In this study, vaginal smear classes were interpreted as estrous-cycle states (see Fig. 1). The original study [18], however, directly compared vaginal smear classes of females (i.e., L, LN, LC, N, NL, NC, C, CL, CN). Because the number of smear classes used was k=9, a match of two out of five animals on any given day may indicate a small but significant degree of synchrony.

Classifying the synchrony level of females according to smear class is a combinatorial matching problem, but simpler than

General discussion

This study failed to detect synchrony among pairs of sibling females that were housed together. There are several possible explanations for the failure to detect synchrony including the possibility of a Type II error. To assess the plausibility of a Type II error, further analysis of the phenomenon of estrous synchrony was performed. Because estrous synchrony among female Norway rats has been reported in only one other study [18], that study and the present study were necessarily the focus of

Acknowledgements

This work was initially supported by the Center for the Integrative Study of Animal Behavior at Indiana University, Bloomington, and the theoretical development by NIH (through a subcontract with Indiana University) at the University of California, Davis. I thank two anonymous referees whose comments greatly improved the presentation of this material.

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Evidence-based research has revealed how physiological and emotional responses to acute stress are adaptive. However, under conditions of unpredictable or protracted stress, health and drug vulnerability can be compromised. In this study, we examined anxiety-like behavioral responses of 4th generation adolescent male and female Long Evans rats selectively bred for high (HAn) and low (LAn) anxiety-like behavior when housed in an isolated environment (IE) versus a social environment (SE). After 35 days in IE or SE, animals were tested in the elevated plus maze (EPM), injected with amphetamine (AMPH: 0.5 mg/kg, IP) in the locomotor activity (LMA) chamber, measured for basal and post air puff-stressor core body temperature and blood pressure. Following select rearing, SE reduced the anxiogenic response in HAn rats with females displaying the lowest anxiety-like behavior in the EPM. During habituation in the LMA, IE rats remained active, while post-AMPH injection HAn females were hyperactive, followed closely by LAn females. Our findings from the post-stressor physiological measurements indicate that temperature differences due to environment are observed only in the SE females. We also observed group differences for diastolic (DBP) and systolic (SBP) blood pressure. HAn IE males experienced higher DBP and SBP but LAn IE females only experienced higher SBP. Not only do our findings corroborate earlier work on HAn/LAn lines but the findings obtained from this research offer new insights about the role of environment and the role of sex in (1) modulation of anxiety-like behavior, (2) AMPH sensitivity, and (3) basal and stress-induced physiological changes.
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Chemical warfare nerve agents (CWNAs) are highly toxic compounds that cause a cascade of symptoms and death, if exposed casualties are left untreated. Numerous rodent models have investigated the toxicity and mechanisms of toxicity of CWNAs, but most are limited to male subjects. Given the profound physiological effects of circulating gonadal hormones in female rodents, it is possible that the daily cyclical fluctuations of these hormones affect females' sensitivity to the lethal effects of CWNAs, and previous reports that included female subjects did not control for the stage of the hormonal cycle. The aim of the current study was to determine the 24-hour median lethal dose (LD₅₀) of the CWNA sarin in male, ovariectomized (OVEX) female, and female rats during different stages of the estrous cycle (diestrus, proestrus, and estrus). Additionally, baseline activity levels of plasma acetylcholinesterase, butyrylcholinesterase, and carboxylesterase were measured to determine differences among the groups. Results indicated that females in proestrus had a significantly higher LD₅₀ of sarin compared to OVEX and estrous females. Although some sex differences were observed in the activity levels of plasma esterases, they were not consistent and likely not large enough to significantly affect the LD₅₀s. These results suggest that hormonal cyclicity can influence the outcome of CWNA-related studies using female rodents, and that this variability can be minimized by controlling for the stage of the cycle. Additional research is necessary to determine the precise mechanism of the observed differences because it is unlikely to be solely explained by plasma esterase activity.
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Although the rats (Ratus norvegicus) most commonly used in scientific studies have been domesticated for more than one hundred years [1], their physiology and behavior are still not fully understood. Although multi-day cycles in female rat hormonal levels that correspond with changes in sexual behavior are well documented [1–5], to our knowledge no one has previously described male hormonal levels and sexual behavior across consecutive days. Past experiments documenting the sexual behavior and hormonal levels of male rats have focused mainly on males that reach sexual exhaustion during one prolonged interaction with one or more females [6–9].
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Such an effect was reported for humans (Stern and McClintock, 1998) and has been inferred in earlier studies reporting synchrony in humans (Weller and Weller, 1993). However, other studies or re-analysis of previous studies, have found no evidence of cycle synchrony in humans (Schank, 2001b; Strassmann, 1999; Whitten, 1999; Wilson, 1987, 1992), non-human primates (chimpanzee: (Matsumoto-Oda et al., 2007); Mandrill: (Setchell et al., 2011); macaque (Furtbauer et al., 2011)), rats (Schank, 2001a) or hamsters (Schank, 2000). It might be that cycle synchrony occurs only in very restricted and contextually-specific circumstances (McClintock, 2002) but, on balance, the evidence suggests that ovarian synchrony does not exist as a biologically meaningful phenomenon (Schank, 2001b, 2006).
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Do Norway rats (Rattus norvegicus) synchronize their estrous cycles?

Abstract

Section snippets

Subjects

Results

Discussion

Methods

Methods

General discussion

Acknowledgements

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