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      Sex differences in variability across timescales in BALB/c mice

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          Abstract

          Background

          Females are markedly underinvestigated in the biological and behavioral sciences due to the presumption that cyclic hormonal changes across the ovulatory cycle introduce excess variability to measures of interest in comparison to males. However, recent analyses indicate that male and female mice and rats exhibit comparable variability across numerous physiological and behavioral measures, even when the stage of the estrous cycle is not considered. Hormonal changes across the ovulatory cycle likely contribute cyclic, intra-individual variability in females, but the source(s) of male variability has, to our knowledge, not been investigated. It is unclear whether male variability, like that of females, is temporally structured and, therefore, quantifiable and predictable. Finally, whether males and females exhibit variability on similar time scales has not been explored.

          Methods

          These questions were addressed by collecting chronic, high temporal resolution locomotor activity (LA) and core body temperature (CBT) data from male and female BALB/c mice.

          Results

          Contrary to expectation, males are more variable than females over the course of the day (diel variability) and exhibit higher intra-individual daily range than females in both LA and CBT. Between mice of a given sex, variability is comparable for LA but the inter-individual daily range in CBT is greater for males. To identify potential rhythmic processes contributing to these sex differences, we employed wavelet transformations across a range of periodicities (1–39 h).

          Conclusions

          Although variability in circadian power is comparable between the sexes for both LA and CBT, infradian variability is greater in females and ultradian variability is greater in males. Thus, exclusion of female mice from studies because of estrous cycle variability may increase variance in investigations where only male measures are collected over a span of several hours and limit generalization of findings from males to females.

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          Most cited references 36

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          Female mice liberated for inclusion in neuroscience and biomedical research.

          The underrepresentation of female mice in neuroscience and biomedical research is based on the assumption that females are intrinsically more variable than males and must be tested at each of four stages of the estrous cycle to generate reliable data. Neither belief is empirically based. In a meta-analysis of 293 articles, behavioral, morphological, physiological, and molecular traits were monitored in male mice and females tested without regard to estrous cycle stage; variability was not significantly greater in females than males for any endpoint and was substantially greater in males for several traits. Group housing of mice increased variability in both males and females by 37%. Utilization of female mice in neuroscience research does not require monitoring of the estrous cycle. The prevalence of sex differences at all levels of biological organization, and limitations in generalizing findings obtained with males to females, argue for the routine inclusion of female rodents in most research protocols. Copyright © 2014 Elsevier Ltd. All rights reserved.
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            Circadian timing in cancer treatments.

            The circadian timing system is composed of molecular clocks, which drive 24-h changes in xenobiotic metabolism and detoxification, cell cycle events, DNA repair, apoptosis, and angiogenesis. The cellular circadian clocks are coordinated by endogenous physiological rhythms, so that they tick in synchrony in the host tissues that can be damaged by anticancer agents. As a result, circadian timing can modify 2- to 10-fold the tolerability of anticancer medications in experimental models and in cancer patients. Improved efficacy is also seen when drugs are given near their respective times of best tolerability, due to (a) inherently poor circadian entrainment of tumors and (b) persistent circadian entrainment of healthy tissues. Conversely, host clocks are disrupted whenever anticancer drugs are administered at their most toxic time. On the other hand, circadian disruption accelerates experimental and clinical cancer processes. Gender, circadian physiology, clock genes, and cell cycle critically affect outcome on cancer chronotherapeutics. Mathematical and systems biology approaches currently develop and integrate theoretical, experimental, and technological tools in order to further optimize and personalize the circadian administration of cancer treatments.
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              Female rats are not more variable than male rats: a meta-analysis of neuroscience studies

              Background Not including female rats or mice in neuroscience research has been justified due to the variable nature of female data caused by hormonal fluctuations associated with the female reproductive cycle. In this study, we investigated whether female rats are more variable than male rats in scientific reports of neuroscience-related traits. Methods PubMed and Web of Science were searched for the period from August 1, 2010, to July 31, 2014, for articles that included both male and female rats and that measured diverse aspects of brain function. Only empirical articles using both male and female gonad-intact adult rats, written in English, and including the number of subjects (or a range) were included. This resulted in 311 articles for analysis. Data were extracted from digital images from article PDFs and from manuscript tables and text. The mean and standard deviation (SD) were determined for each data point and their quotient provided a coefficient of variation (CV) as a measure of trait-specific variability for each sex. Additionally, the results were coded for the type of research being measured (behavior, electrophysiology, histology, neurochemistry, and non-brain measures) and for the strain of rat. Over 6000 data points were extracted for both males and females. Subsets of the data were coded for whether male and female mean values differed significantly and whether animals were grouped or individually housed. Results Across all traits, there were no sex differences in trait variability, as indicated by the CV, and there were no sex differences in any of the four neuroscience categories, even in instances in which mean values for males and females were significantly different. Female rats were not more variable at any stage of the estrous cycle than male rats. There were no sex differences in the effect of housing conditions on CV. On one of four measures of non-brain function, females were more variable than males. Conclusions We conclude that even when female rats are used in neuroscience experiments without regard to the estrous cycle stage, their data are not more variable than those of male rats. This is true for behavioral, electrophysiological, neurochemical, and histological measures. Thus, when designing neuroscience experiments to include both male and female rats, power analyses based on variance in male measures are sufficient to yield accurate numbers for females as well, even when the estrous cycle is not taken into consideration. Electronic supplementary material The online version of this article (doi:10.1186/s13293-016-0087-5) contains supplementary material, which is available to authorized users.
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                Author and article information

                Contributors
                smarr@berkeley.edu
                azuredominique@berkeley.edu
                irvzuck@berkeley.edu
                brianp@uchicago.edu
                510-642-5148 , kriegsfeld@berkeley.edu
                Journal
                Biol Sex Differ
                Biol Sex Differ
                Biology of Sex Differences
                BioMed Central (London )
                2042-6410
                9 February 2017
                9 February 2017
                2017
                : 8
                Affiliations
                [1 ]ISNI 0000 0001 2348 0690, GRID grid.30389.31, Department of Psychology, , University of California, ; Berkeley, CA 94720 USA
                [2 ]ISNI 0000 0001 2348 0690, GRID grid.30389.31, Department of Integrative Biology, , University of California, ; Berkeley, CA USA
                [3 ]ISNI 0000 0004 1936 7822, GRID grid.170205.1, Department of Psychology, , University of Chicago, ; Chicago, IL USA
                [4 ]ISNI 0000 0001 2348 0690, GRID grid.30389.31, The Helen Wills Neuroscience Institute, , University of California, ; Berkeley, CA 94720 USA
                Article
                125
                10.1186/s13293-016-0125-3
                5301430
                © The Author(s). 2017

                Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License ( http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver ( http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

                Funding
                Funded by: FundRef http://dx.doi.org/10.13039/100000071, National Institute of Child Health and Human Development;
                Award ID: HD081957
                Award ID: HD050470
                Award Recipient :
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                © The Author(s) 2017

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