According to a review by Dewsbury et al. (1980), males were significantly heavier than females at 90 days of age in 10 of the 13 muroid rodents, and the other 3 species were considered monogamous. All microtine rodents in their data (M. montanus, M.
pennsylvanicus, M. ochrogaster, and M. canicaudus), and the Taiwan field voles in the
current study, showed sexual dimorphism in body weight. In all cases, males are heavier than females. Among the 5 microtine species, the sexual dimorphism of the gray-tailed vole (M. canicaudus) was the smallest, and males had the small testes (relative to body size) and provided paternal care (Wolff et al. 1994). Although the gray-tailed voles exhibited above-mentioned characteristics of monogamy, the mating system was considered polygamy or promiscuity based on the social organization and the home ranges overlap pattern of the species (Wolff et al. 1994). Unlike the gray-tailed vole, the montane vole (M. montanus) and the meadow vole (M. pennsylvanicus) showed the largest sexual dimorphism, and the mating systems were considered polygamy or promiscuity (Madison 1980, Dewsbury 1981, McGuire and Novak 1986, Boonstra et al.
1993). The sexual dimorphism of the prairie vole (M. ochrogaster) was small, though not the smallest. The species had many characteristics of monogamy and was
considered monogamous during most situations (Thomas and Birney 1979, Getz et al.
1981). Although the Taiwan field vole had a very similar sexual dimorphism as the prairie vole, in light of the mating system of gray-tailed vole, we would need more evidence to establish the mating system of Taiwan field vole as monogamous.
The main purpose of this study had been to investigate the mating system of Taiwan field voles with partner preference test, and examine the effects of steroid hormone concentration on partner preference. I found males did not exhibit significant partner preferences, but females did. The preferences were not influenced by the concentration of steroid hormones. Sexual dimorphism was strong, yet the magnitude was similar to the monogamous prairie vole. In conclusion, my study showed that mating system of the Taiwan field voles is not strictly monogamy.
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28Tables 1. Body weight (in grams) of the Taiwan field vole at 90~180 days of age compared to three other microtine species at 90 days of age. Values in parentheses give the weight difference in relation to female weight. Body weight (mean ± sd)Sex Differences SpeciesLittersMaleFemaleMale − Femalep Reference M. kikuchii8 43.19 ± 3.3837.03 ± 3.546.16 ± 3.18 (16.6%)<.001This study M. ochrogaster3243.5 ± 6.337.3 ± 3.86.2 (16.6%)0.001Dewsbury et al. 1980 M. pennsylvanicus2553.8 ± 6.744.2 ± 9.79.6 (21.7%)0.001Dewsbury et al. 1980 M. montanus2542.4 ± 5.131.4 ± 5.011.0 (35.0%)0.001Dewsbury et al. 1980
Independent variable df Estimate SE Square p Partner Preferences of Males
Partner/Stranger 1 -1.20 1.02 1.40 0.24 Partner Preferences of Females
Partner/Stranger 1 -2.35 1.14 4.22 0.04*
*: p < 0.05
30
linear regression analyses using time as dependent variable and partner/stranger as independent variable. Dependent variableIndependent variableR-squaredfSSMSF Partner Preferences of Males !!!!!! Time in the chamberPartner/Stranger0.021 907.27 907.27 0.37 Time in physical contactPartner/Stranger0.08 1 3230.83 3230.83 1.30 Partner preferences of Females!!!!!! !Time in the chamberPartner/Stranger0.251 8037.798037.795.01 !!Time in physical contactPartner/Stranger0.12 1 2943.142943.141.96 *: p < 0.05
31
Results of categorical logistic regression analyses using outcome (preferred/non-preferred) as dependent variable and hormone Dependent variableIndependent variablesdfEstimateSEWald Chi- SquareAdjusted -preferred
Progesterone Mean before pairing1 -0.00050.0030.02 CV before pairing1 0.001470.020.004 Value during pairing1 0.0001580.00070.05 Estradiol Mean before pairing1 0.003080.0030.93 CV before pairing1 -0.01130.06 0.04 Value during pairing1 0.001020.0010.60 Corticosterone Mean before pairing1 0.0007270.003 0.07 CV before pairing1 -0.01170.04 0.10 Value during pairing1 0.000650.0010.26
32
5. Effects of hormone (progesterone, estradiol, and corticosterone) concentration of partner/stranger on the outcomes of male prefe tests. Results of linear regression analyses using outcome (time in each chamber) as dependent variable and hormone concentra independent variable. Dependent variable Independent variablesR-squaredfSSMSFAdjusted Time in each chamber
Progesterone!!!!!! Mean before pairing0.04521 1193.28 1193.28 0.661.00 CV before pairing0.03671 969.90969.900.531.00 Value during pairing0.01601 423.38 423.38 0.231.00 Estradiol!!!!!! Mean before pairing0.01471 388.67388.670.211.00 CV before pairing0.00071 18.6118.610.011.00 Value during pairing0.10841 2861.38 2861.38 1.70 0. Corticosterone!!!!! Mean before pairing0.01611 425.36425.360.231.00 !CV before pairing0.00171 45.8045.800.021.00 !!Value during pairing0.00591 154.76154.760.081.00
33 ntration of partner/stranger on the outcomes of male preference Dependent variableIndependent variablesR-squaredfSSMSFAdjusted Progesterone!!!!!! Mean before pairing0.01331 374.86374.860.19 CV before pairing0.00141 40.5740.570.02 Value during pairing0.00281 79.51 79.51 0.04 Estradiol!!!!!! Mean before pairing0.02021 568.19 568.19 0.29 CV before pairing0.03311 932.03932.030.48 Value during pairing0.03941 1109.071109.070.57 Corticosterone!!!!!! Mean before pairing0.13411 3776.68 3776.68 2.17 !CV before pairing0.04281 1206.421206.420.63 !!Value during pairing0.04061 1144.341144.340.59
34
Effects of hormone (testosterone and corticosterone) concentrations of partner/stranger on the outcomes of female preference Results of categorical logistic regression analyses using outcome (preferred/non-preferred) as dependent variable and hormone concentration as independent variable. Dependent variableIndependent variabledfEstimateSEWald Chi- SquareAdjusted Preferred/Non-preferred
Testosterone Mean before pairing1 0.001590.007170.05 1.00 CV before pairing1 0.02780.06050.211.00 Value during pairing1 0.006980.008190.73 1.00 Corticosterone Mean before pairing1 0.005340.005360.990. CV before pairing1 -0.02550.06030.18 0.67 Value during pairing1 0.007390.005291.950.
35 female preference tests. Dependent variableIndependent variablesR-squaredfSSMSFAdjusted Testosterone!! !!!!! Mean before pairing0.00161 24.3424.340.011.00 CV before pairing0.00131 20.9520.950.011.00 Value during pairing0.01441 224.76224.760.121.00 Corticosterone!!!!!!! Mean before pairing0.06971 1084.581084.580.61.00 CV before pairing0.00001 0.0880.0880 1.00 Value during pairing0.11771 1830.231830.231.071.00
36 female preference tests. Results of linear regression analyses using outcome (time in physical contact) as dependent variable and hormone concentration as independent variable. Dependent variableIndependent variablesR-squaredfSSMSFAdjusted Time in physical contact
Testosterone!! !!!! Mean before pairing0.00141 22.5422.540.01 CV before pairing0.00131 19.8819.880.01 Value during pairing0.00001 0.600.600 Corticosterone!!!!!! Mean before pairing0.35581 5591.385591.384.42 CV before pairing0.11391 1790.351790.351.03 Value during pairing0.23881 3752.693752.692.51