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Multi-male mating, probability of conception, and litter size in the prairie vole (Microtus ochrogaster)
Behavioural Processes 58 (2002) 105– 110 www.elsevier.com/locate/behavproc
Multi-male mating, probability of conception, and litter size in the prairie vole (Microtus ochrogaster) Jerry O. Wolff a,*, Aimee S. Dunlap b b
a Department of Biology, The Uni6ersity of Memphis, Memphis, TN 38111, USA Department of Biological Sciences, Northern Arizona Uni6ersity, Flagstaff, AZ 86011, USA
Received 11 September 2001; received in revised form 23 January 2002; accepted 25 January 2002
Abstract We conducted a mating experiment in the laboratory using prairie voles, Microtus ochrogaster, to document that multi-male mating (MMM) can occur in this supposedly monogamous species and to test two hypotheses for the advantages of MMM in female mammals. The two hypotheses are that MMM (1) increases the probability of pregnancy and (2) increases litter size. We also tested the hypothesis that multiple copulations, rather than multiple partners, increases litter size and/or probability of pregnancy. Females were given a choice of mating with any of three males, each of which was tethered in a separate compartment. The mate choice bouts were recorded on videotape. We recorded the number of copulations and number of males with which females mated over a 24 h period. Litter size and probability of pregnancy were not significantly different for females that mated with one, two or three males. Increasing numbers of copulations, independent from the number of males, also did not increase litter size but did significantly increase the probability of pregnancy. MMM, at least in prairie voles, must serve some function other than increasing litter size and probability of conception. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Litter size; Mate choice; Monogamy; Multiple-male mating; Prairie voles; Pregnancy; Promiscuity
1. Introduction Sexual selection theory proposes that males increase their fitness by mating with as many females as possible, whereas females should be more selective and mate with the ‘best’ male (Trivers, 1972). In nature, however, many female mammals are promiscuous, mating with more * Corresponding author. Tel.: +1-901-678-2758; fax: + 1901-678-4746. E-mail address: [email protected] (J.O. Wolff).
than one male during a given estrous period (Smith, 1984; Birkhead and Møller, 1998). As many as nine hypotheses have been proposed to explain the advantages of multi-male mating (MMM) for female mammals (e.g. Ginsberg and Huck, 1989; Møller and Birkhead, 1989) but few empirical data are available to discern among them. Two hypotheses that have been tested are that MMM increases probability of pregnancy and litter size. Hoogland (1998) found that in Gunnison’s prairie dogs, Cynomys gunnisoni, MMM
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increased the probability of conception when mating with more than two partners and resulted in a larger litter size. The mechanism by which MMM increases conception rates or litter size is not known, but may be to guard against male sterility or sperm depletion (see Haig and Bergstrom, 1995) or just to stimulate ovulation. Hoogland, however, was not able to address whether increased litter size and conception rates were a function of MMM or if multiple copulations with the same partner would have the same effect. M. Fleming (1995, personal communication) found that in mink, Mustela 6ison, both increasing numbers of partners and numbers of copulations with the same partner increased litter size. Therefore, the number of copulations, and not necessarily number of partners, may affect litter size and probability of pregnancy. The objective of this study was to first document if MMM occurred in the prairie vole, Microtus ochrogaster, a reportedly, socially monogamous species (Getz et al., 1993; Carter and Getz, 1993; but see Evans and Dewsbury, 1978). Second, we attempted to test two hypotheses for MMM: that MMM (1) increases the probability of conception and (2) results in a larger litter size. Alternatively, increased numbers of copulations with the same partner, and not necessarily additional partners, could affect the probability of pregnancy and litter size. We conducted a laboratory study in which we gave females the choice of mating with up to three males and then recorded frequency of copulations, number of mating partners, pregnancy rates, and litter sizes.
2. Methods All animals used in this experiment were from the prairie vole colony at the University of Memphis and were first or second generation offspring of wild-caught outbred voles from Shelby County, Tennessee. Animals were housed singly in standard polycarbonate mouse cages and kept on a 14-h light:10-h dark photoperiod with food and water provided ad libitum. All animals used were 67– 227 days old. We conducted the experiment using a four-
chambered apparatus following the design and methods of Berteaux et al., (1999). This apparatus consisted of a wooden box divided into four chambers. Each of the three adjacent front chambers was 25×25 cm and the rear chamber was 25× 75 cm and extended the length of all three front chambers. A 10× 10 cm doorway provided access from the larger back chamber to each of the three front chambers; the front chambers were not accessible to each other. The front wall of the apparatus was made of Plexiglas to allow observation of the three chambers. Food, water, and bedding were provided in each chamber. The photoperiod was maintained as in the lab colony. To initiate a trial, three unrelated, sexually experienced adult males of similar age and weight (9 2 g) were used for each experiment. One male was tethered in each of the smaller chambers to a steel bar using a flexible wire with a swivel clip attached to a collar consisting of a thin plastic cable tie. Males had free movement within their chambers but could not enter the rear or each other’s chambers. The openings to the back chamber initially were blocked with sliding doors made of hardware cloth. An unrelated, nulliparous female that was unfamiliar with any of the males was placed in the back chamber for a 24 h acclimation period. We assumed that females would use this time to assess the three males and become hormonally primed for mating (Hasler and Conway, 1973). Females were used just once (N = 47); a few males that were used twice were always paired with new males and at least 3 weeks later. All trials were initiated by 09:00 h. Following the 24 h acclimation period, the doors were removed giving the female access to each of the males. We videotaped the apparatus for the next 24 h using a time-lapse VCR and a video camera with night vision. At the end of the 24 h, voles were returned to their cages and the females were observed for pregnancy and parturition. Each videotape was watched; the number of copulations (as defined by the number of mounts with thrusting; Dewsbury and Hartung, 1982) and mating partners was recorded. We could not detect ejaculation or insemination in this study, however, a paternity analysis is currently being
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conducted (S. Mech et al., submitted). In all trials, mating had continued until no mating attempts were recorded for at least 2 h (satiety typically is defined as 30 min with no intermissions; Evans and Dewsbury, 1978). We used univariate ANOVA to test for differences in numbers of copulations and litter size for females mating with one, two or three males. When P values were significant, we used Tukey’s HSD to test for differences between groups. Logistic regression was used to test for a relationship between numbers of copulations and probability of pregnancy; linear regression was used to correlate numbers of copulations with litter size.
3. Results Of 47 trials conducted, 21 females mated with one male, 16 with two males and 10 with three males. The numbers of copulations ranged from 1 to 167. Litter sizes ranged from two to five. Thirty-one of 47 females became pregnant and gave birth. The mean number of copulations performed by a female varied with whether she mated with one male (X= 23.5, S.D.=16.12), two (X = 65.9, S.D. =43.70), or three (X =81.4, S.D.=23.71) males (F2,44 = 9.03, P =0.0005). The females who mated with only one male received significantly fewer copulations than the other two groups. There was no difference between two and three males (Tukey’s HSD test). Using logistic regression, females were more likely to become pregnant and give birth with larger numbers of copulations (G 21 = 5.055, P = 0.025). The probability of becoming pregnant was 78% for 25 copulations, 88% for 50 copulations, and 99% for \70 copulations. However, data analyzed by a log-linear model revealed that the number of males with which a female mated had no significant effect on the probability of pregnancy for one (P = 0.619, n=21), two (P = 0.688, n = 16), or three (P=0.700, n =10) males (all G 22 B0.869). Mean litter size was not correlated with the numbers of copulations (R 2 =0.128, P = 0.307). Similarly, litter size did not differ significantly for females that mated with one (X = 4.0, S.D.=
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0.707, n= 13), two (X= 4.0, S.D.= 0.632, n= 11), or three (X= 3.7, S.D.= 0.787, n= 7) males (F= 1.39, P= 0.265). The largest litters (five young) were born to mothers who mated with one male (three of five) or two males (two of five; Fig. 1).
4. Discussion Female prairie voles mated with more than one male in 26 of 47 (55%) of the mating trials. These results are similar to those of Berteaux et al., (1999) who found that 29 of 39 (74%) female meadow voles, M. pennsyl6anicus, mated with more than one male in similar trials. In meadow voles, the percentage of females mating with one, two, or three males was 21, 55 and 24%, respectively, compared with 45, 34, and 21%, respectively, in our study with prairie voles. Thus, polyandrous mating in the supposedly socially monogamous prairie vole was relatively high, at least in this laboratory setting, and similar to that of the promiscuous meadow vole. MMM by female prairie voles also was demonstrated in the laboratory using a different procedure (Evans and Dewsbury, 1978) but with similar results to ours. Other than one field study in which multiple paternity was found in one of three litters (Carter and Getz, 1993), virtually nothing is known about MMM in prairie voles in natural environments,
Fig. 1. The relationship between numbers of copulations and litter size for female prairie voles. A litter size of zero indicates that females did not get pregnant.
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but it is thought to be uncommon (Carter and Getz, 1993; Getz et al., 1993). However, our current field study has shown several males in the vicinity of females at the time of conception (unpublished). As has been shown in our study and those of other vole species (e.g. Dewsbury and Hartung, 1982; Salo and Dewsbury, 1995; Berteaux et al., 1999), females play an active role in mate choice and seeking multiple partners. Increased numbers of copulations increased the probability of pregnancy. This difference was slight, however, in that overall, 70% of females conceived regardless of numbers of matings or mating partners. The probability of conception was 78% for females with \ 25 copulations, 88% with \50 copulations, 99% with \70 copulations, and one female, conceived with only one mating from a single male. Other laboratory studies on copulatory behavior of prairie voles have shown 42 to 65 mounts with thrusting to satiety (Gray and Dewsbury, 1973) and 6– 39 mating bouts with one male over a 24 h period (Roberts et al., 1999). In meadow voles, females that mated with one, two, or three males copulated a mean total of 10.7, 25.6, and 33.4 times, respectively, (Berteaux et al., 1999) compared with 23.5, 65.9, and 81.4 times, respectively, in our study. Data on copulation frequency for voles are not available from wild populations, but laboratory evidence suggests that 25 copulations are usually sufficient for ovulation (Dewsbury, 1975; Gray et al., 1974, 1977). The larger numbers of copulations observed in prairie voles than meadow voles may be due in part to their differences in mating systems (Dewsbury, 1981). Prairie voles are socially monogamous (Carter and Getz, 1993; Getz et al., 1993) and may require greater amounts of copulatory stimulation than other promiscuous voles for ovulation, conception, and perhaps pair bonding (Gray et al., 1974, 1977; Roberts et al., 1999). Females did not increase their litter size or their pregnancy rate by increasing the number of males with which they mated when controlling for differences in number of copulations. Similarly, the number of copulatory events also did not have a significant effect on litter size although increasing numbers of copulatory events, independent of male number, did increase a female’s probability
of becoming pregnant. Our results do not support either of the two hypotheses we tested for MMM in female prairie voles: that MMM increases litter size or increases a female’s probability of becoming pregnant. Our sample size is relatively small in that very few (n= 10) females mated with three males. However, the mean litter sizes for females mating with one, two, or three males are very similar to each other (4.0, 4.0 and 3.7, respectively) and a power analysis revealed that we would need 70 females in each mating category to detect a 0.25 pup difference in mean litter size. If anything, the trend of litter sizes tended to decrease for females mating with three males. Since the means are so close, we think that even with a larger sample size, litter sizes would not differ significantly when females mated with multiple partners. Maternal effects may also affect litter size, however, all of our females were of similar age, sexual experience, and parity. While Hoogland (1998) found increased litter sizes in multiply-mating female Gunnison’s prairie dogs, MMM did not affect litter size or reproductive success in other species of ground squirrels and prairie dogs (Schwagmeyer, 1986; Murie, 1995; Hoogland, 1995). In fact, the paucity of examples to demonstrate an increase in litter size with multiple mating partners (references in Hoogland, 1998) suggests that this may not be a primary function of promiscuity in female mammals. We do not know the benefits to female prairie voles for mating with multiple partners. Gaining greater paternal care or obtaining material benefits are not good explanations for MMM because additional mates do not provide additional parental care. Alternative hypotheses such as sperm competition (Haig and Bergstrom, 1995) or increasing genetic variation of offspring are possible, but unlikely as a universal explanation for MMM (van Schaik and Keppeler, 1997; Jennions and Petrie, 2000). MMM did not decrease sexual harassment in this study in that males were tethered and could not interact with each other or pursue the females. Additional copulations were solicited by the females and not forced by males. In that MMM is common among rodents (Agrell et al., 1998), primates (van Schaik and Keppeler,
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1997) and other species in which infanticide has been reported (Ebensperger, 1998), we propose that one primary function of mating with multiple partners by females is to confuse paternity and deter neighboring males from killing offspring (Agrell et al., 1998; Ebensperger, 1998) though this remains to be tested. Further arguments for and against alternative benefits of MMM in mammals such as voles are summarized in Berteaux et al. (1999). Our results are interesting in that they confirm that MMM occurs in rodents even in a supposedly monogamous species and females actively solicit extra matings (see also Evans and Dewsbury, 1978). Our results are from a laboratory study and need to be confirmed from the field, but preliminary data from our work suggest that MMM also occurs in the field (unpublished). We were unable to support the hypotheses that MMM functions to increase litter size or probability of pregnancy. The probability of pregnancy does correlate positively with number of matings, but independent of number of partners. We are currently conducting a paternity analysis from this study to determine those parameters associated with mate choice, timing of conception, and reproductive success of males as well as how mating with multiple partners affects paternity (S. Mech et al., submitted). These experiments comply with the current animal welfare laws of the USA.
Acknowledgements This work was supported by NSF grant 9996016. We thank Steve Mech, Michael Ferkin, Shawn Thomas, John Hoogland and anonymous reviewers for helpful comments on the manuscript. S. Mech assisted with the statistical analysis.
References Agrell, J., Wolff, J.O., Ylo¨ nen, H., 1998. Infanticide in mammals: strategies and counter-strategies. Oikos 84, 507 –517. Berteaux, D., Bety, J., Rengifo, E., Bergeron, J., 1999. Multi-
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ple paternity in meadow voles (Microtus pennsyl6anicus): investigating the role of the female. Behav. Ecol. Sociobiol. 45, 283 – 291. Birkhead, T.R., Møller, A., 1998. Sperm Competition and Sexual Selection. Academic Press, London. Carter, C.S., Getz, L.L., 1993. Monogamy and the prairie vole. Sci. Am. 268, 100 – 110. Dewsbury, D.A., 1975. Diversity and adaptation in rodent copulatory behavior. Science 190, 947 – 954. Dewsbury, D.A., 1981. An exercise in the prediction of monogamy in the field from laboratory data on copulatory behavior on 42 species of muroid rodents. The Biologist 63, 128 – 162. Dewsbury, D.A., Hartung, T., 1982. Copulatory behavior of three species of Microtus. J. Mamm. 63, 306 – 309. Ebensperger, L.A., 1998. Strategies and counter-strategies to infanticide in mammals. Biol. Rev. 73, 321 – 346. Evans, R.L., Dewsbury, D.A., 1978. Copulatory behavior of prairie voles (Microtus ochrogaster) in a two-male situation. Behav. Biol. 24, 498 – 508. Fleming, M., 1995. Multiple paternity and female reproductive success in mink, Mustela vison. 75th Meeting Am. Soc. Mammal. Univ. Vermont 20 – 24 June. Getz, L.L., McGuire, B., Pizzuto, T., Hofmann, J., Frase, B., 1993. Social organization of the prairie vole (Microtus ochrogaster). J. Mamm. 74, 44 – 58. Ginsberg, J., Huck, U., 1989. Sperm competition in mammals. Trends Ecol. Evol. 4, 74 – 79. Gray, C.D., Dewsbury, D.A., 1973. A quantitative description of copulatory behavior in prairie voles (Microtus ochrogaster). Brain Behav. Evol. 8, 437 – 452. Gray, C.D., Zerylnick, M., Davis, H.N., Dewsbury, D.A., 1974. Effect of variations in male copulatory behavior on ovulation and implantation in prairie voles (Microtus ochrogaster). Horm. Behav. 5, 389 – 396. Gray, C.D., Kenney, A.M., Dewsbury, D.A., 1977. Adaptive significance of the copulatory behavior pattern of male meadow voles (Microtus pennsyl6anicus) in relation to induction of ovulation and implantation in females. J. Comp. Phys. Psychol. 91, 1308 – 1319. Haig, D., Bergstrom, C.T., 1995. Multiple mating, sperm competition and meiotic drive. J. Evol. Biol. 8, 265 – 282. Hasler, M.J., Conway, C.H., 1973. The effect of mature males on the reproductive state of female Microtus ochrogaster. Biol. Reprod. 9, 426 – 436. Hoogland, J.L., 1995. The Black-Tailed Prairie Dog: Social Life of a Burrowing Mammal. University of Chicago Press, Chicago. Hoogland, J.L., 1998. Why do Gunnison’s prairie dogs copulate with more than one male. Anim. Behav. 55, 351 – 359. Jennions, M.D., Petrie, M., 2000. Why do females mate multiply? A review of the genetic benefits. Biol. Rev. 75, 21 – 64. Mech, S.G., Dunlap, A.S., Hodges, K.E., Wolff, J.O. Unguarded female prairie voles solicit multiple male matings. Behav. Proc. (Submitted).
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Møller, A., Birkhead, T., 1989. Copulation in mammals: evidence that sperm competition is widespread. Biol. J. Linn. Soc. 38, 119 – 131. Murie, J.O., 1995. Mating behavior of Columbian ground squirrels. I. Multiple mating by females and multiple paternity. Can. J. Zool. 73, 1819 –1826. Roberts, R.L., Wolf, K.N., Sprangel, M.E., Rall, W.F., Wildt, D.E., 1999. Prolonged mating in prairie voles (Microtus ochrogaster) increases likelihood of ovulation and embryo number. Biol. Reprod. 60, 756 –762. Schwagmeyer, P.L., 1986. Effects of multiple mating on reproduction in thirteen-lined ground squirrels. Anim. Behav.
34, 297 – 298. Salo, A.L., Dewsbury, D.A., 1995. Three experiments on male choice in meadow voles (Microtus pennsyl6anicus). J. Comp. Phys. 109, 42 – 46. Smith, R., 1984. Sperm Competition and Evolution of Animal Mating Systems. Orlando, Academic press. Trivers, R.L., 1972. Parental investment and sexual selection. In: Campbell, B. (Ed.), Sexual Selection and the Descent of Man 1871 – 1971. Aldine, Chicago, pp. 136 – 179. van Schaik, C.P., Keppeler, P.M., 1997. Infanticide risk and evolution of male – female association in primates. Proc. R. Soc. London 264, 1687 – 1694.
Multi-male mating, probability of conception, and litter size in the prairie vole (Microtus ochrogaster) Jerry O. Wolff a,*, Aimee S. Dunlap b b
a Department of Biology, The Uni6ersity of Memphis, Memphis, TN 38111, USA Department of Biological Sciences, Northern Arizona Uni6ersity, Flagstaff, AZ 86011, USA
Received 11 September 2001; received in revised form 23 January 2002; accepted 25 January 2002
Abstract We conducted a mating experiment in the laboratory using prairie voles, Microtus ochrogaster, to document that multi-male mating (MMM) can occur in this supposedly monogamous species and to test two hypotheses for the advantages of MMM in female mammals. The two hypotheses are that MMM (1) increases the probability of pregnancy and (2) increases litter size. We also tested the hypothesis that multiple copulations, rather than multiple partners, increases litter size and/or probability of pregnancy. Females were given a choice of mating with any of three males, each of which was tethered in a separate compartment. The mate choice bouts were recorded on videotape. We recorded the number of copulations and number of males with which females mated over a 24 h period. Litter size and probability of pregnancy were not significantly different for females that mated with one, two or three males. Increasing numbers of copulations, independent from the number of males, also did not increase litter size but did significantly increase the probability of pregnancy. MMM, at least in prairie voles, must serve some function other than increasing litter size and probability of conception. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Litter size; Mate choice; Monogamy; Multiple-male mating; Prairie voles; Pregnancy; Promiscuity
1. Introduction Sexual selection theory proposes that males increase their fitness by mating with as many females as possible, whereas females should be more selective and mate with the ‘best’ male (Trivers, 1972). In nature, however, many female mammals are promiscuous, mating with more * Corresponding author. Tel.: +1-901-678-2758; fax: + 1901-678-4746. E-mail address: [email protected] (J.O. Wolff).
than one male during a given estrous period (Smith, 1984; Birkhead and Møller, 1998). As many as nine hypotheses have been proposed to explain the advantages of multi-male mating (MMM) for female mammals (e.g. Ginsberg and Huck, 1989; Møller and Birkhead, 1989) but few empirical data are available to discern among them. Two hypotheses that have been tested are that MMM increases probability of pregnancy and litter size. Hoogland (1998) found that in Gunnison’s prairie dogs, Cynomys gunnisoni, MMM
0376-6357/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S0376-6357(02)00022-0
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increased the probability of conception when mating with more than two partners and resulted in a larger litter size. The mechanism by which MMM increases conception rates or litter size is not known, but may be to guard against male sterility or sperm depletion (see Haig and Bergstrom, 1995) or just to stimulate ovulation. Hoogland, however, was not able to address whether increased litter size and conception rates were a function of MMM or if multiple copulations with the same partner would have the same effect. M. Fleming (1995, personal communication) found that in mink, Mustela 6ison, both increasing numbers of partners and numbers of copulations with the same partner increased litter size. Therefore, the number of copulations, and not necessarily number of partners, may affect litter size and probability of pregnancy. The objective of this study was to first document if MMM occurred in the prairie vole, Microtus ochrogaster, a reportedly, socially monogamous species (Getz et al., 1993; Carter and Getz, 1993; but see Evans and Dewsbury, 1978). Second, we attempted to test two hypotheses for MMM: that MMM (1) increases the probability of conception and (2) results in a larger litter size. Alternatively, increased numbers of copulations with the same partner, and not necessarily additional partners, could affect the probability of pregnancy and litter size. We conducted a laboratory study in which we gave females the choice of mating with up to three males and then recorded frequency of copulations, number of mating partners, pregnancy rates, and litter sizes.
2. Methods All animals used in this experiment were from the prairie vole colony at the University of Memphis and were first or second generation offspring of wild-caught outbred voles from Shelby County, Tennessee. Animals were housed singly in standard polycarbonate mouse cages and kept on a 14-h light:10-h dark photoperiod with food and water provided ad libitum. All animals used were 67– 227 days old. We conducted the experiment using a four-
chambered apparatus following the design and methods of Berteaux et al., (1999). This apparatus consisted of a wooden box divided into four chambers. Each of the three adjacent front chambers was 25×25 cm and the rear chamber was 25× 75 cm and extended the length of all three front chambers. A 10× 10 cm doorway provided access from the larger back chamber to each of the three front chambers; the front chambers were not accessible to each other. The front wall of the apparatus was made of Plexiglas to allow observation of the three chambers. Food, water, and bedding were provided in each chamber. The photoperiod was maintained as in the lab colony. To initiate a trial, three unrelated, sexually experienced adult males of similar age and weight (9 2 g) were used for each experiment. One male was tethered in each of the smaller chambers to a steel bar using a flexible wire with a swivel clip attached to a collar consisting of a thin plastic cable tie. Males had free movement within their chambers but could not enter the rear or each other’s chambers. The openings to the back chamber initially were blocked with sliding doors made of hardware cloth. An unrelated, nulliparous female that was unfamiliar with any of the males was placed in the back chamber for a 24 h acclimation period. We assumed that females would use this time to assess the three males and become hormonally primed for mating (Hasler and Conway, 1973). Females were used just once (N = 47); a few males that were used twice were always paired with new males and at least 3 weeks later. All trials were initiated by 09:00 h. Following the 24 h acclimation period, the doors were removed giving the female access to each of the males. We videotaped the apparatus for the next 24 h using a time-lapse VCR and a video camera with night vision. At the end of the 24 h, voles were returned to their cages and the females were observed for pregnancy and parturition. Each videotape was watched; the number of copulations (as defined by the number of mounts with thrusting; Dewsbury and Hartung, 1982) and mating partners was recorded. We could not detect ejaculation or insemination in this study, however, a paternity analysis is currently being
J.O. Wolff, A.S. Dunlap / Beha6ioural Processes 58 (2002) 105–110
conducted (S. Mech et al., submitted). In all trials, mating had continued until no mating attempts were recorded for at least 2 h (satiety typically is defined as 30 min with no intermissions; Evans and Dewsbury, 1978). We used univariate ANOVA to test for differences in numbers of copulations and litter size for females mating with one, two or three males. When P values were significant, we used Tukey’s HSD to test for differences between groups. Logistic regression was used to test for a relationship between numbers of copulations and probability of pregnancy; linear regression was used to correlate numbers of copulations with litter size.
3. Results Of 47 trials conducted, 21 females mated with one male, 16 with two males and 10 with three males. The numbers of copulations ranged from 1 to 167. Litter sizes ranged from two to five. Thirty-one of 47 females became pregnant and gave birth. The mean number of copulations performed by a female varied with whether she mated with one male (X= 23.5, S.D.=16.12), two (X = 65.9, S.D. =43.70), or three (X =81.4, S.D.=23.71) males (F2,44 = 9.03, P =0.0005). The females who mated with only one male received significantly fewer copulations than the other two groups. There was no difference between two and three males (Tukey’s HSD test). Using logistic regression, females were more likely to become pregnant and give birth with larger numbers of copulations (G 21 = 5.055, P = 0.025). The probability of becoming pregnant was 78% for 25 copulations, 88% for 50 copulations, and 99% for \70 copulations. However, data analyzed by a log-linear model revealed that the number of males with which a female mated had no significant effect on the probability of pregnancy for one (P = 0.619, n=21), two (P = 0.688, n = 16), or three (P=0.700, n =10) males (all G 22 B0.869). Mean litter size was not correlated with the numbers of copulations (R 2 =0.128, P = 0.307). Similarly, litter size did not differ significantly for females that mated with one (X = 4.0, S.D.=
107
0.707, n= 13), two (X= 4.0, S.D.= 0.632, n= 11), or three (X= 3.7, S.D.= 0.787, n= 7) males (F= 1.39, P= 0.265). The largest litters (five young) were born to mothers who mated with one male (three of five) or two males (two of five; Fig. 1).
4. Discussion Female prairie voles mated with more than one male in 26 of 47 (55%) of the mating trials. These results are similar to those of Berteaux et al., (1999) who found that 29 of 39 (74%) female meadow voles, M. pennsyl6anicus, mated with more than one male in similar trials. In meadow voles, the percentage of females mating with one, two, or three males was 21, 55 and 24%, respectively, compared with 45, 34, and 21%, respectively, in our study with prairie voles. Thus, polyandrous mating in the supposedly socially monogamous prairie vole was relatively high, at least in this laboratory setting, and similar to that of the promiscuous meadow vole. MMM by female prairie voles also was demonstrated in the laboratory using a different procedure (Evans and Dewsbury, 1978) but with similar results to ours. Other than one field study in which multiple paternity was found in one of three litters (Carter and Getz, 1993), virtually nothing is known about MMM in prairie voles in natural environments,
Fig. 1. The relationship between numbers of copulations and litter size for female prairie voles. A litter size of zero indicates that females did not get pregnant.
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but it is thought to be uncommon (Carter and Getz, 1993; Getz et al., 1993). However, our current field study has shown several males in the vicinity of females at the time of conception (unpublished). As has been shown in our study and those of other vole species (e.g. Dewsbury and Hartung, 1982; Salo and Dewsbury, 1995; Berteaux et al., 1999), females play an active role in mate choice and seeking multiple partners. Increased numbers of copulations increased the probability of pregnancy. This difference was slight, however, in that overall, 70% of females conceived regardless of numbers of matings or mating partners. The probability of conception was 78% for females with \ 25 copulations, 88% with \50 copulations, 99% with \70 copulations, and one female, conceived with only one mating from a single male. Other laboratory studies on copulatory behavior of prairie voles have shown 42 to 65 mounts with thrusting to satiety (Gray and Dewsbury, 1973) and 6– 39 mating bouts with one male over a 24 h period (Roberts et al., 1999). In meadow voles, females that mated with one, two, or three males copulated a mean total of 10.7, 25.6, and 33.4 times, respectively, (Berteaux et al., 1999) compared with 23.5, 65.9, and 81.4 times, respectively, in our study. Data on copulation frequency for voles are not available from wild populations, but laboratory evidence suggests that 25 copulations are usually sufficient for ovulation (Dewsbury, 1975; Gray et al., 1974, 1977). The larger numbers of copulations observed in prairie voles than meadow voles may be due in part to their differences in mating systems (Dewsbury, 1981). Prairie voles are socially monogamous (Carter and Getz, 1993; Getz et al., 1993) and may require greater amounts of copulatory stimulation than other promiscuous voles for ovulation, conception, and perhaps pair bonding (Gray et al., 1974, 1977; Roberts et al., 1999). Females did not increase their litter size or their pregnancy rate by increasing the number of males with which they mated when controlling for differences in number of copulations. Similarly, the number of copulatory events also did not have a significant effect on litter size although increasing numbers of copulatory events, independent of male number, did increase a female’s probability
of becoming pregnant. Our results do not support either of the two hypotheses we tested for MMM in female prairie voles: that MMM increases litter size or increases a female’s probability of becoming pregnant. Our sample size is relatively small in that very few (n= 10) females mated with three males. However, the mean litter sizes for females mating with one, two, or three males are very similar to each other (4.0, 4.0 and 3.7, respectively) and a power analysis revealed that we would need 70 females in each mating category to detect a 0.25 pup difference in mean litter size. If anything, the trend of litter sizes tended to decrease for females mating with three males. Since the means are so close, we think that even with a larger sample size, litter sizes would not differ significantly when females mated with multiple partners. Maternal effects may also affect litter size, however, all of our females were of similar age, sexual experience, and parity. While Hoogland (1998) found increased litter sizes in multiply-mating female Gunnison’s prairie dogs, MMM did not affect litter size or reproductive success in other species of ground squirrels and prairie dogs (Schwagmeyer, 1986; Murie, 1995; Hoogland, 1995). In fact, the paucity of examples to demonstrate an increase in litter size with multiple mating partners (references in Hoogland, 1998) suggests that this may not be a primary function of promiscuity in female mammals. We do not know the benefits to female prairie voles for mating with multiple partners. Gaining greater paternal care or obtaining material benefits are not good explanations for MMM because additional mates do not provide additional parental care. Alternative hypotheses such as sperm competition (Haig and Bergstrom, 1995) or increasing genetic variation of offspring are possible, but unlikely as a universal explanation for MMM (van Schaik and Keppeler, 1997; Jennions and Petrie, 2000). MMM did not decrease sexual harassment in this study in that males were tethered and could not interact with each other or pursue the females. Additional copulations were solicited by the females and not forced by males. In that MMM is common among rodents (Agrell et al., 1998), primates (van Schaik and Keppeler,
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1997) and other species in which infanticide has been reported (Ebensperger, 1998), we propose that one primary function of mating with multiple partners by females is to confuse paternity and deter neighboring males from killing offspring (Agrell et al., 1998; Ebensperger, 1998) though this remains to be tested. Further arguments for and against alternative benefits of MMM in mammals such as voles are summarized in Berteaux et al. (1999). Our results are interesting in that they confirm that MMM occurs in rodents even in a supposedly monogamous species and females actively solicit extra matings (see also Evans and Dewsbury, 1978). Our results are from a laboratory study and need to be confirmed from the field, but preliminary data from our work suggest that MMM also occurs in the field (unpublished). We were unable to support the hypotheses that MMM functions to increase litter size or probability of pregnancy. The probability of pregnancy does correlate positively with number of matings, but independent of number of partners. We are currently conducting a paternity analysis from this study to determine those parameters associated with mate choice, timing of conception, and reproductive success of males as well as how mating with multiple partners affects paternity (S. Mech et al., submitted). These experiments comply with the current animal welfare laws of the USA.
Acknowledgements This work was supported by NSF grant 9996016. We thank Steve Mech, Michael Ferkin, Shawn Thomas, John Hoogland and anonymous reviewers for helpful comments on the manuscript. S. Mech assisted with the statistical analysis.
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