The contingency table of partner preference tests indicated that female, but not male, Taiwan field voles showed significant preference for the cohabitation partner (Table 2). What does such a result mean? I can think of three potential explanations.
1. The result is inconclusive, because sample size is small.
2. The result is inconclusive, because cohabitation is too short.
3. The result supports that Taiwan field vole mating system may be polygyny.
In response to explanation 1, I admit the sample sizes were indeed very small.
However, both preference parameters: the variations in the time spent in
partner/stranger chambers and the time spent in physical contact with partner/stranger had substantial variations. Power analyses indicated that the sample sizes required for generating enough power to reject null hypothesis were very high. Nevertheless, I did observe differential variation between males and females. We should consider it a biological reality. In response to explanation 2, there is no empirical study available that indicates the length of cohabitation required for examining partner preference. The only related study (Williams et al. 1992) I know of indicated that 24-hr cohabitation is sufficient to generate the preference for partner in prairie voles. Prairie vole is a
“model” monogamous species. Its mating system has been examined in the field and laboratory in numerous studies (Thomas and Birney 1979, Getz et al. 1981, Carter et al.
1995). I would like to argue that the use of 24-hr cohabitation, a conservative design, should be employed in the partner preference test. My notion was that if Taiwan field voles were as monogamous as prairie voles, we should be able to see partner preference in both males and females (Williams et al. 1992). Finally, I would like to propose explanation 3. I want to consider that the high variations of and differential variations
between males and females are biological reality, i.e., not a fraud due to experimental design. Such experimental results suggested that male Taiwan field voles have a strong potential to mate with multiple females when there is an opportunity. In a sense, the results support polygyny because females remain with one mate, while males do not.
Less than 3% of mammalian species examined show monogamy. Compared to other taxonomic groups, the requirement for females to stay with young (pregnancy and milk-nursing) gives males the opportunity to mate with other females in mammals.
Thus, it takes unique environmental situation for monogamy to occur. Monogamy may occur only when there is no environmental “polygamy potential” or there is no
opportunity to take advantage of what “polygamy potential” the environment affords (Wittenberger and Tilson 1980). For example, when male parental care is necessary for female reproductive success, or when an individual has less fitness with two or more mates than with one, the mating system is likely to show a “monogamy pattern”
(Wittenberger and Tilson 1980). Many ecological factors, such as resource availability and dispersion, mate availability and dispersion, and predation risk may affect mating system (Orians 1969, Kleiman 1977, Emlen and Oring 1977, Wittenberger and Tilson1980). There were many evidences that the mating system of the same species might be different in varied situation, such as habitats, seasons and population density (Getz et al. 1987, McGuire et al.1993, Cushing et al. 2001, Parker et al. 2001). The mating systems of a species may display intraspecific variations in response to different environmental conditions (Lott 1984). For example, the above mentioned monogamous prairie voles may display a polygamous mating system during winter breeding season
Kansas (Fitch 1957, Gaines and Johnson 1984, Swihart and Slade 1989). In contrast, promiscuous meadow voles may develop monogamous-like behaviors selective partner preference and stranger-directed aggression during the colder months of the year or under low population density during the summer breeding season (Madison et al.
1984, Parker et al. 2001). I propose that the mating system of Taiwan field vole is not monogamy, but polygyny. At least, it is strongly flexible depending on environmental conditions. In this study, all experimental voles were housed in the breeding room. They did not suffer stresses from food, weather and predators. Monogamy might not be necessary for Taiwan field voles. Only under some conditions, say circumstances that require bi-parental care, does monogamy arise.
Do field situations push the Taiwan field vole toward a monogamous mating pattern? Nearly all field studies on the ecology of Taiwan field voles came from
populations in a Yushan cane grassland in the He-huan Mountain. In such a habitat, Dr.
Liang-Kong Lin and colleagues found supports for a monogamy mating system (Wu 1998, Chen et al. 2006, Yang 2011, Wu et al. 2012). As mentioned earlier in the
introduction, I was not convinced by the available information. In the most recent study (Yang 2011), they found that paternal care was beneficial for the growth and
development of pups under a cold condition. However, the pup survival did not decrease without paternal care. In contradiction to their studies, my results support a non-monogamy mating system in Taiwan field voles. Three additional studies lend similar supports. Quan et al. (2010) in a 6-week daily trapping study found home range overlaps among multiple adult males and females, although it was in the alpine forest habitat. Second, Liang (2012) showed that trapping at nest sites often capture more than a pair of adults, particularly during early breeding season (March~June). Finally, the
density of receptors for OTR binding in the nucleus accumbens, a positive indication of pair bonding, was low in Taiwan field voles. The level was between those of prairie voles and meadow voles.