Bryja, Josef., Patzenhauerova, Hana., Albrecht, Tomas., Mosansky, Ladislav., Stanko, Michal., Stopka, Pavel. (2008). Varying levels of females promiscuity in four Apodemus mice species. Behavioral Ecology and Sociobiology, 63, 251 – 261.
Summary by Moses Galvez, Natasha Grabowski, and Alex Zolad
For Dr. Mill’s Psyc 310 Class, Spring 2011
The purpose of this study is test the varying degrees of promiscuity in a mice species and what were factors that led to the differences. Three common mating systems for animals found in nature are monogamy, promiscuity, and polygamy. Although humans are generally monogamous, animals have evolved many types of mating systems because of the environmental conditions that render advantages to find optimal reproductive strategies. In this article, polygamy is the primary mating system for the mice known as Apodemus because of the various environmental conditions that it lives in. The males are not limited by the much slower reproductive rate of females, as in monogamy, since they can mate with multiple females. For females, it provides genetic diversity for offspring deters infanticide (since the males are not sure if they may be theirs or another’s) and promotes successful fertilization.
The primary purpose of this research study is to “assess the role of ecological factors that may have shaped the evolution of particular mating systems” (Bryja, Patzenhaurova, Albrecht, Mosansky, Stanko, Stopka, 2008, pp 251). Researchers studied the mice species known as Apodemus and selected four of its sub-species to further evaluate the varying degrees of promiscuity found in Apodemus uralensis, Apodemus. flavicollis, Apodemus. sylvaticus, and Apodemus. agrarius. The reason for selecting this mice species is that it lends itself to research because it has four sub-species that can be found in varying environmental conditions across central Europe. The researchers will measure the environmental and ecological factors on promiscuity among the four types of mice utilized in the study. Studying these factors will allow them to make inferences about mating behavior based on which of the factors influences varying degrees of promiscuity the most. The researchers believe that it is not enough to discover that a promiscuous, or multiple male mating system (MMM), is at work, but also to study the, “species specific variation in the rate of MMM”(Bryja et al,. 2008, pp 252).
Researchers specifically tested the hypothesis that multiple paternities are a result of forced copulations stating that “it should be more frequent in smaller females that are assumed to have lower ability to defend male attacks” and secondly that fertilization by several males leads to increased litter sizes. Furthermore, testes size of males was only measured to confirm “level of multi-male mating”(Bryja et al,. 2008, pp 255) . Researchers also tested for the effects of season and abundance “on the mean observed number of fathers per litter””(Bryja et al,. 2008, pp 255). The operational definitions in thus study include; season is the number of days from the beginning of the year to the capture data, litter size is the number of embryos extracted from a female uterus and the operational definition of abundance is the number of individuals captured per 100 trap nights.
Important points of discussion prior to the study are: if there is “low abundance, the rate of female promiscuity may be lower (males are unavailable or monopolization of females by males)” and “high population may enable dominant males to exert their control more effectively in species with rigid hierarchies by preventing copulations from subordinates, so even multiple paternity would be lower here too”(Bryja et al,. 2008, pp 258). These hypothetical situations point out that abundance rates in species is not enough to suggest whether a population would have a high or low degree of promiscuity. Therefore, this is a fundamental reason that is driving their research to fin stronger predictors of promiscuity in a species.
The findings reported that multiple paternities was apparent in each of the sub-species and that there was a relative size difference in testes between sub-species. The results rendered were as such: Apodemus uralensis were found to have up to “two males that could be identified as the fathers of a particular litter”(Bryja et al,. 2008, pp 254). This species was also found to have the smallest size of testes and also found to be the least promiscuous of the four sub-species. Software analysis of data calculated that 43.5% were found to be of multiple sired litters. Furthermore, the probability of MMM strongly increased in season and abundance with this sub-species . For the Apodemus flavicollis, “up to two males could be identified as the fathers of a particular litter” and MMM was significantly associated with higher litter size(Bryja et al,. 2008, pp 254). For the Apodemus sylvaticus three males sired 9.1 % of analyzed litters and for the Apodemus agrarius “Three males sired 20.6 % of analyzed litters”(Bryja et al,. 2008, pp 254). The final two species were found to have the biggest testes and the most promiscuous mating behavior with 69.2%. Lastly, the probability of MMM strongly increased in season for Apodemus. agrarius and sylvaticus.
Upon conducting this experiment, researchers calculate that the varying degrees of promiscuity range from 45.2 % and 69.2 % and may be a result of females benefiting from this mating system. Researchers note that females may benefit from this promiscuous mating style in ways that include, “stimulation and onset of estrous cycles in female mice, assurance for successful fertilization, higher litter sizes, increased genetic diversity, and increased probability of finding genetically complementary gametes”(Bryja et al,. 2008, pp 256). As studied in class, the reasons for promiscuity in this species of mice insures that the genes of both males and females will successfully move forward due to high litter sizes and defenses against environmental conditions and diseases at the molecular level because of genetic diversity. Therefore, promiscuous behaviors in this species may in fact be in large part due to both males and females benefiting from these mating systems for each of the sub-species in the various environmental regions.
Closer study of sub-species variation reveals that the member Apodemus. uralensis reflected a low level of promiscuity. This begs the question, if promiscuity benefits males and females due to several factors, if so, our groups asks the question that why does one sub-species actually have lower levels of promiscuity if the benefits are apparent in the other members of Apodemus. Researchers conclude that for this particular sub-species, “ sex is not a commodity” due to lack of “allogrooming for sex as do males with females in Apodemus. sylvaticus”(Bryja et al,. 2008, pp 257). This oddity is credited due several factors that may include natural habitat, smaller testes size, and “frequency of interaction with other males is probably more limited”(Bryja et al,. 2008, pp 257). Larger studies are needed to further investigate each of these factors to gain a better understanding of why levels of promiscuity appear to be much lower for Apodemus. uralensis.