The purpose of this study is to investigate the possible plasticity of primate sexual behavior as an adaptive trait, especially with regards to sex ratios in primate groups. A number of studies indicate that the early experiences of infant mammals can have a pronounced effect on behavior and neurology later in live (Bales et. al. 2007). However, it is possible that primates posses a high level of malleability when it comes to sexual behavior, which allows them to adapt to changing situations. It is possible that females posses a higher level of sexual behavior plasticity than males, for several reasons (Baumeister 2000). Dominance hierarchies appear to be an important aspect of sexuality, at least in non-human primates, and it may be the case that they mediate the relationship between sex ratios and sexual behavior. This study seeks to test the broad idea of primate behavioral plasticity through the specific method of gender ratio manipulation.

Nunn and Pereira (2000) demonstrate that it is possible to conduct a primate study that requires the careful recording of key facts about each individual primate, and they propose some factors that may be important to the determination of sex ratios in the wild. Since these factors appear to influence sex ratios, it is possible that they are the ultimate causes of changes in sexual behavior due to changes in sex ratios. In order to test this hypothesis, it will be necessary to test them under controlled conditions. One might do so by observing how different levels of nutrition affect sexual behavior in the sense that mate competition decreases or increases, or the subjects tend to be more or less polygamous. Also, in order to ascertain if changing sex ratios cause a shift towards or away from the LRE hypothesis that found support in Nunn and Pereira (2000), dominance ranks should be recorded. Different group sizes could be compensated for by simply dividing the number of females the individual is submissive towards on a regular basis by the number of females in the group that individual belongs to. Demographic statistics relating to the Nunn and Pereira (2000) test of the LRE hypothesis should also be recorded.

Ringtailed lemur and (presumably) young lemur in a zoo:

Ringtailed lemur being fed by hand:

Method

The proposed study will include 60 ring-tailed lemurs currently housed at the Duke Lemur Center as subjects. All of the subjects will be lemurs born during the same breeding season, if possible. First, six groups will be isolated from each other, and each group will have a gender ratio that closely resembles the ratio that was previously existing in the center. The subjects will be randomly selected from these six groups. The first three groups will include lemurs that live in manipulated groups from the time they are born, while the second three groups will live in groups that are manipulated after they are adults. The ten lemurs in group one will live in a group with a ratio of one male to nine females. The ten lemurs in group two will live in a group with a ratio of five males to five females. The ten lemurs in group three will live in a group with a ratio of nine males to one female. The last three groups will be subdivided in the same way, so that group four will have a low male to female ratio, group five will be equal, and group six will have a high male to female ratio. The manipulated ratios will be achieved by randomly removing non parental lemurs from the six previously divided groups until the needed ratio is left. This subtraction method will help avoid an increase in male on male violence that might result from new group formation. The subjects will live in their manipulated groups for a period of three months. The experimenters will record several measures of the effects of the gender ratio manipulation. Male and female dominance will be measured in two ways. The first method will record the number of confrontations each lemur participates in during randomly selected days, and how many times that lemur is the submissive party at the end of each confrontation. This number will be divided by the total number of confrontations participated in to result in a confrontation dominance score. The other method of recording dominance will be to observe how many individuals a lemur is submissive towards on a regular basis and divide that number by the total number of lemurs in the group. The number of sexual partners for each individual during randomly selected days will be recorded. The number of offspring for each individual will be observed, and DNA analysis will be used to determine the paternity of offspring. Choosiness of each individual will be evaluated by noting how many times that individual rejects an offer to copulate during randomly selected days. At the end of the three month period of living in the manipulated groups, each individual will be placed in an enclosed area with an unfamiliar member of the opposite sex for a period of six hours. Immediately following the six hour period of cohabitation, subjects will be placed in a testing chamber where they will have a choice of spending time with the now familiar individual, an unfamiliar opposite sexed lemur, or in an empty section of the chamber. The experimenters will record the location of the experiment subject and the amount of time spent near the side of the familiar or unfamiliar lemur.

Ringtailed lemurs calling, drinking, and possibly licking algae:

Expected Results

It is expected that males who rank higher in dominance will have more partners and more offspring than other males, and will also be found to be choosier than lower ranked males. Dominant females will be expected to have more female offspring, and also be choosier. It is predicted that females will be more likely to prefer a familiar companion than males. It is further predicted that adult and neonate manipulation groups will show the same differences in dominance from other groups of the same category, as well as the same number of partners on average. Females in the skewed ratio groups will be expected to show more of a difference from those in the equal ratio groups in dominance, number of partners, and choosiness.

Ringtailed lemurs in the Edinburgh zoo: