Mating strategies and mating systems


	MATING STRATEGIES AND MATING SYSTEMS	A highly vigilant male blue monkey.

One of my main interests is the connection between social behavior and reproduction, which reflects a "functional" or "ultimate" orientation toward behavior. I have conducted a long-term study of reproductive strategies and the mating system in blue monkeys (Cercopithecus mitis) which developed from surprising serendipitous discoveries we made about their behavior some time ago (when I was a graduate student). This species had been viewed as exemplifying a "harem polygynous" mating system, as individuals are normally found living in groups containing a single adult male, multiple adult females, and young. Our observations revealed, however, that the resident male is not the only male to mate in the group, and he does not necessarily achievemost matings. Other males visit the group for varying periods and in varying ways (some surreptitiously, others more conspicuously), and mate with the group’s females. Females appear to play an active role in determining patterns of mating as they persistently solicit the attentions of certain males (usually newcomers) while avoiding others. This behavior seems unexpectedly dynamic for a species once thought to exemplify harem polygyny, but its interpretation has been difficult because the reproductive consequences of being a resident male or a temporary intruder are not known. The fact that female mating is not closely tied to conception (see Karen Pazol’s work) makes things even more complicated!

So we (Jeff Hatcher, Don Melnick and I) tried to pin down these consequences by assessing paternity genetically, using microsatellite loci, and by relating paternity to behavioral strategies of adult males and females. Our data come from multiple groups over multiple years. It’s been challenging to work with fecal samples, which can be gathered non-invasively, but which provide only degraded DNA. Nonetheless we have found that while residents may sire -- in a given group and year -- more offspring than any other individual males, they are losing about half the paternities in their own groups to outside males. This loss occurs whether or not the group is experiencing a 'multi-male influx' breeding season that year. Note that the resident male is invariably socially dominant, yet it seems impossible for him to keep a monopoly on female matings, especially in a visually dense habitat and when groups are large. Females show every indication of having a sexual interest in unfamiliar males as well.

The results of this research are relevant to understanding the importance of co-residence with females in the lives of males. Behavioral ecologists studying many animal species have generally viewed male social strategies (i.e. whether they live with females) as direct consequences of their mating strategies. In birds, however, it is now well known that social and reproductive units often do not coincide, and there is increasing evidence, primarily behavioral, for a similar non-correspondence in mammals, including primates. Where such disjunction occurs, new questions arise about why males reside with females at all, and what other factors might determine their reproductive success. An emphasis on immediate reproductive benefits for males is perhaps overly simplistic, especially in long-lived animals like primates for whom survival is a major component of lifetime reproductive output: males and females provide services to one another in various currencies. In terms of factors other than residence that influence male reproductive output, dominance rank and female choice are two factors whose effect we are trying to evaluate empirically in our study system. Current work on female choice is being done together with Mary Blair, a Ph.D. student in our department. In addition, Su-jen Roberts is developing a project to examine variation in paternity monopolization across groups of different size, across time in a male's tenure as resident, and with respect to varying patterns of female sexual activity.

Our behavioral data from multiple years can also be used to understand the variation in the occurrence of multi-male influxes during the breeding season. It now seems clear that female defendability is critical: when more females are sexually active, a single male has a harder time monopolizing them. A second variable that is important is male intruder pressure, but we are still unsure of the factors cause variation in this parameter.