Lisa Mueller

Writer’s comment: As an anthropology student, I listened to several professors explain how females of many animal species influence the direction of evolutionary change through their choices of particular mates. Often cited was a famous study showing that certain female birds prefer mates with ridiculously long tail feathers—even if the feathers are glued on researchers! However, each time this favorite topic was addressed, I was disappointed to hear that female choice among my nonhuman primate cousins was considered an insignificant evolutionary force. I always wondered how this could be. I thought it inconceivable that females had no interest in actively choosing a suitable mate. Why should females bear less selective impact than males? The feminist in me set out, in English 104E (Scientific Writing), to prove the experts wrong, and I was pleasantly surprised at the overwhelming amount of evidence that agreed with me.
- Lisa Mueller

Instructor’s comment: Lisa Mueller wrote this review of literature for my English 104E: Scientific Writing course. The assignment was to write a review paper that would bring readers up to date on a scientific topic or problem of critical importance, synthesizing material from a number of journal articles. In her review, Lisa very clearly lays out the line of research on the topic of “female mate choice” in nonhuman primates, letting readers know what is known on this topic, what is not known, and what paths future research will likely take. Her paper is a well-organized, clear, and articulate review on a topic that will interest a wide spectrum of readers.
- Linda Bates, English Department



Introduction

     Darwin argued that certain characteristics could not have evolved by natural selection (cited in Harcourt, 1995). These traits, he proposed, do not perpetuate the survival of the individual, but rather they increase the reproductive success of the individual, and hence, the traits must have evolved by some other process. Darwin called the process sexual selectionand explained that it consists of two separate aspects: male-male competition and female mate choice. Male-male competition for females, or intrasexual competition, acts as a significant selective force because the stronger, larger male with more fierce tactics and weapons almost always wins the fight and thus wins the mating. Female mate choice, formally known as intersexual selection, is a strong evolutionary force in many species because it is the female who decides which males will mate, and hence, contribute genes to future generations.
     Female choice has been considered a less important evolutionary force in nonhuman primates by those who insist that the female does not actively choose her mate, but rather, she merely submits to matings with the winner. The purpose of this review is to present recent research indicating that female, nonhuman primates are indeed exerting some influence on which male genes are let into the pool. I will present results of studies of the last decade showing that these females are choosing the fathers of their offspring by directly initiating copulations, refusing matings with unfit or unattractive males, and performing pre-copulatory penis inspections. I will also discuss anti-sperm responses in the female reproductive tract that female primates have obtained through their mammalian heritage. These responses select against undesired spermatozoa. Finally, I will provide new evidence that in at least one nonhuman primate species, female choice has a greater influence on male reproductive success than male-male competition.

Initiating Copulations

     It is surprising that female mate choice has developed secondary importance to intrasexual competition despite overwhelming evidence of female initiation of sex in nearly every primate species studied (Smuts, 1987). Females in the majority of species display their interest in a particular male by presenting their hindquarters to him just prior to copulation (Hrdy & Whitten, 1987). According to one research group, several species have evolved even more elaborate means of informing males of their mating prospects (Hrdy & Whitten, 1987). Ruffed lemurs, Varecia variegata,slap desired males, rub their bodies against them, and then quickly dart away. Callithrix jacchus,the common marmosets, flick their tongues, arch their backs, crouch, and piloerect their fur. Grabbing and jerking the head of an attractive male is the preferred technique of Barbary macaques, while Patas monkeys, Erythrocebus patas,blow into their cheek pouches, drool, and hold their tails up in a curled position. Female orangutans touch their mouths to the genitals of chosen mates while hanging from above (Hrdy & Whitten, 1987).
     Smuts (1987) suggests that in some instances, solicitation of sex by female primates is subtle and is often unnoticed by researchers who incorrectly assumethat only males influence the likelihood of copulation. One study of captive rhesus macaques revealed that males were successful at copulating with a female only if she previously glanced at him and maintained a particular posture (Smuts, 1987). However, because these gestures allowed the macaques to communicate with one another from relatively far distances, observers overlooked the gestures entirely and wrongly suspected that females were receptive to any males who attempted to mount them (Smuts, 1987).

Refusing Matings

     Females of virtually all primate species have also been witnessed refusing the mating attempts of males (Smuts, 1987). Often an uninterested female will walk away from a mounting male, or she may sit down, preventing penetration (Smuts, 1987; Nishida, 1997). Aggression may even be used by females to discourage unwanted copulations in some primate species: bushbabies (Galagospp.), fat-tailed dwarf lemurs (Cheirogaleus medius),vervets (Cercopithecus aethiops),talapoins (Miopithecus spp.),and gelada baboons (Theropithecus gelada)(Smuts, 1987). Rape has been observed in just one of the approximate two hundred species of primates (Smuts, 1987). The fact that only orangutan males use such force to acquire matings is a clear indication that the female’s choice is significant in the vast majority of nonhuman primates (Smuts, 1987).

Pre-copulatory Penis Inspections

     In a recent study detailing mating behaviors of the chimpanzees of the Mahale Mountains National Park in Tanzania, Toshisada Nishida (1997) was the first to report that female chimps are occasionally observed performing a pre-copulatory penis inspection, or “erection check,” to determine the condition of potential mates. Nishida (1997) describes the phenomenon: