Session 11: Sexuality and Reproduction

1.  Key Concepts:

For most mammals, sexual behavior and reproduction are tightly linked, spatially and temporally.  However, anthropoid primates (monkeys and apes) often participate in sexual behavior during periods when it is unlikely or impossible for conception to occur.  Thus, in these species, sexuality and reproduction are not synonymous.  Sexual behaviors have been studied in the context of dominance hierarchy, stress reduction, and confirmation of social interactions.

2.  Terms & Definitions:

Sexuality

Sexual behavior

Reproduction

The production of offspring 

Anisogamy

Differential expenditure, directly expressed as differential size, of sex gametes by males and females.  Trivers' definition would extend the impact of this difference in gamete size into the realm of parental care as an explanation of why females so often are the major care givers.

 3.  The Behavioral Ecological Basics: 

Primates exhibit strong parental investment.  The implicit assumptions of ecological/economic models, measures of energy in and out of a system, and the role of limiting factors such as resources, mates, etc., as well as the strong role of anisogamy in mammals, including primates, inform our ability to understand how sexual behaviors fit into the broader context of social dynamics in primates.  If the limiting sex (females) is constrained by distribution of resources, the non-limiting sex (males) must map to the distribution of the limiting sex.  In general, the limiting sex is female due to the energetic expenditure of producing large ova.  In other words, males want lots of sexual partners, and females want a good mate.

But…

• What are the actual costs of sperm?  Millions must be produced for each ejaculate, sperm production machinery must be maintained and in good working order.
• How does one measure “reproductive effort”?
  
• Variance in reproductive success potentially greater for non-limited sex (males) than for the limiting sex (females), as in most sexually reproducing systems.

Intrasexual selection/competition:

Sexual selection and competition behaviors occur throughout the duration of mating behaviors.  Listed below are some strategies used by males and females throughout the reproductive period.

• Direct competition to gain access to mates: physical confrontations, mate accumulation, display competition
• Scramble competition to obtain sufficient health state: “fatted male” for example, female nutrition

• Harassment (unclear as to actual rate and effectiveness)
• Mate guarding (for serial copulators)

Post-mating:

• Sperm plugs/copulatory plugs

• Sperm competition
• First to mate
• Most sperm (testes size and sperm output)
• Female evacuation of sperm
• Infanticide: Bruce effect and post-partum infanticide

Internal Gestation and Lactation: Costs to mammalian females:  There are important physiological differences between the sexes, and this results in differential investment in offspring.  Gestation is moderately costly, but lactation is extremely costly.  Resource diversion to offspring, while relevant in order to remain evolutionarily fit, can come at the expense of the health of the mother.  In some species, this results in lactational amenorrhea.  In other species, the result is highly dependent young.

4.  Challenges to mammalian reproduction:

Resources:

Females must acquire sufficient energy for gestation and lactational periods.  Males, however, must have sufficient energy for spermatogenesis and competition for access to females.  Seasonality of food resources, infant caretaking and reproductive inhibition all contribute to declines in resource availability to adults

Anatomy: complementary genitals

Behavioral physio-chemical stimulation prior to copulation is required in some species.  Variation in mechanistic copulatory patterns, as in felids, primates, rats, is required and is thereby a limiting factor for between species or subspecies reproductive behaviors.

Physiological signals: Sex swellings:

Photo by lemai13. Some rights reserved.

Photo by Emily Walker. Some rights reserved.

5.  Primate sexual behavior:

Females: 

Primate sexual behavior can be broken down into two major categories.  The first—proceptive behaviors—are those behavioral cues that individuals display or exhibit in order to announce their receptivity to sexual behavior.  The second category of behaviors includes selectivity, physiology, and copulatory patterns that focus on those behaviors surrounding copulatory behaviors.

Males:

Male primate penises are relatively generic.  Penis morphology in many animals matches specifically with females and is limiting factor in intercourse.   Testes size, however, varies wildly.  Volume and mating pattern within a species or group are highly correlated.  Large testes – multimale:multifemale. Sperm plugs common in prosimians (and a few monkeys).  Some true plugs; some simply amount of sperm/ejaculate that form blocking mass.  Males also participate in proceptive and receptive behaviors.

6.  Sexual intercourse in non-human primates:

Copulatory pattern: 

Unlike the remainder of the mammalian world, primates utilize a variety of copulatory patterns.  These include:

• ventral-dorsal
• ventral-ventral
• positional variants
• Other sexual behavior: oral-genital, manual genital stimulation.

Socio-sexual behavior: 

Plays a major role in some species of primates in maintaining rank, status, and hierarchy, reconciliatory behaviors, and conflict avoidance.  These behaviors also include social negotiation and bond reinforcing as well as "play" behaviors in immatures.

Other sexual behavior:

•  Paraphilias/Fetishes?

• potentially only in apes.
• Object fixation in captivity.
• Cross-species sexual attraction.
• Object use in self-stimulation.
• Context-dependent masturbation.

Homosexual behavior:

Although female-female and male-male sexual behaviors have been documented in many primate species, including humans, chimps, orangutans, and macaques, there remains a substantial debate about the frequency and significance of these behaviors. For example, A. Dixson argues some homosexual behavior occurs, C. Campbell argues that plenty of behavior occurs, but no patterns emerge so the significance is low, and P. Vasey argues that homosexual behaviors are quite common in some species and present in many more.

7.  Additional Material:

Primates in Perspective.  2007.  C.J. Campbell. A. Fuentes. K.C. MacKinnon. M. Panger. S.K. Bearder.  Oxford University Press.

Chapter 25: Primate Sexuality and Reproduction - Campbell

Chapter 27: Mate Choice - Manson