Psychology 351 – Animal Behavior

Alcock, Chapter 10 – The Evolution of Reproductive Behavior

Introduction
- Typically males do the courting and females do the choosing.

The Evolution of Differences in Sex Roles
- Fundamental Difference Between the Sexes : Males produce sperm and females produce eggs.
  • In virtually all sexes, eggs are larger than sperm, which are usually just large enough to contain the male’s DNA and enough energy to fuel the journey to the egg.
- Differences in Male Sex Roles : males almost always have enough gametes (sperm) to fertilize the eggs of a great many females. A male’s contribution of genes to the next generation generally depends directly on how many sexual partners he has : the more mates, the more eggs fertilized, the more descendants produced.
- Differences in Female Sex Roles : females do not need to maximize their sexual encounters because their reproductive success is typically limited by the number of eggs they can mature, not by any shortage of partners.
- Operational Sex Ratio : the ratio of sexually receptive males to receptive females. There are many more sexually active males than females at any given time, creating a male-biased operational sex ratio.
- Parental Investment : expenditures of time and energy and risks taken by a parent to help one offspring. Increases the possibility that an existing offspring will survive to reproductive age.
  • In the animal world, the general rule is that females male a larger parental investment per offspring than males do. Example : Mammals – females nourish the embryos within their bodies, then care for the babies when they are born. Males typically impregnate a female and depart.

Testing the Evolutionary Theory of Sex Differences
- Sex Role Reversal : for species in which males make a larger parental investment, the operational sex ration should be biased toward females, leading to female competition for mates and careful mate choice by males.

Sexual Selection and Competition for Mates
- Sexual Selection : the advantage which certain individuals have over others of the same sex and species, in exclusive relation to reproduction (Darwin). A subcategory of natural selection, focusing on selective consequences of sexual interactions within a species.
- Components of Sexual Selection : Intra-Sexual Selection – the members of one sex compete with one another for access to the other sex.
- Dominance Hierarchy : individuals that live together in groups interact aggressively for a while before sorting themselves out from top dog to bottom mutt.

Alternative Mating Tactics
- In some species, lower-ranking males can and do develop friendships with particular females as long as they show a willingness to provide protection for her and her infant.
- Males also form friendships with other males. These alliances help collectively confront a stronger rival that has acquired a partner.

Conditional Mating Strategies
- Conditional Strategy : the ability to adopt the “inferior” tactic can be adaptive if small or otherwise handicapped individuals gain more fitness than they would if they were to try to use the tactics of their dominant opponent.

Distinct Mating Strategies
- Distinct Strategy : when males are either territorial or subordinate satellites on another’s territory, a trait that is passed on to male offspring.

Sperm Competition
- Sperm Competition : competition among males with respect to the fertilization success of their sperm. A very common phenomenon in the animal kingdom, no matter whether the fertilization is internal or external.

Mate Guarding
- Mate Guarding : by standing guard over a mate, a male can sometimes physically prevent his partner from mating again, thereby improving the odds that his sperm will not have to compete with those of other males for her eggs.
- Advantages : extra egg fertilizations with one female.
- Costs : the loss of opportunities to seek out other females.

Sexual Selection and Mate Choice
- Mate Choice : can create sexual selection pressure. In most species it is exercised primarily by females, a fact that has great evolutionary significance for males.
- Attributes of Males that Females Favor : Nuptial Gift – a gift of food that the male presents to the female so that the female will copulate with him. Males of some other animals offer parental care for the female’s offspring, rather than a nuptial gift.

Mate Choice Without Material Benefit
- Good Parent Theory : aspects of male color, ornamentation, and courtship behavior are sexually selected indicators of a male’s capacity to provide parental care.
  • Example : for male birds, being able to sing a very rapid trill might constitute a demonstration of the male’s physiological state, an honest signal of the quality of the neuromuscular system and overall condition.

Male Courtship Signals Mate Quality
- Heightened sensory simulation provided by males during courtship often does influence mate selection by females.
- Healthy Mate Theory : female preferences are focused on male courtship displays and ornaments that serve as indicators of a potential sexual partner’s health or parasite load.
- Good Genes Theory : male courtship displays and ornaments provide information to females that enables them to choose males with viability-enhancing genes.
- Runaway Selection Theory : discriminating females acquire sperm with genes whose primary effect is to influence their daughters to prefer the male traits their mother found attractive and to endow their sons with attributes that will be preferred by most females.

Sexual Conflict
- Chase Away Selection Theory : exploitation and conflict between the sexes, rather than cooperation and mutual benefit.

Presentation Notes:

“The Reproductive Behaviors of the Brown Tree Snake”
(Boiga Irregularis)

I. Mate Choice
A. Elaborate courtship ritual
-Females can reject mates they’ve deemed unfit by lifting their tails and releasing cloacal excretions.
-Females engage in head-lifting behaviors in courtship, so they can assess the males’ success in pre-courtship combat with other males.
B. Courtship
-Males sense pheromones via their vomeronasal organs by flicking their tongues
-The courtship sequence (of males) includes head jerking, chin-rubbing along the female’s back, snout probing, and chasing behaviors
II Copulation (Sex)
A. The process
-Once a male has mounted the female, he attempts to align his cloaca (opening) with hers by intertwining tails.
-The female must gape her cloaca in order for the male to insert his hemipene.
-The entire mating process takes about 15-20 minutes to complete.
B. Sperm Competition
-Like most snakes, when males feel they are in competition with other males, Brown Tree Snakes can leave a waxy plug in the female’s cloaca after copulation to prevent other males from successfully copulating with her.
C. Sexual Conflict
-Unlike many other species, male Brown Tree Snakes can’t circumvent female mate choices in order to accomplish fertilization.
-The female must willingly gape her cloaca in order for the male sex organ (hemipene) to be able to enter..
-Females can successfully reject males with bodily excretions and body language.