Male and Female Reproductive Strategies

1. Male and Female Reproductive Strategies Selection Processes

2. Natural Selection • Natural selection is the differential survival and reproduction of organisms as a function of their physical attributes. • Because of their phenotypes, which are due to traits that make up an individual, some individuals do better than others. • The concept of selection is central to Darwin's theory of evolution, and forms the cornerstone of many theories in the field of animal behavior.

3. Selection • Selection is defined as a relationship between fitness and phenotype • Fitness is defined using terms of three kinds of SELECTION MODES: • directional selection in which a trait is linearly related to fitness, • stabilizing in which there is an optimal value for the trait of interest, and • disruptive in which individuals with the smallest and largest values of the trait have the highest fitness and individuals with intermediate traits are at a fitness disadvantage.

4. Selection Modes

5. Selection Modes • Directional Selection - favors individuals possessing extreme values of a trait (like long necks in giraffes, coloration in peppered moths), which causes the population to move in a particular direction. • Example: If a climate becomes colder, a population may evolve in a consistent direction in response - thicker fur

6. Selection Modes • Stabilizing Selection - acts against individuals who deviate too far from the average, favors the average. • Example: sizes in lizards: large lizards may be subject to predation, small lizards have a hard time defending territories, natural selection favors the average

7. Selection Modes • Disruptive Selection - adapts individuals in a population to different habitats. Its similar to directional selection, but it favors either extreme, not just one extreme. Disruptive selection may occur in an area that provides different resources. • Example: Galapagos finches had a variety of food choices, smaller birds fed on small seeds, larger birds fed on large seeds, natural selection favors both the large and the small bird, but not the average bird (who would compete for both resources)

8. Sexual Selection • Charles Darwin distinguished sexual selection as a variance in the number of mates. • Sexual selection acts to refine characters in the phenotypes between males and females types. • The action of sexual selection can take the same three modes that are discussed on the previous slide for natural selection. • Directional, stabilizing or disruptive selection.

9. Sexual Selection II • Darwin viewed male sexual of “ornaments” as a curious evolutionary puzzle that begged explanation. • An ornament is a physical form, gesture or ritual performed to obtain attention from a female. • Natural selection tends to produce individuals that are well adapted to their environment. • Sexual selection does not adapt the individual to the environment but does enhance traits involved in mate acquisition.

10. An Example of an Ornament • A grouse (like a pheasant) has an elaborate mating ritual. • Males impress females by taking a few steps forward, wings raised, lowered and brushed twice against stiff, feathers of white pouch, • Produces loud swishing noises • Utter distinct series of sounds by vocalizing and popping two air sacs within pouch 

11. Ornamentation Theory

12. Sexual Selection III • Sexual selection produce individuals with elaborate ornaments that costs • Cost is in maintenance of characteristic • Ornament may lead to survival cost • Thus, sexual selection has capacity to evolve maladaptive traits

13. Sexual Selection IV • Darwin's theory of sexual selection gives plausible explanation for many bizarre ornaments • Theory refined by scientists expresses that; • Females are limiting sex and invest more in offspring than males • Females may be unavailable because of care for young • Males tend to be in excess in animal populations, males tend to develop ornaments for attracting females or engaging other males in contests.

14. Female Choice or Male-Male Competition • Darwin was the first to realize that sexual selection by female choice or male-male competition was likely to lead to the evolution of structures and rituals that often times reduce survival. • In the case of male-male competition, the direct costs of such structures arise. • While lethal fighting is rare in the animal kingdom it still does occur. • If battle is not lethal and males fight for a female, the war of can leave both males in a weakened state and result in their death.

15. Selection and it’s Leads • Sexual Selection leads to several things: • Competition between males, or competition between females for a higher rate of reproductive fitness. • Mating Systems that result in traits (physical or ritualistic) that make you more “desirable” to be selected over peers. • Elaborate courtship behaviors to win over your mate for the right to reproduce. • Dominance, fighting and other social cast rituals in animal populations for the right to reproduce. • Territoriality to have space and resources to give and or occupy for reproductive fitness. • Predation and Anti-predation behaviors that make you more equipped to reproduce or supply for your mate. • Parenting behaviors that lead to the success of reproduction

16. Female Choice • Also referred to as inter-sexual selection, in which females choose males based upon elaborate male ornamentation or behavior. • Why do Females Choose and Males Display? • The role of the female choice in promoting sexual selection is largely tied to the the observation that females invest more heavily in reproduction, making them a limiting resource. A female that is taking care of young or gathering energy to provision eggs is not interested in mate choice after she copulates with the male. • While females may invest in prolonged care in young, the male is free to move on and mate with other females or compete with other males for access to females. Thus, females should be quite choosy about their mate because males are present in abundance. • The role of parental investment in mate choice has been powerfully tested in animals with sex-role reversal where the male is primarily responsible for parental care.

17. Male Competition • Also referred to as an intra-sexual selection, in which males compete for territory or access to females, or areas on mating grounds where displays take place. • Male-male competition can lead to intense battles for access to females where males use elaborate ornaments. • While, females are more often than not the limiting sex, this is not always the case in the animal kingdom. • Some mating systems like the male pipefish are limiting because they brood offspring in a pouch. • In this case females compete for mates, and in some species of pipefish, females are more brightly colored than males. It is often the exceptions that prove the rule in biology, and reverse sexual dimorphism in pipefish is a case in which females have become the sexually-selected sex.

18. Operational Sex Ratio • The number of sexually receptive males to receptive females in a population. • Male competition usually arises because of the customarily strong male bias to this ratio. • When receptive females are scarce, they represent a limiting resource for males.

19. Mate Guarding • Males attempt to control female reproduction habits by guarding them from other males interacting with them. • Male Dunnocks (a sparrow) will peck at the cloaca of his partner after seeing another male around her to enduce her to eject any sperm that may had been deposited in her. • There are many examples of male guarding (an aggressive behavior) in animal populations.

20. Male Types Found in the Wild • Male strategies are usually broken down into the following: • Aggressive males that tend to have many females and are called polygynous. The flip side of polygynous is a polyandrous mating system in which females have multiple male partners. • Regular old males that are monogomous and usually have one mate (the standard by which we measure male success). • Sneaker or Satellite males that are usually not very conspicuous, but live around the edges of the more dominant males in the population. • The kinds of males present in the population also depend on the kind of care that the species provides to its young. For example, male birds tend to be monogamous and may also participate in the rearing of young. However, in fish, the parental male types need not be monogamous, for a parental male fish could entice many females to spawn in the nest that he guards.

21. Polygamy and Polyandry • There are subcategories of polygynous and polyandrous mating systems: • The subcategories are defined in terms of whether the mating systems are resource based or non-resource based. • In a resource based polygynous mating system a male defends some resource that might sustain several females. • In a non-resource based mating system like a lek, females aggregate in regions with sole purpose of choosing a mate. Males aggregate in these locations and display in order to gain access to females. • Sexual selection theory and the relatively low parental investment that males make in offspring would suggest that polygynous mating systems should predominate in the animal kingdom, but most males are monogamous.

22. Lek • Polygynous males feature a defense to a territory too small to offer useful resources to visiting females. • Females come to these territories to find and select a mate. • After copulating the female departs and may never see her partner again. Male white-bearded manakin form leks for female selection.

23. Monogamy • Organisms may not permit males to have more than one partner. Females may be so widely distributed that males cannot monopolize more than one female. In such cases, searching for another mate may be very costly. • In real animal populatiolns, a female is occupied with another male, because the costs of giving up control of the territory may be so restrictive that the resident males maintain a strong home-field advantag for that female. That would pay off for a male (to remain with a single mate) because the cost of searching multiple mates might mean it’s life. • In addition, if the male were to abandon the female, she is very likely to mate again with a rival male. A male may enter into a monogamous relationship to mate and guard the female. • Finally, it might pay for a male to remain at the nest with a female, and participate in the rearing of young, because this enhances the male's fitness more than moving on to find other mates. • The evolution of monogamy and the further evolution of male parental care is driven by the ecology of the organism and the distribution of resources in the environment for both the male and the female.

24. Controlling Reproduction

25. Sexual Harassment and Forced Copulation (Rape) • Conflict between the sexes is most readily apparent when males use force or harassment to overcome sexually resistant females. • Male water striders will force themselves onto a female until she accepts that she cannot dismount him and thus mates with him.

26. Parenting and Parental Investment • Once copulation takes place a male may stay or leave. • Parental investment refers to the time, energy and risks that a parent invests in one offspring that reduce the chance that the parent will have more offspring in the future. • Females usually make more of a parental investment that males. • Males usually compete for mates because of inequalities in parental investment and inequalities in the operational sex ratio. • Sex role reversal is when the operational sex ratio is reversed to females.

27. Parental Control by Females and Males • Females control parental investment into offspring by: • Adjusting how hard they work to bring food to offspring fathered by certain males. • Abort embryos fathered by certain males in order to reproduce with a new partner. • Mating with several males to prevent one male from investing into the offspring. • Going through a false estrous (heat) to prevent some males from reproductive desire. • Males control parental investment into offspring by: • Infanticide - Males kill young animals despite resistance from mothers.