Chapter 18 Evolution of reproductive behavior (1st lecture) The Satin Bowerbird has an unusual courtship ritual Male constructs an female elaborate “avenue” bower, contaiing colorful objects that he has collected. When a female arrives, he performs a energetic dance while emitting a medley of male buzzes, screeches and imitations of other bird songs. The female assesses the male based on: bower attributes, the male’s dance, and location of the bower If the female decides to mate with him, then she will enter his bower, copulate, and fly away, never to see him again. She will incubate her eggs and raise young on her own. The male will continue in this manner across the 8-month breeding season, mating with as many females as possible. The design of the bower varies greatly, both among and within species Playhouse bower Different populations of Amblyornis inoratus construct different types of bower, perhaps reflecting different “aesthetic tastes” of females within each population Maypole bower Evolutionary relationships among 14 of the 19 species of bowerbird, based on similarities in their mitochondrial cytochrome b gene Note that 2 bowerbird species share the ancestral trait of not building a bower Maypole builders Presumably, the hypothetical species Y built a Avenue simple bower. builders The ensuing adaptive radiation illustrates a fantastic example of divergent evolution What are the features of the bowerbird courtship ritual that represent evolutionary conundrums? The males make absolutely no parental investment Each male mates multiple females, but each female mates only one male Only the males do the courting The female bowerbirds are extremely choosy, whereas the males are relatively indiscriminate One salient feature of sexual reproduction is that male and female parents rarely invest equally in offspring Parental investment Any investment by a parent into an offspring that increases that offspring’s chances of surviving to reproduce, at the cost of the parent’s ability to invest in additional offspring Parental investment occurs in many guises A female cicada-killing wasp has just paralyzed this cicada, and is dragging it to her nest to feed her offspring A male frog carries his tadpole offspring on his back A female eared grebe is protecting her young A male crocodile is transporting recent by carrying them on her back hatchlings from the nest to the river’s edge The asymmetrical parental investment often begins before fertilization Females produce limited number of eggs, and males produce trillions of sperm e.g., in humans, each female is born with a total of ~ 200 eggs, whereas each male ejaculate contains ~ 350 million sperm cells Eggs are enormous and energetically costly to produce, relative to sperm Post-fertilization asymmetry Females are more likely to raise and care for offspring What are the consequences of this asymmetry? In a typical species (e.g., the satin bowerbird), where females make a larger parental investment: A. Female reproductive success is limited by the ability to provide pre- and post-fertilization parental investment B. Male reproductive success is limited by access to females and their relatively scarce number of eggs This line of reasoning generates two predictions Prediction 1a In species with asymmetrical parental investment (e.g., lions), the gender with the lowest investment should have high the highest variance in lifetime reproductive success Females Males 18 Average = 4 Average = 6 15 Variance = 8 Variance = 21 12 9 6 3 Percentage of breeders 0 01234567891011121314 01234567891011121314 Number of offspring surviving to 12 months of age Indeed, among wild lions in the Serengeti, some males fail to reproduce and others have exceptionally high reproductive success (here, measured in terms of surviving offspring). Females, in contrast, exhibit less variation and a slightly lower average level of reproductive success. Prediction 1b In a species with a more balanced parental investment, the sexes should have a similar variance in lifetime reproductive success Lifetime reproductive success in a population of monogamous birds: Florida Scrub Jays Note that both sexes have a similar distribution Prediction 2 The gender with the greatest parental investment (usually females) should have a lower potential rate of reproduction This generates a biased operational sex-ratio (e.g., more sexually receptive males than females) An important evolutionary consequence of a biased operational sex-ratio is sexual selection What is sexual selection? A form of natural selection that occurs when individuals vary in their ability to compete with others for mates (the competition for mates component of sexual selection) or in their attractiveness to members of the opposite sex (the mate choice component of sexual selection) As with natural selection, when the variation among individuals is correlated with genetic differences, sexual selection leads to genetic changes in the population over time. Note that sexually selected traits do not enhance survivorship, only reproductive success Sexual selection is thought to be one factor favoring the evolution of sexually dimorphic traits Female and male peacocks For this species, the primary Male and female bighorn sheep dimorphic trait is feather color and In this species, the males are larger morphology. The male’s and possess enormous horns. The feathers are thought to enhance male’s large size and horns are thought its attractiveness to females--i.e., to enhance its ability to fight conspecific function in intersexual selection. males, and thereby establish larger harems of females during the mating season--i.e., function in intrasexual selection. Intrasexual selection Competition within one sex (usually males), with the winner gaining access to the opposite sex. Competition can take place before mating, where males will have extended verbal and/or physical contests. High ranking males of the southern elephant seal are able to establish the largest female harems, and as a result, copulate many more times than low ranking individuals do Intrasexual selection can also take place after mating when females have mated with multiple males within a short period of time Cloaca-pecking in dunnocks Prior to copulating with a female, the male repeatedly pecks her cloaca, sometimes causing her to eject sperm from a previous copulation. Once this happens, the male mates with the female. Intersexual selection Individuals of one sex (usually males) “advertise” that they are worthy of an investment: then females of the other sex (usually females) choose among them In many species, males are not only sexually dimorphic, but also display extravagant traits and/or behaviors that have no obvious survival value Presumably, this male’s tail feathers enhance his attractiveness to females.