1Sexual behavior and mating systemsWhy sexual reproduction?Benefits of sexual reproduction: offspring are more variedmutations are reducedCosts of sexual reproduction: loss of 50% of genesenergetic cost of producing gametestime and energy invested in courtship and mating
2Why two sexes?Gamete sizeProto-maleProto-femaleDisruptive selectionSexual reproductiondoes not require it:isogamyinmicroorganisms;anisogamyin mostplants & animalsParker, G. A. et al. (1972) The origin and evolution of gamete dimorphism and the male–female phenomenon. J. Theor. Biol. 36, 181–198.Sex Ratio•Sex ratio is usually 1:1 …. why?•R. A. Fisher (1930) proposed that the common 1:1 sex ratio can be explained in terms of selection acting on the individualA.B.
31:1 sex ratioR. A. Fisher (1930)Lots of females in population: advantage to produce malesLots of males in population: advantage to produce femalesBonellia viridisFerguson & Joanen (1982): alligator sex determined by the temperature of the egg during the 2nd and 3rd weeks of incubation. 30°C or below produces females; 34°C or above produce males. At 32°C, 87% of the hatchlings were female.
51:1 sex ratio .. exceptionsAcarophenax tribolii:viviparous miteFemale produces a brood of 1 son and about 20 daughters.The son mates with his sisters inside the mother and dies before he is born!Hamilton, W.D. (1967) Science156: 477-488.Mating systems•Evolutionary view–Energetics & Economics•Egg vs. Sperm–Greater cost of reproduction for the female–Female is the more “selective” sex (sexual selection)–“Ideal” mating systems•Male acquires as many mates as possible and invests little in parental care•Female selects “fit” male and leaves parental duties to him, while acquiring resources required to produce more offspring
7Sex differences in the maximum number of offspring produced – from Krebs & Davies (1993)** from 27 pregnancies* Moulay Ismail the Bloodthirsty, Emperor of Morocco source: Guinness Book of World Records69**888*Man1424Red deer8100Elephant sealFemaleMaleSpeciesMaximum number of offspring produced during lifetimeMoulay Ismail, Emperor of Moroccofrom 1672-1727: 888 offspring?Moulay Ismail548 sons and 340 daughters through a harem of 500 women during his life. 1.2 copulations/day for 62 years