Transcript Document
According to natural selection, what are both females and males selected to do? • Survive and reproduce • Certain traits favor survival and reproduction. • When those traits have a genetic basis, they are passed on to successful offspring. • Genes that code for successful traits or behaviors carry on over many generations - (evolution is a change in frequencies of genes over time) In terms of sexual selection, in most species… • Males do best to maximize the number of matings. Females are a limiting resource for males. • Females do best to maximize number of successful eggs/pregnancies. Males are not a limiting resource for females. Leads to an asymmetry between the sexes and to conflicts of interest orangutan Mating systems Greek terms • Terms ending with -gamy refer to number of partners or characteristics of gametes • Terms ending with -gyny refer to women • Terms ending with -andry refer to men • mono - “one” ; poly - “many” Having one mate • Monogamy - having one mate per breeding season (lifetime monogamy, or serial monogamy) • Seems to always involve some “cheating”. Having many mates • Polygyny (male fertilizes the eggs of many females in a breeding season) • Polyandry (female has several partners in a breeding season) • Promiscuity (informal term) implies no durable associations, mating a matter of temporary contact (also called polygynandry) Polygyny -one male, many females • Polygyny - poly (many) gyny (women) • Very common in animal kingdom (97% of mammals) • Categories of polygyny: – – – – Female defense Resource defense Leks Scramble competition Female defense polygyny • Males fight for access to females • Females are clustered or can be herded - easy to monopolize/guard. • At an extreme, males have harems (elephant seals) Resource-defense polygyny • Males have resource-rich territory to attract females. • Best (most fit) males have best resources/territories • Cichlid fish (eggs in snail shells) • Topi (african antelope) females gather on green patches of grassland • Female resources can be clumped, and males can guard those. Resource-defense polygyny Female resources clumped, males can defend these and attract females • Cichlid fish (eggs in snail shells) • Topi antelope females- green grassland • More resource, more females/male. When females dispersed widely across the landscape…. • Scramble competition polygyny, OR • Lek polygyny When females cannot be herded, or guarded in one place, males have to find them, or attract them from far away. Scramble competition polygyny • Females dispersed widely and hard to find. • males compete to outrace rivals to find receptive females. Scramble competition polygyny Examples: Flightless firefly (brief call by female) Horseshoe crabs Male wood frogs (all females receptive on same single night each year) Lek Polygyny • Males display in groups on small territories, offer no resources or parental care, only “good genes” and selection of males to choose from. • Mannakin birds and W.African hammer-headed bats (females travel together looking for widely scattered food • Feeding ecology of females makes in hard to monopolize them. Why the group display? Lek Polygyny Leks: why display in groups? • Theories Display where females tend to aggregate (“hot spot”) Display near dominant male (“hot shot”) Display with other males so females can pick among rivals (“female preference”) Display in groups to avoid predators (“safety in numbers”) Lek Polygyny - stand near dominant male “hot shot” Lek Polygyny - females have a preferred gather-place “hot spot” Puzzle of monogamous males • When do you find it? When females remain receptive after mating or are widely dispersed/hard to find --guard your mate. When offspring survival requires two parents assist your mate. Monogamy Mating with one partner for breeding season Rare in mammals More common in birds - why? Male birds can incubate, feed young Young require two parents. Female-enforced monogamy • Burying beetle - pair bury a dead mouse together, lay eggs, care for larvae. • Males call other females to mouse - when? Monogamy: associated with promiscuity • EPCs (extra pair copulations)- promiscuity 20%-50% of nestlings in songbird nests are from EPCs. Advantage to males of promiscuity? Advantage to females of promiscuity? Advantage to females of mating with males other than primary partner (EPCs): • Fertility assurance (“better sperm” hypothesis) • Genetic compatibility hypothesis (no “duds” or “best” male per se, based on match of female and male genes (eg, MHC tee shirt study) Benefit of promiscuity: fertility assurance hypothesis Matings outside pair bond: Higher hatch rate (redwing blackbirds) Fewer still born (adders) Blue tit bird - males of stayat -home females are more attractive Promiscuity: material benefits • Females can forage on territories of males they have mated with. Promiscuity:genetic incompatibility hypothesis • Harlequin beetleriding pseudoscorpion Genetic incompatibility hypothesis: females mate with more than one male to hedge their bets against “mismatched’ genes. • Harlequin beetleriding pseudoscorpion (Jeanne Zeh) Genetic incompatibility hypothesis Experiment 1 (Zeh): when females mated to several males. Results: Less embryo failure Higher offspring survival Harlequin beetle-riding pseudoscorpion Experiment 2 (Zeh): when different females mated to same males. Results: No “good” and “dud” males. Depended on particular male-female genetic match or genetic compatibility . Dunnock (songbird) case study Diverse mating types within the same species • males with 1 female, • males with 2 females, • female with 2 males, • male and female promiscuity on one territory Dunnock (songbird) case study Dunnocks eat seed and small insects. Males mate-guard a group of females if food is concentrated enough. More concentrated food, more monogamy. Dunnock (songbird) case study Experiment 1. Added food in clumps over years. Female territory size decreased by 40 feet. Males guarded females in territory. Female monogamous to one males (polygyny) Dunnock (songbird) case study But… female behavior interfered with male ability to monopolize females. Females chased off other females (from food and male parental care). Females solicited copulations from “beta” males. Illustrates how distribution of females and resources and conflict between the sexes affects evolution of mating systems.