Mating Games and Signalling • Searching vs signalling for mates • Hybrid mating avoidance Chapter 23: Mating Signals • Courtship control and persistance • Mechanisms of mate choice • Postcopulation signals • Role reversal BGYD45 2003:10 1 BGYD45 2003:10 2 Male mating strategies Consequences of anisogamy • Males produce large numbers of sperm, and can fertilize many females • Males compete for access to females • Sedentary males compete by sperm competition • Mobile males can either search for females, defend resources, or wait for females to encounter them BGYD45 2003:10 3 BGYD45 2003:10 4 1 Sexual selection and signalling Mobility Game • Mate attraction and courtship signalling is Search Mate Recover influenced by – The operational sex ratio – The male mating strategy – The relative importance of intrasexual (male- Simulation model: each sex minimizes cycle time in male competition) vs intersexual (female mate order to maximize fitness choice) selection Search = move until encounter mate Stategies = P(move) on each time-step (0 ≤ P ≤ 1) BGYD45 2003:10 5 BGYD45 2003:10 6 Mobility game Mate Continuous asymmetric scramble among adults with equal sex ratio. searching Each adult rotates between mate searching, gamete production, courtship, copulation, parental care, and recovery. patterns Each sex seeks strategy that minimizes its cycle time given partner behavior. Males tend to search Which sex should search and which should signal? Operational sex ratio favors sex with shortest gamete + recovery time. Females tend to search Can go either way, but males more when there is resource likely to search overall. defense since males are tied to resources Why? What conditions favour one or the Non-searching sex emits other. attraction signals Exaggerated signals are given by males due to sexual selection BGYD45 2003:10 7 BGYD45 2003:10 8 2 Mating Decisions Hybrid mating game Discrete asymmetric scramble • After finding a potential mate Mates encounter and must decide – Go through with it or not? whether or not to mate Offspring viability decreases • Mistakes usually more costly to female with level of hybrid incompatibility – Therefore sexual conflict ESS decision thresholds vary with the ratio of investment in male to female gametes, gm/gf Females become more choosy as their investment relative to a male increases and the sex ratio is male-biased Whether mating and fertilization occurs in the conflict area depends on a male’s ability to force matings and female’s ability to control fertilization after mating. BGYD45 2003:10 9 BGYD45 2003:10 10 Character displacement in Male vs Female Attraction Signals damselflies Expect species specificity of mate attraction signal when females emit the signal since they have more to lose from a hybrid mating • Lower costs for males, higher for females, of mis-matings – Female attraction signals should be more specific – Best examples of sexual character displacement are from female signalling systems BGYD45 2003:10 11 BGYD45 2003:10 12 3 BGYD45 2003:10 13 BGYD45 2003:10 14 Sexual Conflict Sexual arms race in water striders. • 2 regions in conflict zone Variation among – Stronger selection on males to mate species in relative – Stronger selection on females to reject investment of males and females • Which sex can control copulation? in “tools” to control copulation. BGYD45 2003:10 15 BGYD45 2003:10 16 4 Courtship persistence games Courtship duration patterns • Discrete courtship persistence game • Females control courtship in male resource defense – ESS1: male persists, female passive, when cost of rejecting is high for female and self-advertisement systems – ESS2: female rejects and male nonpersistent, when cost of – courtship is typically prolonged and involves many male persisting is high for male display behaviors – ESS3: male persists and female rejects: when costs of rejecting – e.g. most birds, lekking and paternal care species and persisting are both low • Sexual arms race – Females also control courtship in predatory species, e.g. – Stable ESS requires variation in arms level spiders, preying mantis, lions – Males typically win given their ability to invest in armaments (more • Males control courtship in female defense systems to spend on this because sperm is cheap) – courtship is often short or absent, females may even solicit • Sexual war-of-attrition matings to insure fertility – Females decide contest duration based on perceived value of male – some insect males mate with females before eclosion • Sexual sequential assessment game – Predicts variation in duration of courtship interactions based on – some sea slugs use “love darts” value (quality) of male relative to cost of resistance for female BGYD45 2003:10 17 BGYD45 2003:10 18 Mountain Zebra Przewalski’s Horse Grevy’s Zebra Wild Ass No permanent association between males Single-male harems. Little courtship. and females. Males must convince receptive females when they encounter them. BGYD45 2003:10 19 BGYD45 2003:10 20 5 Why accept or reject? Mechanisms of mate choice • Quality of potential mate • Fisherian runaway - predicts arbitrary traits – Signals? – Honesty? • Good genes - heritable • Evolution of conspicuous attraction signals • Direct benefits – non-heritable • Condition dependent indicator traits • Revealing indicator traits • Testing these models BGYD45 2003:10 21 BGYD45 2003:10 22 Direct benefits of female preferences Fisher’s Runaway process for male frequency in Australian frogs • If females exhibit preference for a male trait • And selection does not act on females • Then sons and daughters will carry genes for both the preference and the trait • This creates a genetic correlation between preference and trait • And leads to geometric increase until further increase in the male trait is opposed by natural selection BGYD45 2003:10 23 BGYD45 2003:10 24 6 Condition dependent traits Arbitrary traits Only males in good condition can make a large investment in a trait which then has less affect on their survival BGYD45 2003:10 25 BGYD45 2003:10 26 Revealing trait A couple of bizarre things All males attempt to display the trait and pay the same cost, but the effect of the trait is less in low quality males BGYD45 2003:10 27 BGYD45 2003:10 28 7 Population variation in bowers BGYD45 2003:10 29 BGYD45 2003:10 30 Postcopulation signals • Synchronizes orgasm – Humans, gibbons? • Females advertise status to other members of social group – Hammer-headed bats, chimps • Females incite competition among males – Elephant seals • Advertise mate guarding by males – Little brown bats BGYD45 2003:10 31 BGYD45 2003:10 32 8 Role-reversal • In some groups the extra investment by males (over and above gamete production) can more than balance the overall asymmetry in reproductive investment • Male parental care – Birds, fish, amphibians, insects – Usually roles are fixed BGYD45 2003:10 33 BGYD45 2003:10 34 BGYD45 2003:10 35 BGYD45 2003:10 36 9 Kawanaphila nartee Australian katydid Live in grass trees. BGYD45 2003:10 37 BGYD45 2003:10 38 Role reversal • Male choosy about mating – sing less – reject females • Females compete for access to males • Protein in spermatophore is very valuable during food shortage. Lots to eat when grasstrees are in bloom: typical sex roles At other times, severely protein-limited: role-reversed BGYD45 2003:10 39 BGYD45 2003:10 40 10.