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Indirect Benefits: Mate Choice and “Good Genes” Why do females sometimes show preferences for males that offer nothing but sperm? What indirect benefits do such males offer? “Good Genes” Hypothesis Females choose mates whose genes improve their offspring’s fitness. Prediction: Females allowed to choose males with ‘good genes’ should produce more fit offspring than females not allowed to choose. gray treefrog house mouse barn swallow “Good Genes” in House Mice female Each female is given a choice between two males. Males are designated as preferred (P) or nonpreferred (NP). A female is permitted to mate with either a P male or a NP male. Offspring production and viability then monitored for both P mated and NP mated treatment groups. Some Results: 1. P-mated females produced more litters than did NPmated females. 2. At 30 days survival was greater for P-sired offspring. 3. P-sired male offspring had larger home ranges. (from Drickamer et al. 2000) Evidently, females are choosing males with ‘good genes’. But there is still a problem: How can females determine if a male has good genes? Hey, look at me!! Hypothesis: Males indicate their quality in some way. They sing, they dance, they display ornaments. Case Study: Call duration in Hyla versicolor Some males have long calls: Some males have short calls: Eastern gray treefrog green form In choice tests, females prefer long calls over short calls by 3:1. gray form Call duration in Hyla versicolor and offspring viability Fitness Measure High Food Low Food Larval Growth Long Callers > Short Callers Long Callers > Short Callers Larval Survival No Difference No Difference Welch et al., 1996 data Evidently, females sometimes choose mates with traits that signal high genetic quality. But… there are two more questions: How can mate choice for ‘good genes’ lead to such exaggerated male traits? What prevents a male from deceiving a female as to his genetic quality? Hmm… if I look C-GA-G-T, will she know I’m really G-T-T-G-A? Zahavi’s Handicap Hypothesis Exaggerated male traits are essentially handicaps. Female prefer males with these handicaps… … because such males have demonstrated an ability to survive despite their handicaps!! Hypothetical Example Peahens prefer peacocks with absurdly long tails because such males have demonstrated an ability to survive despite such tails. At first glance, Zahavi’s idea seemed ridiculous! Wow, how attractive am I!! But Zahavi’s handicap model can work if… … the signals used are costly, such that only the highest quality male can pay the cost of the very longest tail. Darn! And these styrofoam antlers seemed like such a good idea… “Bright Birds and Parasites” Birds are susceptible to parasites, both external and internal. Natural selection should favor female choice for parasiteresistant males. How to choose? Parasitized birds have dull plumage. Therefore, natural selection should favor female choice for males with brighter-than-average plumage. “Bright Birds and Parasites” cont’d But why do males evolve to be so extremely bright? Because parasites evolve ways to defeat resistance. Once a new parasite form arises, natural selection again favors males who can resist the parasite. In other words, females who choose males with still brighter plumage. Thus, coevolution between host and parasite leads to brighter and brighter males. Tail Length in Barn Swallows and Parasite Resistance Males tails are longer than female tails. Females prefer long-tailed males 1. Females prefer to pair up with males with long tails. 2. Females that paired with short-tailed males were more likely to engage in extra-pair copulations. 3. Long-tailed males were more likely to engage successfully in extra-pair copulations. So what’s so important about long tails? No. mites on offspring Long Tails as an Indicator of Resistance to Parasitic Mites P < 0.001 Mail tail length Tail length of an offspring’s biological father is inversely correlated with mite load. What we hear: What a female Hyla versicolor hears: Bud- weis- -I got great DNA!