The behavioral ecology of animal reproduction

Transcription

1 The behavioral ecology of animal reproduction I. Introduction to Behavioral Ecology Behavioral ecologists test hypotheses about the adaptive function of a behavior Behavioral Ecology Behavioral ecologists test hypotheses about the adaptive function of a behavior Adaptive function? Behavioral Ecology Behavioral ecologists tests hypotheses about the adaptive function of a behavior Adaptive function? Contribution of a behavior to the fitness of an organism

2 Behavioral Ecology Adaptive function: the contribution of a behavior to the fitness of an organism Fitness? Behavioral Ecology Adaptive function: the contribution of a behavior to the fitness of an organism Fitness? The relative contribution of an individual to the gene pool of the following generation II. Introduction to sexual selection What s sexual selection? What s sexual selection? Genetic variation among individuals in mating success

3 Intrasexual vs. Intersexual selection Intrasexual selection? Variation in ability to compete within the same sex for the opportunity to mate Intrasexual vs. Intersexual selection Intersexual selection? Variation in mate attraction and mating success Intrasexual vs. Intersexual selection Intersexual selection The power to charm Charles Darwin Intrasexual vs. Intersexual selection Sexual selection is part of natural selection

4 Intrasexual vs. Intersexual selection Sexual selection is part of natural selection Which sex is more particular about mates? Mate choice Which sex is more particular about mates? It depends... Trivers hypothesis: The sex that invests the most resources in reproduction should be the choosiest about mates

5 Trivers hypothesis: The sex that invests the most resources in reproduction should be the choosiest about mates Trivers hypothesis If little investment per mating - maximize the number of matings Why? Trivers hypothesis If little investment per mating - maximize the number of matings If large investment per mating maximize the success of each mating Mate choice Trivers hypothesis If little investment per mating - maximize the number of matings If large investment per mating maximize the success of each mating -so individuals should choose mates carefully!

6 What kinds of resources are invested in reproduction? What kinds of resources are invested in reproduction? stored nutrition Mate choice What kinds of resources are invested in reproduction? stored nutrition time and energy How would you test Trivers hypothesis (that the sex that invests most is choosier)?

7 Example: Nuptial gifts in mormon crickets Mate choice Example: Nuptial gifts in mormon crickets Which sex of mormon cricket would you predict is choosier about mates? Males deliver huge sperm packet (up to 30% of male body weight) to females. Females consume the external portion. Example: Nuptial gifts in mormon crickets Males choose females on basis of body size Mean weight of females Rejected Accepted Males choose females on basis of body size Heavier females carry more eggs - so increase male reproductive success Mean weight of females Rejected Accepted

8 What s the basis of the choice? Mate choice What s the basis of the choice? i. Direct estimates of reproductive potential (e.g. female body size in mormon crickets) What s the basis of the choice? i.direct estimates of reproductive potential ii. Material benefits (e.g. territories) What s the basis of the choice? i.direct estimates of reproductive potential ii. Material benefits (e.g. territories)

9 What s the basis of the choice? i. Direct estimates of reproductive potential ii. Material benefits (e.g. territories) iii. Indirect estimates of reproductive potential E.g. bright plumage, elaborate calls, songs etc. Mate choice iii. Indirect estimates of reproductive potential Do female birds actually choose males that can do silly dances, sing elaborate songs or have brilliant feathers? Mate choice Yes they do... Tail manipulation experiment in long-tailed widow birds Average no. of nests per male Artificially Normal Artificially lengthened shortened Why don t widowbirds have tails as long as the ones made by the experimenters? Average no. of nests per male -why Darwin made a distinction between sexual selection and selection for viability

10 Why do females choose on the basis of ornament and display? Two nonexclusive hypotheses Why do females choose on the basis of ornament and display? Two nonexclusive hypotheses 1) The good genes hypothesis: Showiness is correlated with the presence of genetic alleles in males that lead to greater offspring fitness Why do females choose on the basis of ornament and display? Two nonexclusive hypotheses 1) The good genes hypothesis: One aspect : males with inherited resistance to parasites tend to have brighter plumage, etc. Why do females choose on the basis of ornament and display? Two nonexclusive hypotheses 1) The good genes hypothesis: One aspect : males with inherited resistance to parasites tend to be brighter colored etc. Example: turkey snoods!

11 Males with inherited resistance to parasites tend to be brighter colored etc. Example: Turkey snoods And males with big snoods are more resistant to parasites Snood length Males with inherited resistance to parasites tend to be brighter colored etc. Example: turkey snoods Female turkeys prefer males with big snoods snood Log Coccidea no. per gram feces Why do females choose on the basis of ornament and display? 2) Runaway sexual selection occurs when a) a slight preference by females for showy trait leads to greater reproductive success of males with trait (even if the trait is not correlated with any other good trait e.g. size, agility, resistance to parasites) Two nonexclusive hypotheses 1) The good genes hypothesis 2) Runaway sexual selection

12 2) Runaway sexual selection occurs when a) a slight preference by females for showy trait leads to greater reproductive success of males with trait (even if the trait doesn t directly increase the fitness of the offspring) b) a genetic association of preference by females for showy trait and showiness of males develops Why do females choose on the basis of ornament and display? 1) The good genes hypothesis 2) Runaway sexual selection What s the difference between these hypotheses? are defined by the relative maximum number of mates of males and females I. Polygyny - the dominant mating system in animals with internal fertilization poly - many gyn-female

13 I. Polygyny The maximum no. of mates for males is greater than the maximum no. of mates for females I. Polygyny The maximum no. of mates for males is greater than the maximum no. of mates for females Why not the average? I. Polygyny The maximum no. of mates for males is greater than the maximum no. of mates for females Why not the average? When there are equal numbers of each sex, the average number of mates must be equal I. Polygyny The maximum no. of mates for males is greater than the maximum no. of mates for females Why not the average? When there are equal numbers of each sex, the average number of mates must be equal So is the variation in mating success greater for one sex than the other? Which one?

14 III. i. Polygyny In polygynous systems, the variation in reproductive success is greater in males I. Polygyny Why is polygyny the most common mating system? Because females often invest more in reproduction than males Fewer gametes - sometimes eggs big relative to body size, and in mammals, both gestation and lactation performed by female I. Polygyny I. Polygyny So how does greater female investment lead to polygyny? -same logic as for mate choice So how does greater female investment lead to polygyny? - same logic as for mate choice If little investment per mating - maximize the number of matings If large investment per mating maximize the success of each mating

15 II. Monogamy (or monogyny) Maximum number of mates for males = maximum number of mates for females = 1 II. Monogamy (or monogyny) Maximum number of mates for males = maximum number of mates for females = 1 Monogamy may evolve when rearing offspring requires more than one individual II. Monogamy (or monogyny) Monogamy may evolve when rearing offspring requires more than one individual Common in birds Humans generally monogamous An aside: What about human beings - how much of behavioral ecological theory can be applied to us? No question that human behavior molded in part by evolutionary history But humans unique in animals in having social and cultural conventions that often override evolutionary fitness considerations

16 III. Polyandry Poly - many, andr- male Maximum number of mates for females is greater than the maximum number of mates for males III. Polyandry A likely example from earlier in this lecture? III. Polyandry Examples? Mormon cricket, In some birds, females defend territories, some with many males, males build nests and raise the young Sexual cannibalism An observation that needs explaining (at least to us)...

17 Sexual cannibalism In some animals, especially spiders, females eat mates, during or after mating... Sexual cannibalism What s the hypothesis the narrator proposes? Sexual cannibalism What s the hypothesis the narrator proposes? Males are victims of the predatory instinct of females This hypothesis suggests an evolutionary constraint - males are not able to devise effective strategies to avoid being eaten Sexual cannibalism evolutionary constraint - the optimal trait or behavior (e.g. being able to always avoid getting eaten) cannot be selected because there is no genetic variation for it

18 Sexual cannibalism Sexual cannibalism What s the hypothesis the narrator proposes? Males are victims of the predatory instinct of females What s the hypothesis the narrator proposes? Males are victims of the predatory instinct of females In other systems, it appears being eaten is adaptive for the male Sexual cannibalism In other systems, it appears being eaten is adaptive for the male Example: the Australian red back spider (close relative of the black widow) Males summersault into the jaws of females chelicerae Sexual cannibalism Example the Australian red back spider (close relative of the black widow) Males only likely to find one mate per lifetime - if they re eaten more of their sperm is transferred