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Lecture 13 Leks, Adaptation, Phylogenetic Tests

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Lecture 13 Leks, Adaptation, Phylogenetic Tests Lecture Outline Reasons for Lek Polygyny Alternative mating strategies Adaptive versus Non-adaptive hypotheses Using phylogenetics to test hypotheses in behavioral ecology Lek Polygyny 1. Lek Polygyny: Sometimes males advertise to females with elaborate visual, acoustic, or olfactory displays. Males do not hunt for mates. Females watch males display at territories that do not contain food, nesting sites, or anything useful. 2. Sometimes males aggregate into groups and each male defends a small territory that contains no resources at all-sometimes just a bare patch of ground. a. When territories are clumped in a display area = a lek. 2. Male mating success is highly skewed on lecks a. Manakins: males jump between perches, snapping. feathers. At a lek of 10, there were 438 copulations. One male = 75%, second male = 13%, six others = 10%. Leks Mating success if usually strongly skewed on leks with the majority of matings going to a small proportion of males. 3 Leks 1. Leks have been reported in only 7 species of mammals and 35 species of birds 2. Thought to occur when males are unable to defend economically either the females themselves or the resources they require a. In both antelope and grouse, the lekking species are those with the largest female home ranges. b. In Uganda kob, topi and fallow deer, males lek at high population densities but defend territories or harems at low densities. Why Lek? 1. Why do males all congregate to display? Lots of competition. 2. Hotspot hypothesis: females tend to travel along certain routes and males congregate where routes intersect. 3. Hotshot hypothesis: subordinate males cluster around highly attractive males hoping to interact with females attracted to the hotshots. 4. Test: Remove males that were successful in attracting females. a. Hotspot prediction = other males will move into best site. b. Hotshot prediction = subordinates will leave site. 5. Example: Great snipe. Removal of dominant male caused subordinates to leave. Removal of a subordinate resulted in another subordinate coming in. Why Lek? 6. Female preference hypothesis: females prefer groups of large males where they can more quickly compare the quality of males. 7. Test: Look at whether reproductive success varies with number of males at lek. 8. Example: Ruffs (a sandpiper). More males attract more females up to a point. After 6 or more males, the dominant’s reproductive success goes down. Why Lek? 9. Reducing male predation hypothesis: As lek size increases likelihood of any one male being predated decreases (dilution effect) a. Tungara frogs are safer from bat predation when calling in large choruses (panel a). 10. Increased female attraction hypothesis: By displaying together males provide greater attraction for females and draw them in from a greater distance. a. Tungara frog payoffs per individual male increased with lek size (panel b). 11. These hypotheses are not exclusive and many be important depending on species and spatial scale. Alternative Mating Systems 1. Monopolization of females by males, leaves many males without a mate. 2. This can favor the evolution of sneaking strategies. Often get “female mimics.” 3. Traits favored for sneaking may differ from those of territorial males. Often different morphs become markedly distinct. 4. Selection is often frequency dependent selection. The rare types have a fitness advantage. This leads to the stable maintenance of multiple morphs. a. Prediction: over time the fitnesses of the strategies should be about equal. 5. Can be polymorphic, polyphenic, or plastically determined. Polymorphic side-blotched lizard strategies Orange Morph: Territory Usurper Large size Large territories Tradeoff: Low survival to next year Yellow Morph: Sneaker Blue Morph: Mate Guarder Mimics females Small territories Not territorial Sinervo & Lively 1996, Zamudio & Sinervo 2000, Sinervo et al 2000, Sinervo & Svensson 2002, Sinervo & Clobert, 2003, Sinervo et. al. 2006 Selection on males is frequency dependent: rock-paper-scissors Alternative Mating Strategies Marine Isopod Morphs Shuster and Wade 1. Male types are a polymorphism. a. Alpha males: Have pincers and defend sponges. b. Beta males: Female mimics c. Gamma males: Invest in sperm, dive into matings. 2. Mating success approximately equal: Alpha = 1.51 mates, Beta = 1.35 mates, gamma = 1.37 mates. Dung Beetle Mating Strategies Emlen & Nijhout 1. Male types are a polyphenism. Morphology (horns/no horns) is plastically determined by the amount of dung the larva was raised on. 2. Mating behavior is variable: defend or sneak. 3. Developmental tradeoff: Big horns can mean small eyes! Plastic Natterjack Toad Mating Strategies 1. Male types are plastic. Males make the “best of a bad job” = makes best of poor circumstances by adopting alternative strategy. 2. Large males call loudly and attract the most females. 60% were callers, but got 80% of matings. 3. Small males are “satellites” and attempt to intercept females. Will switch behaviors when they get bigger. Also, if remove the large males, then they will call. Lecture Outline Reasons for Lek Polygyny Alternative mating strategies Adaptive versus Non-adaptive hypotheses Using phylogenetics to test hypotheses in behavioral ecology Adaptive and Non-adaptive behavior 1. Adaptation: A trait, or integrated suite of traits, that increases the fitness (reproductive success) of its possessor. 2. Gould and Lewontin (1979) criticized the “adaptationist program” = hypothesizing that all traits are adaptive. 3. For example, it was hypothesized that the small forelimbs of Tyrannosaurus rex were used to stimulate females. Is this testable? 4. Hypotheses suggesting that a trait is an adaptation must be tested! Is the giraffe’s long neck an adaptation? 1. Hypothesis: Giraffe’s have long necks in order to forage from tall trees. 2. Test: Where do they forage? Where do giraffe’s forage? They forage below their maximum height. They prefer to feed at shoulder height. Is the giraffe’s long neck an adaptation? 1. Hypothesis 2: Giraffe’s have long necks for fighting in male-male competition. 2. Test: Do long necks affect sexual selection? 3. Males with larger size, thicker necks, and more massive horns were more successful at displacing rivals. 4. Females seem to prefer larger males Should also test non-adaptive explanations 1. Differences between populations may not be adaptive. a. To test: Are differences in spotting between giraffe populations could be due to genetic drift = selectively neutral? 2. Trait 1 may evolve because of selection on trait 2; Trait 2 is an adaptation, trait 1 is due to correlated evolution/development a. To test: Female giraffes could be tall because selection for large males spread genes for large size. 3. Traits may be currently useful for reasons different for which they evolved (exaptations). a. Hypothesis to test: Long necks may provide some benefits to foraging but evolved because of sexual selection. The reverse could also be true. 4. Adaptations are not perfect. Tradeoffs may be interesting too. a. Long necks make it difficult for giraffe’s to drink. Adaptation vs Exaptation 1. Definition 1: Adaptation = A trait, or integrated suite of traits, that increases the fitness (reproductive success) of its possessor. 2. However, traits can have current utility (i.e. increase fitness), but may not have been selected for that function. 3. Definition 2: Adaptation = a trait shaped by natural selection for its current use. 4. Exaptation (pre-adaptation): a. Trait arises nonadaptively, but is co-opted for useful function later. b. Trait arises adaptively, but is co-opted for a new use. Examples of Exaptation 1. Spandrels 2. Feathers and flight 3. Snail that lays eggs in the center of its spiral 4. Dancing in cockatoos Lindberg and Doberteen 1981 Behavior of the Day! Meet Snowball the cockatoo! http://www.youtube.com/watch?v=hF7xTTvU2sM http://www.nature.com/scitable/blog/student-voices/we_got_the_beat_and Historical Hypotheses 1. Definition 2: Adaptation = a trait shaped by natural selection for its current use. 2. This is a historical hypothesis: Must reconstruct evolutionary history to test it. 3. Also important to observe how evolution proceeds over time. Lecture Outline Reasons for Lek Polygyny Alternative mating strategies Adaptive versus Non-adaptive hypotheses Using phylogenetics to test hypotheses in behavioral ecology Overview of Phylogenetics 1. A phylogeny, or evolutionary tree, represents the evolutionary relationships among a set of organisms or groups of organisms, called taxa (singular: taxon). 2. The tips of the tree represent groups of descendent taxa (often species). 3. The nodes on the tree represent the common ancestors of those descendents. Overview of Phylogenetics 4. Two descendents that split from the same node are called sister groups. In the tree below, species A & B are sister groups — they are each other's closest relatives. 5. Outgroup — a taxon outside the group of interest. 6. All the members of the group of interest are more closely related to each other than they are to the outgroup. 7. The outgroup is useful when constructing evolutionary trees and determining how phenotypic traits have evolved over time. Using phylogenies to understand evolution 1. Phylogenies can reconstruct the order of evolutionary changes. a. Example 1: Spider behavior Orb-weaving spiders, spin intricate and orderly webs. Other spiders spin disorderly cobweb-like webs. Hypothesis on which came first? The Evolutionary History of Spiders and Web Making Cobweb Orb webs evolve (species in yellow) Cobweb Cobweb Dimitrov et al 2012 Tangled in a sparse spider web: single origin of orb weavers and their spinning work unravelled by denser taxonomic sampling Using phylogenies to understand evolution 1. Hypothesis: Complex orb webs evolved from less complex cob webs. 2. Results from phylogenetic analysis: Orb-weaving was the ancestral state. Cobweb-weaving evolved from spiders with more orderly webs. Reject hypothesis. 3. Side lesson: Evolution does not always go from less to more complex. Sensory bias in Swordtails Test for the preference in closely related species that don’t have the trait.
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