This week in Animal Behaviour. This week in Animal Behaviour. Why polyandry? Why polyandry?

Transcription

1 This week in Animal Behaviour This week in Animal Behaviour Oct. 21 project 1 write-up due in PDF form on Moodle by midnight Oct. 21 raw data due (either in class or yellow drop box outside 3520)... 10% penalty on write-up if raw data not turned in Oct. 23 online discussion #3: Networking crows Lecture 16: Mating systems I Lecture 17: Mating systems II Lecture 18: Parental care LAB: Project 1 help session Text chapters 7-8 Why polyandry? What causes sex role reversal? Why polyandry? Examples: red phalaropes, wattled Jacanas...Galápagos hawks are flexible

2 Multiple mating: the costs Mating with multiple males can potentially expose females to costs, such as the time spent searching, risk of lost investment of current mate, STDs... Evidence: comparative study of immune system in primates Why mate multiply? measure of white blood cells degree of polyandry Multiple mating: genetic benefits 1. Fertility insurance: multiple mates might reduce the risk of having an infertile partner for a mate e.g. red-winged blackbirds, Gunnison s prairie dogs Multiple mating: genetic benefits 2. Good genes: females might seek out several males in order to secure superior genes from at least one of them, either to improve offspring viability or attractiveness e.g. blue tits, yellow-tooth cavy mean # offspring # of mates

3 Multiple mating: genetic benefits 3. Compatible genes: mating with several males could increase the genetic variety of sperm available, and increase the chance that a female will obtain some sperm with DNA that is an especially good match for her genotype e.g. bluethroats wing-swelling response (mm) Multiple mating: material benefits 4. Infanticide reduction: having more mates increases paternity uncertainty, decreasing the likelihood of losing offspring to infanticidal males e.g. lions, Hanuman langurs WPY EPY Multiple mating: material benefits Polygyny 5. Resources: more mates can increase the resources and/or parental care received from sexual partners e.g. pierid butterflies reproductive output mates per female

4 Polygyny is common It can take several forms... Scramble competition e.g. garter snakes Female defence polygyny: e.g. grey seal Resource defence polygyny: e.g. Juan-Fernandéz fur seal Lek polygyny: e.g. peafowl, sage grouse Scramble competition Polygynous males search for and acquire several widely scattered mates e.g. garter snakes, horseshoe crabs, wood frogs, ground squirrels time searching (min) Female defence polygyny Polygynous males directly defend several females e.g. red deer, elephant seals, greater spear-nosed bats Resource defence polygyny Polygynous males acquire several mates attracted to resources that are under the males control e.g. water striders, African cichlids, antlered flies

5 The influence of resources The polygyny threshhold model resource dispersion predation female dispersion resource defence polygyny male dispersion Males arrive and settle Then, females arrive sequentially 1st female takes best territory (D) 2nd takes 2nd-best territory (C) 3rd takes 3rd-best territory (B) B E A C G D F costs & benefits of social living female defence polygyny = monogamous = polygynous The polygyny threshhold model The polygyny threshhold model Males arrive and settle Males arrive and settle??? Then, females arrive sequentially When is it beneficial to join as a secondary female? Is undivided male care more important than male quality and/or resources? B E C G A F D Then, females arrive sequentially When is it beneficial to join as a secondary female? Is undivided male care more important than male quality and/or resources? B E A C D??? 2nd F G = monogamous = polygynous The point where it pays to go to an occupied territory = polygyny threshold = monogamous = polygynous

6 The polygyny threshhold model Why don t females change the system? It s about individual decisions each female decides what is best for her the quality of the available habitat leads to polygyny polygyny threshhold Example: yellow-bellied marmot # offspring # females per group Why don t females change the system? Lek polygyny Example: pied flycatchers monogamous male polygynous male Höglund and Alatalo (1995): An aggregated male display that females attend primarily for the purpose of fertilization. Characteristics of the lek: an arena where males gather and females come to mate relative fledging success only mate primary secondary resource-free display site females choose male no male paternal care Leks generally have highly skewed male mating success (e.g % of males get > 50% of matings)

7 Leks are rare ~35 species of birds ( < 0.5%) 7 species of mammals (< 0.2%)...some insects, some fish, maybe some amphibians too? Why lek? Hotspot hypothesis: males cluster because females are located in certain areas, e.g. when females travel repeatedly along certain routes Hotshot hypothesis: subordinate males cluster around attractive males that females tend to visit more often Why lek? Topi antelope are lekking mammals Female preference hypothesis: males cluster because females prefer to visit sites with many males, e.g. for safety, or to be able to compare many males e.g. ruffs overall copulation rate (/min) # males at lek

8 Mating system evolution Threat of infanticide matters Case study: monogamy in prosimians (lemurs, bushbabies, tarsiers) dark blue: year round! " association light blue:!s do not accompany "s yet this does not appear to be the case in other primates AND the best predictor across all mammals seems to be female dispersion Mating system evolution Case study: fairy wrens Radical loss of promiscuity purple-crowned fairy wren promiscuity lost despite no change in breeding biology or ecology

9 Summary Mating systems differ in the advantages they offer for males and females! There is often a conflict of interests between the sexes Ecological factors are thought to be generally important in the evolution of mating systems! However, ultimate explanations can depend on the taxa being studied