The Mating Game: Conflict or Cooperation (BIOL2306) PDF

The mating game: conflict or cooperation between the sexes? Ecology & Evolution BIOL2306  Natural selection determines life histories: i.e. when organisms should breed, how often, & how much to invest. But what about the choice of partner? Animals are either: MONOGAMOUS ( &...

The mating game: conflict or cooperation between the sexes? Ecology & Evolution BIOL2306  Natural selection determines life histories: i.e. when organisms should breed, how often, & how much to invest. But what about the choice of partner? Animals are either: MONOGAMOUS ( & form a pair bond) or POLYGAMOUS (where each sex may have >1 mate) POLYGAMY subdivides into: POLYGYNY 1 male & many females; each female mates with 1 male & is responsible for parental care POLYANDRY 1 female & many males; each male mates with 1 female & is responsible for parental care What percentage of animals are monogamous? 'Faithfulness' has an unusual distribution among animals Why are birds mostly monogamous? Female has to take part in parental care: it increases her fitness to look after her (expensive) eggs, & she cannot risk having them abandoned by male But, what prevents male abandoning female to find another mate? (so he can fertilize more eggs)  Male birds usually invest in parental care because it yields higher potential fitness gains than leaving the first to find another female. There are 3 reasons for this: Eggs must be incubated until they hatch which requires both & Altricial* nestlings (can’t look after themselves) have high demand for food, & their chances of survival are much greater with presence of both parents Females have seasonal synchronization of reproductive cycles (by day length); male has little chance of mating again if he leaves female Altricial * Opposite = precocial nestlings (e.g. chickens, ducks) Although & birds are usually monogamous, females tolerate sharing a male with another female under certain circumstances (i.e. male is polygamous). Why? If male bird holds a high-quality territory that he defends for exclusive use of females (+ offspring male has fathered), then sharing is tolerated by females. Easier for female to rear young without help on a good-quality territory than with help of male on a poor- quality territory Note: sharing is more likely to be tolerated in bird species where the young are precocial. Why? Why are mammals generally polygynous? Females are a limited resource (eggs are limited, sperm are not), so it is an advantage for males to mate many times Reason #2 is ??? 5% of mammals are monogamous - usually territorial species &/or predators: males contribute by defending territory for females & offspring, &/or providing growing offspring with food (= prey) Natural selection & evolution of parental care When parents make an investment in caring for eggs/young & take risks to provide food/resources for them, parental survival (& future fitness) is traded off against offspring survival. But … Maternal parental care Maternal parental care is more common than paternal care. In some instances, maternal care is a result of internal fertilization and the delay between mating and birth (gestation). Other general reasons for maternal care being more common focus on the relative costs to the two sexes of being the caregiver. For males there is uncertainty about paternity, which will reduce the benefit to cost ratio of engaging in parenting. In addition, for males when there are opportunities to mate with multiple females, males that give up that opportunity to engage in parental care will pay too high a price. Paternal care (either with the female or alone) would be selected for only when the payoff is sufficient to outweigh the costs. Maternal care: Membracinae treehoppers (boomcicade) 3 independent origins of female parental care (egg guarding), none of male parental care Paternal Care: fishes In fish male parental care is quite common. Many males mouth brood eggs or care for eggs in nests. Costs of parental care in these cases seem to be lower for males than for females. E.g. because females prefer males that engage in parental care or because males can take care of several egg clutches. Paternal Care: fishes Because, in many fish, costs of parental care are higher for females than they are for males, paternal care may have evolved because males lose less from parental care than females do. E.g. St. Peter's fish. difference is less (mouth brooder) Paternal Care: male water bugs Male water bugs guard and moisten eggs above the water (Lethocerus) or carry eggs on back (Abedus, Belostoma). Abedus eggs do not develop unless aerated by male. Because water bugs are predatory insects (catching fish, frogs and tadpoles) they are large and consequently their eggs are too. This is why oxygenation is necessary. Why only male care? Male water bugs with one clutch of eggs sometimes attract a second female. Also costs of parental care may be disproportionally great for females in terms of lost fecundity. Natural selection & evolution of parental care When parents make an investment in caring for eggs/young & take risks to provide food/resources for them, parental survival (& future fitness) is traded off against offspring survival. But … What evidence is there of costs & trade-offs associated with parental care? How do we know that investment in care will affect parental fitness? Trade-offs studied in Sweden where Great tits (Parus major) produce two broods each year Parent bird Nestlings Linden (1988) examined effects of changing size of 1st brood (by -4, -2, 0, +2 & +4 eggs from original number) on 2nd brood. NB: changing clutch size = changing the cost of parental care Great tits in Sweden 1. Birds with enlarged broods reared the extra young & produced more fledglings than control birds 2. The enlarged broods yielded under- weight fledglings with poor survival; fledglings in reduced broods were heavier than in normal broods & survived better 3. Few adults with enlarged 1st broods produced second broods, while pairs with reduced 1st broods often produced 2nd broods Similar evidence of costs of parental care has been found in other birds … & other animals ….  Chick color affects parental care in mixed broods of coots Control broods were unaltered (orange) or had orange feathers trimmed (black) Experimental broods had 1/2 orange, 1/2 black chicks Chick color likely indicates offspring health Belastoma flumineum (Order Heteroptera) Predatory freshwater insect with extracorporeal digestion carry eggs on back until they hatch No significant difference in feeding rate of each group Significant difference in feeding rate Discriminating Parental Care Misdirecting parental care towards non- offspring obviously would be a costly mistake for any organism. Many animals rear their young in colonies and there is plenty of opportunity for confusion. Yet, as predicted, parental care is usually very discriminating.  Discriminating Parental Care Fig 12.7 Young Mexican free-tailed bats at a creche containing 4000 pups per square meter. Females give birth to a single pup. They use vocal and olfactory cues to identify their offspring from among thousands in the creche. The bats do occasionally make mistakes but the benefits of leaving a baby in a creche (mainly thermoregulatory) appear to outweigh the cost (accuracy from allozyme data: 80%). Discriminating Parental Care Cliff Swallows often nest in large colonies and their young produce much more variable calls than do Barn Swallows, Fig 12.9 which generally nest solitarily. Cliff Swallow parents are also much better at distinguishing between calls than are Barn Swallows.  Adoption: goldeneye duck In some instances adoption may be beneficial to the adopter. E.g. in ducks it is common for females to accept extra eggs laid in their nests and to accept stray ducklings into their broods. This may increase the odds that one’s own young would be saved from predators by the dilution effect. Also, there is little or no cost to adoption because chicks forage for themselves. General points about mating & choice of partners Females invest more per gamete than males, & Female gametes are a limited resource As a result, Males compete among themselves to mate with Females Consequently, natural selection will produce males that are good at seeking out & competing for females But, there is a conflict of interests between the sexes …* *At least, in birds & mammals For males, the 'ideal' mating system is one in which they maximize the number of mates & offspring fertilized For females, the 'ideal' system involves restricting the number of mates of each male, & involving him in parental care  Conflict of interests between the sexes … One method used by males to increase their fitness - even in 'monogamous' birds is to carry out extra-pair copulations (EPCs): mate 'secretly' with other paired females As a result, one male may unknowingly be caring for another males offspring. Males in EPCs thus parasitize the parental effort of other males. Zebra finches (Taeniopygia guttata): 8% of young result from EPCs; up to 35% in some bird species Swallows (Hirundo rustica): 26% of young result from EPCs, but EPCs only 7% of copulations seen (= 'sneaky') Cheating is one way of getting ahead in the ‘battle of the sexes’: sexual cannibalism is a more extreme method, as seen in praying mantis females (Order Mantodea) Sexual cannibalism … Obviously, sexual cannibalism can only evolve in predators Female praying mantis gains fitness by eating her mate - why don't other species (non- herbivores) practice sexual cannibalism? Mantis Alternatively: why do males egg (sometimes) let their mates eat case them? Should a male allow a female to eat him? Probably depends on: a) Number of additional young male would father if female eats him (NB: assumes assurance of paternity) b) Number of times male can expect to mate. If it is low, male might allow female to eat him – e.g. if females are rare, or if males have trouble finding them Active (cursorial) spiders Web-building spider that chase their prey sit-&-wait predator  Main consequence of ‘battle of the sexes’ is that males compete for females EPCs show that 'sneaky' strategies may evolve where males 'parasitize' the parental effort of other males →‘Sneaking' can also occur in instances of courtship where the male exposes himself to risk from predators in order to attract females Large male frogs call to attract females; smaller males wait near them hoping to encounter a female on the way to a calling male Small 'satellite’ males only call if large males are not present 'Satellites' need not always be small individuals: large Males will become 'parasites' upon loudspeakers broadcasting exceptionally loud calls!  + 'parasitism' has also been reported in fish … ‘Satellite' attempts to shed sperm near e.g. Coho salmon (Onchorhynchus an egg-laying in the company of a kisutch) has two forms: competitively- dominant fish dominant 'hooknose' sneaky 'jack' The study species: Tripterygion delaisi Small common rocky shore fish Common name: Black-faced blenny Feeds on benthic invertebrates T. delaisi: reproductive strategy Reproductive period from April to July External fertilization Female lay eggs on nests and male eject sperm directly on them. Larvae hatch and leave T. delaisi: reproductive strategy Two alternative mating strategies: Territorial male: changes colour guards nest Territory defense Female courtship Sneaker male: No change in colour `sneaks` in when female lay eggs Schunter et al. 2014  There is a conflict of interests between the sexes … For males, the 'ideal' mating system is one in which they maximize the number of mates & eggs fertilized For females, the 'ideal' system may involve restricting the number of mates of each male, & involving him in parental care Both sexes have evolved ‘sneaky’ or ‘parasitic’ forms of behaviour to increase their mating success. But there is competition within the sexes, as individuals of each sex are competing to enhance their success (= fitness) relative to members of the same sex References Krebs, J.R. & Davies, N.B. (1993) An Introduction to Behavioural Ecology (Chapter 9; there are also some examples in Chapters 8 & 10 that may be useful for background reading, but there is more detail than you will need) Davies, N.B., Krebs, J.R. & West , S.A. (2012) An Introduction to Behavioural Ecology (4th Edition): Chapters 7, 8 & 9 (more detail than you will need, but includes many additional examples of sexual conflict) Stiling, P. (2012) Ecology: Global Insights and Investigations : Section 4.4 (pg 90-97) Linden, M. (1988) Reproductive trade-off between first and second clutches in the great tit Parus major: an experimental study. Oikos 51: 285-290 Schunter C, Vollmer S, Macpherson E, Pascual M (2014) Transcriptome analyses and differential gene expression in a non-model fish species with alternative mating tactics. BMC Genomics, 15, 167. Brepson et al. (2012) Cheating for sex: inherent disadvantage or energetic constraint? Animal Behaviour 84: 1253–1260 (& the popular science article on ‘satellites’ at here) See the ‘Quick Note’ on sexual conflict at http://www.sciencedirect.com/science/article/pii/S0960982205007530 Plan for the rest of semester LA 25.10.24 12 The mating game: conflict or cooperation among the sexes? BG 29.10.24 13 Case study: is insect colour a defensive adaptation? 1.11.24 BG 14 Optimal animals: what should they eat and where should they eat it? BG 5.11.24 15 What prevents populations from growing infinitely? BG 8.11.24 16 Why do some animals live in groups? AH 12.11.24 17 Co-evolution LA 15.11.24 18 The evolution of sex ratios: why are they usually 1:1? LA 19.11.24 19 Case study 2: how (or why) did the guppy get its spots? LA 22.11.24 20 Evolution of humans: where did we come from? LA 26.11.24 21 Human evolution: meet the relatives LA 29.11.24 22 The Anthropocene Final exam 19th December 2.30-4.30 LYH The final exam for Ecology and Evolution will constitute 50% of the overall assessment for the course. This is a CLOSED book examination proctored on campus. The final examination will consist of 70 multiple-choice questions (70 marks) and three short response questions (30 marks). The exam will be two hours duration. Questions in the exam will be designed to cater to a range of student abilities, and will include some questions that are very straightforward, and others that might require analytical thinking and application of concepts taught in the course, or a good mastery of the relevant material. Optional tutorial on primates and human origins Course Tutorials: Primates and Human Origins– 21.11.24 Lab E 1/F North Wing KBSB 10.00 – 17.20 You can drop in any time during these hours Demonstrators will help you work through the material