Mating Systems
Evolution of mating systems and parental investment Parental investment into – gametes – embryo – offspring (and grandchildren) • Protection (predators, weather etc.) • Feeding • Teaching Investment carries a cost, investment into present offspring reduces the number of offspring in the future Evolution of mating systems and parental investment Investment carries a cost, investment into present offspring reduces the number of offspring in the future If true: – – – Which sex would invest (conflict between the sexes) How much a parent would invest (conflict between the parent and offspring How much investment one offspring should force (conflict among offspring) Evolution of mating systems and parental investment Equal large investment needed → monogamy Small investment: → promiscuity The sex investing less can have more offspring But if sex ratio is 1:1 → conflict within the sex If males invest less → polygyny If females invest less → polyandry Evolution of mating systems and parental investment But if sex ratio is 1:1 → conflict within the sex Mechanism of conflict in polygamy: mating systems – Resource defence – Female (male) defence – Hierarchy (e.g. lek polygyny) – Scramble competition Whom to mate with (conflict between the sexes) – The sex investing more → has less offspring → more choosy What to choose: whatever increases fitness Evolution of mating systems and parental investment Factors in choice: whatever increases fitness – Good genes→ healthy offspring – Sexy partner → sexy sons – Provisioning → fit offspring – Good match → healthy offspring – Protection → live offspring Conflict between factors: e.g. what if sexy partner seeks extra-pair copulation instead of provisioning? → parent with provider socially, seek EPC for good or sexy genes Evolution of mating systems and parental investment Complications Social and genetic mating system can be different Internal fertilisation: fathers can never be certain → males are less likely to invest into offspring Not real polyandry. Extra Pair Fertilisation (EPF) independent of social mating system Evolution of mating systems and parental investment Complications Social and genetic mating system can be different Internal fertilisation: fathers can never be certain → males are less likely to invest into offspring Ecological constraints may influence mating system • e.g. sparse distribution of food → sparse distribution females → monogamy (male provisioning may be secondary) • Group structure may influence mating system • e.g. if cooperation among males is vital → sperm competition Parental investment into gametes How did anysogamy evolve? Parental investment into gametes How did anysogamy evolve? Parental investment into gametes How did anysogamy evolve? Small + small → low fitness Small + large → medium fitness Large + large → high fitness Selection for large gametes? Simplistic approach: smaller gametes → more can be produced and higher motility If small gametes can avoid to unite with small ones, selection would favor small and large gametes. Parental investment into embryo Investment asymmetry between sexes: Internal fertilisation: females invest more (mammals, birds, some fishes, amphibians, insects) External fertilisation: both sexes can invest equally Genetic asymmetry between sexes: Internal fertilisation does not guaranty genetic fatherhood In which case do you expect paternal care? Parental investment into embryo External fertilisation: more species with paternal care. Stickleback: territorial Reversed sex roles Male care is frequent in fishes, rare in other taxa Some exeptions (waterbugs: Abedus, Belostoma) • Female deposits large eggs on male’s back • Eggs are attractive for other females • Male can carry more batches Tasks for the male • Aeration • Humidity • Male should balance between the two • Why paternal care? Why paternal care? • Predators -> large body size -> large eggs • Producing large eggs is costly for females • Carrying eggs would significantly reduce female’s fitness (and that of males) • Male can easily carry several batches Large water bugs: parental care. Small bugs: no care IN THE NEWS Peru poison frog reveals secret of monogamy Genetic tests have revealed that male and females of one species of Peruvian poison frog remain utterly faithful. After mating, a female frog lays her eggs on the surface of leaves The male frog then takes away the tadpoles that hatch, carrying them one by one on his back to pools of water which collect in bromeliad leaves high up in the branches of trees. If the pools were bigger, the frogs wouldn't have to remain faithful… Monogamy:mate assistance hypothesis When partners staying together can have higher fitness than searching for new mate • Example: hippocampus. Male carries brood in pouch for 20 days • Daily greeting ritual: female checks the condition of brood • Male carries the brood until new eggs are produced by female Monogamy: the female egg laying rate is limiting. Seeking new partner would be costly. Females are not choosy Sex role reversal in pipefish Males care for brood in pouch During the time of male pregnancy females of some species can produce enough eggs to fill 2 male pouches. Male pouch space is in short supply Polyandry: males in these species tend to be choosy (they select females which provide the most eggs) Parental investment into embryo Asymmetry between sexes: females invest more into embryos in species with internal fertilisation (mammals, birds, some fishes) Difference between birds and mammals Birds: once the eggs are layed both sexes could incubate them – cca. 90 % of bird species are socially monogamous – cca. 5% in mammals Monogamy Factors ensuring monogamy: Mate guarding: if distribution of females is sparse it is impossible for males to monopolise more than one female Mate assistance: if investment of both parents is possible and needed for offspring survival (monogamy widespread in precocial or in altricial birds?) Female-enforced monogamy: female prevents polygyny to keep male’s help These 3 hypotheses are not mutually exclusive Female enforced monogamy • Female needs help of male • Prevents male to provision to more females • Example: Yellow-breasted chat (Icteria virens) 11.4 Dual11.4 mate-enforced monogamymonogamy 11.4Dual Dual mate-enforced monogamy mate-enforced Agression toward modell birds (A) Males (B) Females Mate assistance in birds 90% of birds species are socially monogamous Extra pair copulation (EPC) in 90% of monogamous species Mate assistance in mammals Example: California mouse (Peromyscus californicus) • Monogamous. • Some of the males were removed in experiment. • In presence of males 3 times more offspring • („Males exhibit apparently more sexual fidelity than females.” Gubernick and Nordby) Interesting California mouse: postpartum estrus The male should defend territory, copulate and take care of offspring Aggression and copulation: high level of testosterone Testosteron normally inhibits paternal care Testosterone is converted into estradiol in some brain areas by aromatase !!! Trainor and Marler (2001) Mechanism of sexual fidelity Montane vole (Microtus montanus) polygynyous Prairie vole (Microtus ochrogaster) monogamous Social attachment: „love” Experiment Copulation then test. Female in mid chamber. Which females are in love? Prairie vole females spend more time with partner Mechanism of sexual fidelity No difference in circulating oxytocin level But: more receptors in brain N. accumbens, prelimbic cortex Mechanism of sexual fidelity Experimental manipulation of OXI receptors Fidelity developes following first copulation Blocking OXI receptors would inhibit development of fidelity? In which brain areas? OXI antagonist in different brain areas + control Copulation Test of fidelity Mechanism of sexual fidelity N. accumbens, prelimbic cortex, caudate putamen Green columns: time spent with partner Yellow columns: time spent with stranger Love is in the Nucleus accumbens and prelimbic cortex Mechanism of sexual fidelity Males: Vazopressin receptors Prairie vole males express more V1aR (vazopressin 1a receptor) in ventral pallidum (VP) Mechanism of sexual fidelity Prairie vole males without mating or before copulation received arginine vazopressin (AVP), cereb. spine fluid (CSF) or V1 receptor antagonist AVP alone without copulation increased pair fidelity Antagonist prevented fidelity even in mated males Paternel help in apes How to explain monogamy? • Assistance against infanticide? – Females are dominant in most monogamous sp. • Do males care for the offspring? – Males may invest in polygynous sp. • Help in carrying the young? – Independent of the number of offspring Mate assistance: offers protection against infanticide Purple: females carry young male-female pair Yellow:young stays in nest males do not follow Two exceptions: Varecia variegata: antipredatory? Pongo pygmaeus: male-female Mate assistance: offers protection against infanticide a) Long lactation period, no post-partum estrus, female exclusively carries the young infanticide: 49% b) male and female cooperate in carrying the young c) young stays in nest Evolution of mating systems Monogamy: one male – one female Polygyny: one male – more females Polyandry: one female – more males Polygynandry (Promiscuity): more males – more females Wilson's Phalarope (Steganopus tricolor) polyandry Poliandry Rare • Example: spotted sandpiper (Actitis macularia) • Females are bigger and brighter • Arrive earlier, occupy territories • Competition among females • Males settle in females' territories, mating • Female lays 4-4 eggs to nests of several males • Males incubate the eggs, protect chicks Factors favouring polyandry • • • • • Abundance of food at hatching Precocial chicks need not much investment A single parent is not worse than biparental care Sex ratio is male biased Sandpipers can incubate no more than 4 eggs. Origin of polyandry Not in the textbooks: Oring et al. BEHECO. Observation for 17 years 26% of females are monogamous (especially young) Type of polyandry depends on age Yearlings: sequential resource defense Two year old: simultanious polyandry (equivalent proportions of resource-defense and mate-access polyandry.) Older females: primarily simultaneously resourcedefense polyandrous (this is in the textbooks:-) Polygyny • • • • • One male mates with more females Paternal investment is small Common in mammals Competition among males Mechanism – – – – – Scrambling Exhibiting quality Defence of resource Defence of females Establishing hierarchy not mutually exclusive Scramble Competition Polygyny Distribution of females is sparce because of ecological constraints Males cannot monopolise more females (one possible factor in monogamy) Males can visit many females. Males reaching more females in estrous have higher fitness Thirteen-lined Ground Squirrels Males “make the rounds” visiting female burrow sites Male remembers which females are about to enter estrus Experiment: Remove near estrus female or control female, not near estrus Resource Defence Polygyny • Male occupies and defends resource important for females (for offspring) • Mates with females on resource • Bigger, better quality resource – higher fitness Northern elephant seal (Mirounga angustirostris) Males: 2300 kg. Females: 640 kg. Great sexual size dimorphism (SSD) Bulls arrive at terrestrial breeding ground and fight Northern elephant seal Arrival mass correlates with rank Rank correlates with copulatory success Selection for size (Haley et al. 1994) Variance of success: 25-40% large size + age, experience Southern elephant seal (Mirounga leonina) males: 3000 - 5000 kg; 4 - 6 m (length) females: 400 - 900 kg; 2.8 m (length) Larger than Northern species, higher SSD (male 8 times bigger than female) Fierce competiton Harem size: 2-125 females Southern elephant seal Fabiani et al. (2004) Harem size: 2 - 125 females (median 37) Male mating success: huge variance (0 → 80) Southern elephant seal More than one male around females Alpha male, beta male, peripheral males Alternative male strategies: Mating with females at periphery, Raping females entering the sea http://www.eleseal.org/sli/sli_socio.html Rensch rule High SSD – high degree of polygyny Large body mass: higher SSD and higher polygyny (Rensch rule) Degree of SSD correlates with harem size in seals Female defence polygyny – Red deer Mating: September – (October) Rut: dominant stags visit traditional rutting place if successfull they monopolise groups of (related) females Mating: September – (October) Rut: stag copulates with females in estrous (vomeronasal organ) Red deer Male- male competition: Deep voice – large body in most species Trick in red and fallow deer: lowered larynx Fitch and Reby: The descended larynx is not uniquely human As in humans... Costs? More men than women die chawking in restaurants… Red deer Male- male competition: Roaring contest Clutton-Brock: replay experiment Frequent roaring deters weaker stags. Frequent roaring takes energy, indicates strength Red deer Male-male competition: Parallel walk Antler fight Red deer Male-male competition: Parallel walk Antler fight Decision points during contest Red deer Male-male competition: why so fierce? (Mortality can be up to 10%) Most fighting around conception date. Female-defence polygyny: harem Fights and injuries: what is to be gained? Red deer Advantages of fighting for females Hinds: 0-14 calves Stags: 0-24 calves !!! No choice for males but to fight for females Males are not so picky... polygynous males Low threshold for mating Red deer Reproduction succes depends on age: 7-11 year old stags Red deer Limits of dominance Large body Large antler Sexual dimorphism: females develop earlier, males keep growing.. Red deer Mortality of stags and hinds Red deer Femal strategies Fertility depends on condition Red deer Femal strategies Cost of breeding Carcass weight, kidney fat, rump fat. Condition of hinds with calves is poor. Calving: every second year on average in poor habitats Red deer Trade-offs You can’t have everything Natural selection: favors high reproductive success. Among males: competition (ritual and direct fight) high mortality males in poor condition do not breed. Among females: less competition higher mortality with calves females in poor condition do not breed. Would you have a boy, or a girl? Hinds in good condition (high rank) have more sons Gomendio (1990) Nature More fertile males produce more male offspring Gomendio et al (2006) Science IN THE NEWS (2010) …in 2003 showing that expectant mouse moms who consume lowcalorie diets tend to carry more females to term than males— indicating that male fetuses are the more sensitive sex in utero and miscarry at higher rates. That made Rosenfeld wonder if diet causes genes to behave differently in wombs with male or female fetuses. Resource defence polygyny – Roe deer Social groups: Solitary, forming small groups in winter. Territory: males mark and defend area from April Scent glands: preorbital, tarsal, metatarsal. Damage to trees and shrubs Roe deer Cleaning of antlers, marking Carranza & Mateos-Quesada, Oecologia (2001) 126:231–238 Fraying and scratching around the stem of bushes Defoliation by leaf eating Within clearings (2700 m2) more than 50% of leaves were eaten Resulting in: Cover of shrub layer within clearings diminished. Function? Enhanced visual signals? Antipredator strategy? Farming? Roe deer Territory size: few square km Overlaps with homeranges of 1.5 – 3 females Reproduction system: territorial (resource defence) polygyny Mating in July – August Male chases female, female circles conflict Age and antler size together affect territorial status Hoem et al. Eur J Wildl Res (2007) 53: 1–8 Conflict between bucks seldom escalates into direct fight. Resident buck usually wins. If escapes may still hold the territory. For resident: loss is great, takes great risk. For intruder: gain is unlikely, does not take risk Optimal size of territory: Optimal: maximum profit, maximal difference between cost and income Cost is increased by size of territory population density Benefit is increased by size of territory quality of terr. Optimal size: X or X’ Effect of density on territory size Kjellander et al. Oecologia (2004) 139: 478–485 Home range Many roe: black bars Few roe: blank bars buck > doe winter >= summer Few roe > many roe Fluctuating asymmetry (FA) In a roe deer population FA is high with high density Alternative male tactics in lizards side-blotched lizard (Uta stansburiana) Costs of territorial defence Common side-blotched lizard (Uta stansburiana) 3 fenotypes Orange: large territories Blue: strong pair-bond Yellow: female mimic (stone – paper – scissors) Survival of territorial males Barry Sinervo is low Yarrow's spiny lizard males received testosterone capsules. How did territorial behaviour change? több időt töltenek a védelemmel Survival of treated males was law if no extra food was given Costs of territorial defence Direct costs: time, injuries, energy, prredators Indirect costs: highy testosterone → low immunocompetence IN THE NEWS Men have a hard time being just friends. Sexual attraction is regarded cost more by women than men. Discussed in context of differential reproductive strategy IN THE NEWS Singing Mice May Join Humans and Songbirds As Vocal Learners Male mice produce courtship ultrasonic vocalizations with acoustic features similar to songs of songlearning birds. cortex region active during singing, includes a motor that projects directly to brainstem vocal motor neurons and is necessary for keeping song more stereotyped and on pitch depend on auditory feedback to maintain some ultrasonic song features, and that sub-strains with Figure 1. Brain systems for vocalization in birds and mammals. Arriaga G, Zhou EP, Jarvis ED (2012) Of Mice, Birds, and Men: The Mouse Ultrasonic Song System Has Some Features Similar to Humans and Song-Learning Birds. PLoS ONE 7(10): e46610. doi:10.1371/journal.pone.0046610 http://www.plosone.org/article/info:doi/10.1371/journal.pone.0046610 IN THE NEWS Spontaneous human speech mimicry by a cetacean. Ridgway et al. (2012) A white whale, Beluga, imitated human voice Resource defence polygyny in an African cichlid African cichlid fish Lamprologus callipterus exhibits extreme sexual dimorphism (males 13X times larger than females). Females deposit eggs inside an empty snail shells and guards eggs and larvae Resource defence polygyny in an African cichlid Males gather shells into large collections and defend them from rival males. The more shells the male can collect the more nesting sites it provides for females. Up to 86 shells have been recorded in one collection and up to 14 females at once. Extremely large male body size has been selected for because it enables males to collect shells and to defend their territories. Alternative Reproductive Tactics (ARTs) Nest male (bourgeois male): holds territory Dwarf males (parasitic): small male, it's size is 2% of nest male, 25% of female •Hides in the shell with female and seeks to fertilise eggs •92% of young are fathered by nest male •Genetically fixed Sneaker males: mid size males seek to fertilise eggs when nest male is busy •Transitional, opportunistic Alternative Mating Strategies (ARTs) Lake Tanganyika shell-brooding cichlid Telmatochromis vittatus, Les Bourgeois Sneaker males Pirate males Territorial males + 2 parasites Ota et al. (2010): territorial males living under the risk of pirates and sneakers face increased risk of sperm competition and therefore should produce high-quality sperm experiments demonstrated that a visual stimulus of a pirate was enough to induce an increase in sperm longevity in territorial males Sepia apama 4-11 males fight for one female The largest male wins, consorts (c) Alternative tactics: male (m) assumes female colouration and behaviour (takes 10-15 minutes), approaches, inseminates Success rate 30 times in 62 observed cases (DNA fingerprinting) Hanlon (2005) Nature videó Bluegill sunfish Treatment: small sneakers were kept by the tank of territorial males. Control: territorial males did not see sneakers Test: predatory fish was introduced to territory in plastic sack Data: number of attacks to predator Male can identify own offspring by smell at fry age but not in eggs Lek polygyny Fallow deer males either defend territory or move to common rut place „lek” LEK: Swedish leka "to play" Each male defends and marks a small stand within lek Females approach rut place Dominant males in centre position copulate. Lek polygyny Variable: may start with lekking followed by harem holding Lek polygyny (McElligot & Hayden, 2000, Behav. Ecol. Sociobiol.). Life-time mating success of males: only 10% mates cca 3% gained 73% of all matings Lek polygyny Wild Turkey (Meleagris gallopavo) Several males gather, display cooperatively, attract females, dominant male mates with all females. Subordinate males can either go solo, or join coalition. What's in it for subordinates in coalition? Lek polygyny Krakauer (2005) found that cooperative subordinates are related to dominant by r = 0.42 Kin selection Lek polygyny Great bustard (Otis tarda) Males 3x bigger than females „Extended lek”: up to 200 males may gather to perform their courtship displays Larger male territories than in classical leks Females choise is based on male quality and not on territory Olea et al. (2010) found that found that the male birds tended to direct their tail feathers towards the Sun in the morning. Hypotheses for leks Hotspot males aggregate to maximize female encounter rate Hotshot females prefer to select mates in aggregations males aggregate around most attractive male (hotshot) • Expect center male to have highest mating success Kin selection Males aggregate around relatives Black hole females avoid sexual harassment by young males Why Leks? Benefit to - females:window shopping escape harassment by young males reduced predation risk - dominant males: many matings - subordinate males: kin selection AND/OR few matings if possible AND/OR gaining experience Hierarchy within group Wolves: reproduction exclusively by alpha pair (pack is a family) Spotted hyenas: females are dominant over males, high ranking females mate with high ranking males. Pseudopenis (enlared clitoris) is used in communication Pseudopenis Pseudopenis is displayed in communication, however, Pseudopenis Pseudopenis is displayed in communication, however, females urinate, copulate and give birth through clitoris Hierarchy + sperm competiton Pre-copulatory choice. Cooperation of females is needed for copulation (erect penis through flacid clitoris) → female choice Hierarchy + sperm competiton Post-copulatory sperm choice: Females mate with multiple males female's reproductive tract is long, and full of blind alleys and dead ends Only the best sperms reach the ova http://www.vet.cornell.edu/labs/place/publications/11cunha%20et%20al.2003.pdf (Social) mating systems Monogamy Polygamy: polygyny or polyandry • Scramble competition • Resource defence (e.g. territorial) • Female (male) defence (e.g. harem holding) • Hierarchy (e.g. lek polygyny) Sperm competition (sometimes used also for polygynandry) Polygynandry: more males with more females within a social group Polygynadry Hyaenas can drive female and subadult lions off their kills provided they outnumber the lions by a factor of four. At Savuti, groups of female and subadult lions lost almost 20% of their food to hyaenas Larger, year-round stable groups fare better . Polygynadry Hyaenas can drive female and subadult lions off their kills provided they outnumber the lions by a factor of four. At Savuti, groups of female and subadult lions lost almost 20% of their food to hyaenas Larger, year-round stable groups fare better, with males even better. Cooper (2008) African J. Ecol. . Polygynadry Males cooperate to monopolise female groups Reproduction: Oestrous lasts 5 days Lioness may copulate with more than one male Copulation rate: 1/20 min. for 5 days = hundreds of copulations! Why waste energy for copulation? . Polygynadry Sperm competition • Male lions should cooperate to monopolize female group • Direct conflict (fight) would be detrimental for male coalition Sperm competition: males transferring more sperm have more offspring. . Polygynadry Infaticide Young males leave pride and form coalition Male coalitions attempt to take over territory When successful, males kill all cubs . Polygynadry Infaticide (10% of total lion mortality is due to infanticide!) Good for the species? Good for whom? Females get into eostrous when cubs die Males can rule pride for about 2 years, first year is important Cooperation among males and among females DNA analysis: females of pride are allways close relatives, males and females are NOT related males in large coalitions are related male repr. success increasingly skewed with increasing coalition size → relatives act as helpers . Packer et al. 1991. Sperm competition Chimpanzees • Food is densely distributed and plentiful • No need for territoriality for females • Multi-male polygyny polyandrogyny, promiscuity, sperm competition Sperm competition However, there are 3 male mating strategies • opportunistic mating, when females mate consecutively with many males with little overt competition between males • possessive mating, when a dominant male actively attempts to prevent other males from copulating with a female • consorts, when a female and male travel apart from the group for hours or days Sperm competition Male reproductive success is related to rank (priority-of access model) because dominant males copulate with females in periovulatory period Influence of female strategy: females may follow a mixed reproductive strategy, being selective when conception is likely and more promiscuous when conception is unlikely. Stumpf and Boesch 2004. Females prefer dominant males and males who shared meat with them Bluethroat (Luscinia svecica) Immun response of offspring from Extra Pair Fertilisation is superior Sperm competition Because of Extra Pair Copulation sperm competition occurs in all mating systems. Males use various techniques to ensure paternity And to counteract those techniques Females may be able to choose among sperms (spotted hyena) Copulation in the black-winged damselfly enables the male to remove a rival’s sperm before transferring his own by his brush like penis Another female counterdefence Dunnocks (heggemus): females eject sperm of low-status males Davies Nature 1983 In the news (2011) Scientists have discovered that female chickens have a remarkable ability to choose the father of their eggs. Working with feral fowl in Sweden, the scientists found that many matings were forced, as the roosters are twice the size of the hens. Even when unforced, the females still exercised their right to choose by opting to eject the sperm of males they considered to be at the bottom of the pecking order. With the reproductive odds stacked against them, these low-status roosters have fought back by developing larger ejaculates in the hope of increasing their chances of passing on their genes. . Degree of sexual dimorphism Forms of polygyny: •Female defense (red deer harem) •Lek polygyny (fallow deer) •Resource defense (roe deer) Degree of polygyny? Can you guess? Pheasant Partridge Degree of sexual dimorphism Degree of polygyny Variance in male reproduction rate (number of offspring) High variance – high competition. Bigger males (larger antlers) win. Antler size depends on body size (within and among species) Degree of sexual dimorphism Body size: sexual dimorphism Degree of sexual dimorphism correlates with party size in deer species Polygyny threshold model Some females choose to mate with already mated males who will not help them feed their chicks even though unmated males with territories are available. Why would a female do this? Polygyny threshold model: predicts that female will accept role of 2nd mate (polygyny) when superior resources on males territory mean that female would do better there than as 1st mate on a poor territory. Polygyny threshold model Polygyny threshold model: pied flycatcher Each female mated to a polygynous male has lower reproductive success than a monogamous female. However, males' r.s. is higher than that of a monogamous male. Male Pied Flycatchers clearly try to deceive females into polygyny. Not clear yet if females really fooled or have no better alternative. male territory quality ranked according to shade Polygyny Threshold e.g. Lark Bunting choose on the basis of M territory Fitness benefits 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 1 1 1 1 1 1 1 1 2 2 2 3 3 2 2 2 2 2 1 1 1 1 1 1 1 1 1 1 1 1 2 3 4 5 6 7 8 day of female arrival Conflicts Parents are not related genetically Offspring is related to itself by r = 1, to parent by r = ½ Offspring demand more than what parents wish to provide Full siblings are related to each other by r = 1/2 Parent-offspring conflict (POC) Parents pass their genes to next generation by offspring. Is survival of offspring equally beneficial for for parents and offspring? Not. Offspring carry half of the parent genes. R (offspring – offspring) = 1 R (parent – offspring) = 1/2 Robert Trivers: parent-offspring conflict over the amount of food and duration provisioned to young. At some point, a parent will prefer to reserve investment for future offspring rather than investing in the current one, while the current offspring will disagree. Parent-offspring conflict Benefit: to current offspring Cost: reduction in future offspring. The period of weaning conflict ends when both offspring and parent agree that future investment by the parent would be better directed at future offspring rather than to the current offspring. For full siblings, this is when the benefit to cost ratio drops below ½. Parent-offspring conflict For half siblings when B/C ratio drops to 1/4, because the current offspring is less closely related to future offspring. Conflicts Parent-offspring conflict Prediction: in the conflict zone mothers tend to reduce provisioning offspring tend to demand more care Offspring tend to manipulate parents (begging) Should be an honest signal (energy demand) Conflict among siblings Prediction: less competition among full siblings Parental favoritism and siblicide In many cases parents actively discriminate against certain offspring and either allow them to starve or allow their siblings to kill them. In egrets, boobies, pelicans and other birds older siblings attack and drive younger offspring out of the nest where they starve to death. Young great egrets fight while their parent ignores the behavior. At a Brown Booby nest the older chick (under its parent) has driven its smaller sibling from The nest where it will die of exposure and starvation. Parental favoritism and siblicide Parents assist siblicide In Black Eagles incubation begins as soon as the first egg is laid. The first egg hatches 3-7 days before the second and so the older offspring has a huge size advantage over its younger sibling and can easily kill him. Parental favoritism and siblicide In many birds the female spike the earlier laid eggs with high doses of androgens causing high aggressivity in older chicks. Possible functions: Insurance against failure Environmental uncertainty
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