ANT 154BN-09 Infanticide

ANT 154BN-09 Infanticide - ANT 154B Lecture #9: Infanticide...

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Unformatted text preview: ANT 154B Lecture #9: Infanticide 1 Feb 2011 Tuesday, February 1, 2011 1 Midterm Exam* 8 Feb 2011, during class Review Session 4 Feb 2011, Friday, 4:10–6 pm Olson 261 *Limited notes allowed: one, double-sided, letter-sized piece of paper only Tuesday, February 1, 2011 2 Infanticide 1. Why kill infants? 2. Who kills (and who should kill) infants? 3. Counterstrategies 4. Concluding comments Tuesday, February 1, 2011 3 Infanticide >1. Why kill infants? 2. Who kills (and who should kill) infants? 3. Counterstrategies 4. Concluding comments Tuesday, February 1, 2011 4 Why kill: sexual selection hypothesis 1. Killer is new alpha male (coincident disappearance of former alpha) 2. New M is not related to infant; mates mother after killing infant. 3. F’s next birth: 10 months, cf. 2 years if infant lives 4. Hence M increases RS (though 75% FF also mate other MM) Tuesday, February 1, 2011 5 Figure 1. Parental investment in two infants during one inter-birth interval (IBIfull) with and without infanticide. Here we refer to IBIfull as the full expected inter-birth period following the birth of the first infant if that infant were to survive. The top timeline represents the situation where infanticide does not occur, while the bottom timeline includes an infanticide (marked by the ‘X’). Without infanticide, at the end of the IBIfull, there is one weaned infant and a second, newborn infant. With infanticide, the first infant is dead leaving only the second infant which is of age IBIfull minus IBIReduction, where IBIReduction equals the sum of ‘Age Of First Infant At Infanticide’, ‘Waiting Time To Conception’ and ‘Gestation Time’. The grey area represents this reduction in IBIfull (‘IBI Reduction’) before because of the infanticide. IBIReduction/IBIfull is directly proportional to a male’s genetic payoff for committing infanticide. Gestation periods are the same in the two scenarios, while the waiting time to conception may differ between scenarios. Logic of the sexual selection hypothesis Tuesday, animals with seasonal reproduction; however, the sexual selection hypothesis covered by this model seems well-supported in many taxa (van Schaik 2000b; Ebensperger & Blumstein 2007). In the absence of infanticide, after one inter-birth interval (IBI), a male obtains a genetic ‘payoff’ proportional to the expected proportion of his genes in the offspring born at the start February 1, 2011 of that IBI. Hereafter, we use the term IBIfull to refer to the potential full IBI the female would experience if her offspring relatedness later), so his estimated genetic payoff would be his estimation of his own paternity probability, P(p0), weighted by the payoff for having an offspring, in this case 1 (see Table 1 for a definition of terms). Thus, at the end of one IBIfull, in the absence of infanticide, the male’s estimated payoff is merely his (presumably subconscious) estimation of his paternity probability in the & Boyko first offspring (plus some discounted value based on his paternity probability of the newborn second offspring, but as gestation of Marshall 2009 6 Alternative hypotheses: adaptive Population regulation: when high density --> group benefits. * but reduced density can --> infanticide (langurs, blue monkeys) Form of sexual coercion/exploitation, results in female more likely to mate with male in more distant future (a form of sexual selection hypothesis) Resource competition Infants as food Tuesday, February 1, 2011 7 Alternative hypotheses: non adaptive Social pathology: crowding --> maladaptation. * but not in captivity (unless new male) * but occurs in low-density populations Sexual frustration / rage: anger --> kill. * but male may have mated often Biased observation * but # species, observations, replications grow Tuesday, February 1, 2011 8 Distribution of infanticide Tuesday, February 1, 2011 9 Key predictor: ratio of lactation to gestation van Schaik 2000 Tuesday, February 1, 2011 10 Key predictor: ratio of lactation to gestation I I Infanticide by males present No male infanticide lactation/ gestation I van Schaik 2000 Tuesday, February 1, 2011 11 Infanticide in humans Tuesday, February 1, 2011 12 Genetic fathers Stepfathers “But do non-father human males kill infants to hasten ovulation, the main benefit of infanticide by males in primates, or do men, perhaps unusually among primates, commit infanticide mainly to avoid investment in another male’s offspring?” Harcourt 2003 Tuesday, February 1, 2011 13 “Sex-biased infanticide might be a peculiarly human trait.” Harcourt 2003 Tuesday, February 1, 2011 14 Infanticide 1. Why kill infants? >2. Who kills (and who should kill) infants? 3. Counterstrategies 4. Concluding comments Tuesday, February 1, 2011 15 Female philopatry e.g., gelada baboons, redtails, langurs Inf Takeover Inf Tuesday, February 1, 2011 16 Female transfer e.g., gorillas, hamadryas Attack Inf Inf Tuesday, February 1, 2011 17 Two types of infanticide Female philopatry Female transfer Attack Inf Inf Takeover Inf Inf Tuesday, February 1, 2011 18 Infanticide variants 1. male display of power: e.g. gorilla, Thomas’ langur Sexual selection, competition for breeding opportunity 2. female display of power: e.g. chimpanzee Resource competition 3. male reduction of rival power: e.g. chimpanzee inter-community Resource competition? 4. male chimpanzee within communities generally considered to be puzzling (pathology? memory of failure to mate??) Tuesday, February 1, 2011 19 When should males commit infanticide? R.H. Boyko, A.J. Marshall / Animal Behaviour 77 (2009) 1397–1407 Tuesday, February 1, 2011 (upper left area). In this area, infanticide would never be adaptive. Below the ideal line, infanticide would be adaptive if the current infant and next infant required equal infanticide (e.g. Hrdy 1979 males should always seek t Little attention has been p ‘Ideal line’ that promiscuous mating m Do not commit Probability 0.8 that half the genes in e Safe zone confusion come from a m of Never infanticide commit infanticide have noted that some mal offspring via their physica 0.6 having (even when this causes them e.g. Harcourt & Stewart 20 0 sired With gestation from the protective effects 0.4 been neglected in the lite current effects on males’ mating eff Paternity disagreement offspring Commit infanticide is a threat in t 0.2 against it (references in Ag With MM males in these species ma infanticide relatedness confusion. Danger zone Ignoring inclusive fitnes 0 benefit from a single offspri 0.6 0.8 1 0 0.2 0.4 bility of his paternity in that o Probability of siring next offspring that the offspring dies before Probability of siring next offspring focal male’s paternity pro Figure 2. The conditions under which a male would increase his fitness by committing fathered infant j), we find the infanticide under the sexual selection hypothesis. The ‘ideal line’ represents the case Valj, by multiplying pj by the where a male’s estimated paternity in one infant (the ‘current infant’) is equal to his expected paternity in the next infant that a female would conceive if the male were to which is the joint probabilit kill the current infant. Above this line, the male’s probability of siring the female’s next does not die from extrinisic offspring, E(pi), is lower than the probability of his having sired her current infant, p0 Boyko & Marshall 2009 equation, given the terms de 1 Probability of having sired current offspring 20 N Y h When should males commit infanticide? R.H. Boyko, A.J. Marshall / Animal Behaviour 77 (2009) 1397–1407 Tuesday, February 1, 2011 (upper left area). In this area, infanticide would never be adaptive. Below the ideal line, infanticide would be adaptive if the current infant and next infant required equal infanticide (e.g. Hrdy 1979 males should always seek t Little attention has been p ‘Ideal line’ that promiscuous mating m Do not commit Probability 0.8 that half the genes in e Safe zone confusion come from a m of Never infanticide commit infanticide have noted that some mal withfspring via their physica of gestation 0.6 having (even when this causes them e.g. Harcourt & Stewart 20 0 sired With gestation from the protective effects 0.4 been neglected in the lite current effects on males’ mating eff Paternity disagreement offspring infanticide is a threat in t Commit 0.2 against it (references in Ag With MM males in these species ma relatedness infanticide confusion. Danger zone Ignoring inclusive fitnes 0 benefit from a single offspri 0.6 0.8 1 0 0.2 0.4 bility of his paternity in that o Probability of siring next offspring that the offspring dies before Probability of siring next offspring focal male’s paternity pro Figure 2. The conditions under which a male would increase his fitness by committing fathered infant j), we find the infanticide under the sexual selection hypothesis. The ‘ideal line’ represents the case Valj, by multiplying pj by the where a male’s estimated paternity in one infant (the ‘current infant’) is equal to his expected paternity in the next infant that a female would conceive if the male were to which is the joint probabilit kill the current infant. Above this line, the male’s probability of siring the female’s next does not die from extrinisic offspring, E(pi), is lower than the probability of his having sired her current infant, p0 Boyko & Marshall 2009 equation, given the terms de 1 Probability of having sired current offspring 21 N Y h When should males commit infanticide? R.H. Boyko, A.J. Marshall / Animal Behaviour 77 (2009) 1397–1407 Tuesday, February 1, 2011 (upper left area). In this area, infanticide would never be adaptive. Below the ideal line, infanticide would be adaptive if the current infant and next infant required equal infanticide (e.g. Hrdy 1979 males should always seek t Little attention has been p ‘Ideal line’ that promiscuous mating m Safe zone Probability 0.8 that half the genes in e Safe zone confusion come from a m of (never commit Never commit infanticide have noted that some mal offspring via their physica 0.6 having (even when this causes them infanticide) with g. Harcourt & Stewart 20 0 e. gestation sired With gestation from the protective effects 0.4 been neglected in the lite current effects on males’ mating eff Paternity disagreement offspring with tmale is a threat in t infan icide 0.2 against it (ref relatedness erences in Ag With MM males in these species ma relatedness confusion. Danger zone Danger zone Ignoring inclusive fitnes 0 benefit from a single offspri 0.6 0.8 1 0 0.2 0.4 bility of his paternity in that o Probability of siring next offspring that the offspring dies before Probability of siring next offspring focal male’s paternity pro Figure 2. The conditions under which a male would increase his fitness by committing fathered infant j), we find the infanticide under the sexual selection hypothesis. The ‘ideal line’ represents the case Valj, by multiplying pj by the where a male’s estimated paternity in one infant (the ‘current infant’) is equal to his expected paternity in the next infant that a female would conceive if the male were to which is the joint probabilit kill the current infant. Above this line, the male’s probability of siring the female’s next does not die from extrinisic offspring, E(pi), is lower than the probability of his having sired her current infant, p0 Boyko & Marshall 2009 equation, given the terms de 1 Probability of having sired current offspring 22 N Y h Infanticide 1. Why kill infants? 2. Who kills (and who should kill) infants? >3. Counterstrategies 4. Concluding comments Tuesday, February 1, 2011 23 Female counter-strategies: langurs • try to hide • try to counter-attack, prevent male take-over • females with infants copulate (‘pseudo-estrus’) • females do not try to leave (more than 1-2 days) - repelled by other groups perhaps dispersal --> infanticide in new group? Tuesday, February 1, 2011 24 Multiple-mating and the associated long estrus of females = anti-infanticide strategy via paternity confusion? Tuesday, February 1, 2011 25 Chimpanzee swelling cycle 100 % max 0 swelling 0 5 10 15 20 25 Ovulation (average) 30 35 day 40 Menstruation Graham (1970) Tuesday, February 1, 2011 26 Sexual swellings in primates • signal proceptive and receptive behavior • in many OWM and ape species highly exaggerated in size e.g., substantial increase in FF body weight (14% – 25%) • costly increased BW increases locomotor costs fluids diverted from other bodily functions attract parasites (e.g., biting flies, mosquitos) sustain injuries (e.g., cuts, tears) • evolved at least three times independently ... assumed to be adaptive, to serve a “function” Nunn 1999 Tuesday, February 1, 2011 27 Tuesday, February 1, 2011 28 Sexual swellings in primates: correlates • associated with non-seasonal breeding losses in lineages that make transition from nonseasonal -> seasonal • associated with multimale mating systems average 4.3 males/group spp. w/ swellings, 1.8 males/group w/o 71% of multi-M OWM and apes have swellings, 0% of single-M Nunn 1999 Tuesday, February 1, 2011 29 Sexual swellings in primates • longer duration in peak sexual activity spp. w/ swellings mean 10.6 days, spp. w/o mean 4.6 days allows females to mate with more males? Nunn 1999 Tuesday, February 1, 2011 30 Paternity confusion: benefits for females “Hrdy’s (1981) paternity confusion hypothesis applies to situations in which adult males pose a threat to infants unlikely to be their own offspring. … in these situations a female may therefore benefit from nonprocreative matings that induce her copulatory partners to exhibit benevolence or neutrality toward her subsequent offspring” (Manson & Perry 1997). Tuesday, February 1, 2011 31 • protect infants likely to be your own (fighting, vigilance) only works if protection perfect (rare) the male, Valj, by robability the infant is e joint probability that does not die from . This yields the ven the terms defined in • paternity confusion? male strategies evolved in the context of multiple males playing the “game” ) is essentially the ality (3a) will be true for male when the infant is infanticide models instead. In populations , P(INFANTICIDALkj) would ny immigrant. ion of the number of d the infant, the amount close proximity to the of time the infanticidal t and the how successful emales can be. As ends on pkj (the hered infant j) and game, pj may be elow one. While it is groups males could be e sum of their perceived han one, natural select against this, hree-way evolutionary e alpha male, lowerfemale. This tested with explicit game j k Probability infant killed P(INFANTICIDALkj) is essentially the probability that inequality (3a) will be true for another within-group male when the infant is born, though alternate infanticide models could be incorporated instead. In populations with male immigrants, P(INFANTICIDALkj) would presumably be 1 for any immigrant. P(SUCCESSkj) is a function of the number of males willing to defend the infant, the amount of time they spend in close proximity to the infant, the Boyko & Marshall page the infanticidal proportion of time 8 male is with the infant and the how successful j probability (p )defending males and females can be. As under the threat of infanticide without adequate protection. A male’s reproductive success p j (the P(INFANTICIDALkj) depends on k increases nearly linearly withmale k fathered infant j) and probability increasing paternity probability at low paternityis a zero-sum game, pj may be paternity probability but then sharply decreases as other males become optimized at a value below one. While it is infanticidal. As in (a), the shaded zone represents possible that in some and the the area of positive net reproductive valuegroups males could be confused such that the sum of arrow indicates the point at which the reproductive their perceived paternities is greater than one, natural value is maximized. selection likely would select against this, possibly leading to a three-way evolutionary arm’s race between the alpha male, lowerranking males and the female. This supposition could be tested with explicit game theoretic models. The implications of equation (5) are presented in Fig. 3. The value at birth of each infant increases linearly with the paternity probability while the probability another male RS commits infanticide (assuming protector Male anti-infanticide strategies by another male Figure 3. a. A male’s expected genetic representation in an offspring (‘reproductive value’) increases linearly with a male’s paternity probability 0 (p ; dashed line) but this increase can be negated by other males’ infanticidal tendencies, which, aggregated, increase in a logistic fashion (solid line). RS The optimal paternity probability is the point that j maximizes Val , the expected genetic representation in the infant times the probability the infant is not killed (represented by the arrow). The shaded area represents the area of positive net ‘reproductive value’, or the expected mean proportion of his genes in infants surviving to adulthood. b. The relationship j between reproductive success (Val ), in terms Paternity probabilityrepresentation in of maximizing a male’s genetic offspring surviving to adulthood, and paternity Paternity probability 32 s of equation (5) are Tuesday, February e value at birth of each ly with the paternity 1, 2011 Infanticide 1. Why kill infants? 2. Who kills (and who should kill) infants? 3. Counterstrategies >4. Concluding comments Tuesday, February 1, 2011 33 Does infanticide limit group size? ‘‘When infanticide rates increase with the number of reproductive females, females may opt for dispersal, thus keeping total group size small. We propose that indirect evidence previously suggesting the role of food competition in limiting group size might actually reflect infanticide.’’ Crockett & Janson 2000 Tuesday, February 1, 2011 34 Does infanticide limit group size? Assumes: food is not limiting in these species some females stay in large groups, despite increased infanticide risk (due to some other selective force, e.g., predation?) Crockett & Janson 2000 Tuesday, February 1, 2011 35 Are all attacks attempted infanticides? Even in “infanticidal species”, most infants survive takeovers: e.g., red howlers (68%) mantled howlers (60%) hanuman langurs (63%, Jodhpur; 78%, Ramnagar) Proof that counter strategies work? why don’t they work better? why not universal in species at risk? Males are not trying to kill infants? Bartlett 2003 Tuesday, February 1, 2011 36 Infanticide: concluding points 1. Infanticide has become a stock explanation when all else fails. cf. pair bonding in gibbons (van Schaik & Dunbar 1990) the folivore paradox (Steenbeek & van Schaik 2001) male-female relationships in some Lemurs (van Schaik & Kappeler 1997) orangutan social systems (van Schaik & Kappeler 1997, van Schaik 2004) Tuesday, February 1, 2011 37 Infanticide: concluding points 2. More data are needed. quantify fitness costs and benefits for males determine how presence of male protectors influences infanticide risk how effective are counterstrategies? are they best explained as antiinfanticide strategies? Overdorff & Parga 2007 Tuesday, February 1, 2011 38 Infanticide: concluding points 3. Need to separate infanticide risk from infanticide rate. Tuesday, February 1, 2011 39 Infanticide: concluding points 4. Infanticide is predicted to occur under a wider range of conditions than is generally assumed. e.g., after recent rank changes by potential fathers (because strategies are probabilistic, and paternity is confused). Boyko & Marshall 2009 Tuesday, February 1, 2011 40 Take home messages 1. Infanticide is widespread in mammals, and is generally (but not universally) assumed to be adaptive. 2. The primary adaptive hypothesis is the sexual selection hypothesis; alternative adaptive and non-adaptive explanations have been hypothesized. 3. Female and male counterstrategies in the face of infanticide risk may not be as distinct as is generally assumed. 4. Much infanticide in human primates, while still potentially explicable from an adaptive perspective, may occur for different reasons than those assumed to explain infanticide in non-human primates. 5. Infanticide is predicted to occur under a wider range of conditions than has generally been assumed. Tuesday, February 1, 2011 41 Question to ponder Infanticide is quite rare in seasonal breeders (i.e., species that predictably reproduce at the same time each year). Using the logic of the sexual selection hypothesis, suggest an explanation for this pattern. Would you expect infanticide in seasonal breeders to be more or less likely if each female did not reliably produce an infant every single year, depending on her physical condition (compared to species in which every female reliably produced an infant every year). Why? Tuesday, February 1, 2011 42 ...
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