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For more information, see: www.auckland.ac.nz/evaluate Reproduction, Birth, Alloparenting, and Life History Evolution
ANTHRO 201 2018 Week 11 Lecture 11 Week 11: Lecture 11: Alloparenting and Life History
• A review of life history theory and human life history characteristics • A review of changes in relevant aspects of socio- ecology • How are these plausibly related to the evolution of alloparenting among human? Review…
• What is life history theory? Can you give a concise definition? • How are life history characteristics linked to aspects of socio-ecology? • Related important question: – How have relationships changed during the course of hominin evolution? Definition of Life History Theory
• How do members of a species allocate energy through life to accomplish: • survival to and through their reproductive period • growth and development • maintenance of organ systems
• reproduction, and offspring care (if any) • Tasks clearly interlinked and involve trade-offs Common Life History Life History links to Parameters Socio-ecology • Metabolic requirements • Niches • Gestation length • Sizes of home range • Litter size • Anatomy: body size, brain size, • Speed of growth and locomotion, dentition, gut development morphology and patterns of growth • Age at maturity / age at first birth • Diets (quality and quantity) • Inter-birth interval • Social group size and composition • Duration of reproductive period • Intra- and inter-group social • Age mortality profiles etc. relationships etc. Review: life history parameters characterizing humans? • long gestation relative to most mammals, but… • usually born singly and very dependent (secondarily altricial) • weaned relatively early • an extended period of dependency on kin • relatively late puberty and even later first reproduction • relatively short inter-birth intervals & relatively high infant and child survival rates • long life spans that extend well beyond reproductive period in women, and most men Further consideration of evolutionary processes that may have contributed to these characteristics • What do we expect of the last common ancestor of chimps and humans in terms of socio-ecology? – What is a useful way of thinking about these sorts of questions? • Hormonal influences on reproduction? – Hypotheses about loss of estrus in our lineage • Obstetrical dilemma? – Birthing in apes, australopithecines and us • Alloparenting – What is it and what advantages does it provide? Comparison of Behavioural Characteristics
Trait Pan troglodytes Pan paniscus Homo sapiens Last Common Ancestor? Social group Mm-mf Mm-mf Mm-mf Mm-mf communities, communities, communities, communities, fusion fission fusion fission fusion fission fusion fission
Emigration Females Females Females (more Female? often) FF bonds Weak Strong Weak – Moderate ?
MM bonds Strong Weak Strong ?
Territorial Common Infrequent Variable: Context ? defense Specific
Comparison of Behavioural Characteristics (continued)
Trait Pan troglodytes Pan paniscus Homo sapiens Last Common Ancestor? Paternal Slight Slight Strong Slight? investment Tool use Frequent Rare Very frequent Probable Meat eating Yes Yes Yes Yes
Bipedalism Rare Rare-Occasional Habitual Rare-Occasional?
Food sharing Routine Routine Frequent Routine
Something not on list just considered
• Did LCA females exhibit oestrus? • Oestrus = hormone-regulated, cyclical period of sexual receptivity and fertility in females – Associated with changes in genitalia around time of ovulation (e.g. sexual swellings, colour changes) • Oestrus found in genus Pan • Oestrus missing in H. sapiens sapiens • Ovulation in human females concealed Loss of estrus in humans: Hypotheses
1. Paternal investment 2. Reduced infanticide 3. Cuckoldry 4. Reduced aggression / more stable social relationships 5. To counter conception avoidance by female hominins 6. Side effect of bipedalism, or by-product of other evolutionary changes 1. Greater paternal investment • Reduced or conceal oestrus hypothesised to result in males more frequent mating with same female throughout cycle • This increases likelihood of pair-bonding and willingness of male to invest in offspring • What would be advantage of this? 2. Reduced risk of infanticide 3. Increased chances of cuckoldry 1. Inability to reliably determine (non)paternity 2. Females can conceive with males other than their usual mate without his knowledge 3. Reduced risks of infanticide obvious advantage, but cuckoldry? 4. Reduced aggression and formation of more stable social bonds Reduce male-male aggression related to competition for females -> more stable social relations within group 5. To counter conception avoidance • Evolution of brain and intelligence -> female hominins become intelligent enough to consciously avoid pregnancy • Ovulation hidden from males and from females themselves 6. Side effect, or by-product • Of bipedalism, or other evolutionary changes Bipedal Species Giving Birth • Washburn (1960) proposed that the human lineage faced an “obstetrical dilemma” – Hypothesised trade-offs between bipedal locomotion, obstetrics, brain growth, gestation length and altriciality Important Questions • How much of an impact did changes in the shape of the innominate associated with facultative and then obligate bipedalism have on: – Gestation length and status of newborns? – The manner and difficulty of parturition (giving birth) – Whether or not assistance is required during birth?
Average Gestation Lengths & Status at Birth
• Humans: 39 weeks • Chimps: 34 weeks • Gorillas: 37 weeks • But human infants more helpless (less developed; altricial) than great ape infants – Technically we are secondarily altricial as primates as an order are precocial • Human neonates more like foetuses; extero-gestation period (Montagu 1989); secondary altriciality Birth, obstetrics and human evolution
BJOG: An International Journal of Obstetrics & Gynaecology Volume 109, Issue 11, pages 1199-1206, 22 DEC 2003 DOI: 10.1046/j.1471-0528.2002.00010.x http://onlinelibrary.wiley.com/doi/10.1046/j.1471-0528.2002.00010.x/full#f1 Gestation and the Brain
• Has selection acted to constrain gestation length and neurological maturity in humans at birth because of: – counter-vailing selection pressures on hip morphology to maintain efficient bipedal gait? – or because energetic demands on mothers are constrained?
Dunsworth et al. PNAS 2012;109:15212-15216 Variation in maternal energetic investments among hominids.
Dunsworth H M et al. PNAS 2012;109:15212-15216
©2012 by National Academy of Sciences There is more! • Human birthing is argued to differ from birthing in great apes • Human neonates tend to rotate their head and shoulders as they pass through the inlet, midplane and outlet of the birth canal. • They tend to exit the canal facing toward the sacrum, away from the mother (occiput anterior postion) • Argued to increase need for assisted birthing • Is rotation or assisted birthing unique among humans?
Hirata et al. (2011) Mechanism of birth in chimpanzees: humans are not unique among primates Biology Letters 7(5):686-688. VIDEOS: Apes giving birth • Show three videos of female apes giving birth • The first is of a Sumatran orang-utan (Pongo abelii). The second is of a lowland gorilla (Gorilla gorilla). The third is of a chimpanzee • Pay attention to the following: – The position of the mom as she births – The position of the infant during the birthing process – The behaviour of the mother apes during and following the birth What about birthing in australopithecines?
• Assumed to birth easily like apes, but was this true?
Desilva et al. (2017) Neonatal shoulder width suggests a semirotational, oblique birth mechanism in Australopithecus afarensis. THE ANATOMICAL RECORD 300:890–899 Evidence suggests need for semi-rotational oblique birth in A. afarensis
• First of two reconstructions of AL 288-1 pelvis Evidence suggests need for semi-rotational oblique birth in A. afarensis
• Second of two reconstructions of AL 288-1 pelvis Chimp, Australopithecus and Human • Influences of bipedalism – Shift of sacrum relative to acetabulum – Relatively narrower anterior-posterior measurement – A. afarensis particularly wide but shallow pelvic dimensions – Birthing probably not so easy for A. afarensis – Are their implications for social behaviours? Black-and-white snub-nosed monkeys Assistance during birth
• Birth in nonhuman primates generally a solitary activity • However: Turner et al (2009) argue social proximity at parturition is more common in nonhuman primates than previously emphasized • Some cases of assistance (‘midwives’) observed – e.g. in wild snub-nosed monkeys (Ding et al. 2013) – Groups of related females = inclusive fitness, kin selection; strong social bonds
Ding et al. (2009) http://www.sciencedirect.com/science/article/pii/S0376635713000077 White-headed langurs Anthropologist Sarah Hrdy Alloparenting Evolution of alloparenting
• Cooperative breeding = reproductive strategy in which alloparents care for and provision young • Sarah Hrdy: evolution of cooperative breeding preceded large brains in hominins Human vs chimpanzee • Set stage for evolution of large brains brain; made them possible Alloparenting in nonhuman primates
Angolan colobus
Francois’s langur
Hanuman langur Allo-nursing in mammals Evolutionary benefits of alloparenting (Hrdy) • Mothers conserve energy, stay safer and healthier lead longer lives w/ greater reproductive success • Mothers able to reproduce again sooner • Additional carers = better chance of young surviving
Ultimately greater species advantage – faster pace of reproduction and flexibility for young to survive in range of habitats Importance of child-rearing
“So much of the literature on human evolution stresses hunting and intergroup fighting and mate choice and competition for mates that we tend to forget that unless the offspring conceived survive, all these activities are just so much sound and fury signifying nothing.” - Sarah Hrdy http://www.pbs.org/wgbh/nova/evolution/evolution-motherhood.html Alloparenting and cognition Non-maternal care in hunter-gatherers: the Efe • “the most extreme example of alloparenting in a foraging population” (Ivey 2000:857-58) • Women other than the mother hold and breastfeed the infant first; responses to crying include non-maternal nursing • At 3 weeks old, non-mothers account for 39% of infants’ physical contact; 60% at 18 weeks. CHILDHOOD 4 2 Evolution of Slow Life History
. Changes in diet . Larger brains . Smaller guts . Long juvenile period . Increased longevity . Paternal investment (and reduced sexual dimorphism)
Extractive foraging, hunting and complex sociality require intelligence and learning Human reproductive success • Biocultural adaptations • Stages and growth, growth patterns facilitate parental investment • Culture as response to need to nurture, protect, and teach young