Parental Investment Flashcards
Background of parental investment
Can we use the fossil record to better our
understanding of the origin and diversity of parental care?
The fossil record has been underutilized in studying the evolution of social behavior. Fossils from a number of locales were examined for evidence of brood care.
Brood care was found in a number of different species dating from 450–500 million years ago. At least three different brood care strategies were uncovered. The fossil record can be a useful depository of information relating to the evolution of social behavior.
Definition:
“Any investment by the parent in an individual offspring that increases the offspring’s chance of surviving (and hence reproductive success) at the expense of the parent’s ability to invest in other offspring.”
Trivers (1972)
Parental care:
Behaviour by one or both (“bi-parental care”) parents that contributes to the growth, development and survival of offspring.
Most common examples of parental care:
- Guarding
- Food provision
Parental investment can happen at any stage of development such as the:
Prenatal stage - for example birds guarding their eggs, incubating the eggs
postnatal stage - in bears such as food provision or protection of offspring
Types of prenatal care:
parental care involves being present or absent, if there is presence of parental care this can be witnessed by either the male provider, female provider or both. If both are involved then the next stage can be followed whether the female contributes more, female and male contribute equally or the male contributes more.
Prevalence of parental care. Different strategies in different taxons:
- invertebrates (Taxon) - parental care; uncommon, if occurs, usually female
- fish (Taxon) - parental care; absent in most species, male only>bi-parental>female-only (proportions-9:3:1)
- Amphibians (Taxon) - parental care; male-only and female-only equally common. Bi-parental rare
- Reptiles (Taxon) - parental care; female only or both sexes
- birds (Taxon) - parental care; 90% bi-parental, male-only very rare
- Mammals (Taxon) - parental care; 95% female only, 5% bi-parental, no male-only care.
Quality vs. Quality Principle: An inverse (negative) relationship between the number of offspring you can produce and the amount of parental care you can provide to the offspring. You can either invest in the quality or quantity of offspring due to parents having limited resources.
Determinants of different parental strategies
Why are parental strategies different in regards to who is involved,
i.e. male, only, female only, or both.
PHYSIOLOGICAL AND LIFE HISTORY CONSTRAINTS
E.g. Fertilisation type: Internal vs. external
Internal fertilisation:
1) predisposes males to desert
2) causes paternity uncertainty (for males)
Hence, male-only care rare
However sometimes parental care is simply required due to:
1) Costs of parental care/opportunities for male to contribute can result in bi-parental care
- E.g. in species where reproductive success is substantially increased by additional food provision –e.g. birds.
2) Ecological conditions and mating opportunities
- E.g. In many fish species male-only care common when brood protection overlaps with territory protection and associated mating opportunities.
COSTS OF PARENTAL INVESTMENT
Parental investment = conflicts
•Conflict between offspring
Intra-sibling conflict (i.e. with existing siblings)
- Its about genetic relatedness or how much of that relatedness we have between siblings. Offsprings related to them serves more than to any other offspring (due to sharing 100% genes with themselves but only 50% with siblings). Siblicide is an extreme example of intra-sibling conflict. Facultative siblicide which takes places mainly when the mother cannot provide enough resources for both offspring. In Obligate siblicide the killing always takes place, usually younger siblings are abused by older one and dies from starvation. The lower the relatedness between siblings within the brood, the higher the competition,e.g.:
Begging displays (indirect competition)
Ornamentation (indirect competition)
Inter-sibling conflict (i.e. with future siblings)
- offspring is more related to themselves than to future offspring, but the parents is related to each equally so conflict is inevitable. the offspring will demand more than the parents prepared to give. this brings the concept of parent offspring conflict
•Conflict between parents
Conflict over care (usually considered as a type of postzygotic sexual conflict) is common, because the evolutionary interests of male and female parents are rarely identical.
•Conflict between parents and offspring
Inclusive fitness theory predicts that parents should go to great lengths to help their offspring because parents and offspring have an average r of 0.5. Yet there are limits to this aid, as first conceptualized by Robert
Trivers in his parent-offspring conflict theory (Trivers, 1974).
Parent-offspring conflict arises with respect to a parent’s decisions about how much aid to give to any particular offspring. From the perspective of the parent, these decisions are affected both by how much energy is available for helping current offspring, and by how many offspring it is likely to have in the future
The conflict between parent and offspring
arises because, although each offspring will value the resources it receives more than those dispensed to its siblings, all offspring are equally valuable to a parent, in terms of the parent’s own inclusive fitness. This then sets up a zone of conflict between how much offspring want, and how much a parent is willing to give (the former always being greater than the latter). This zone is where parent-offspring conflict takes place
ECOLOGICAL CONDITIONS AND MATING OPPORTUNITIES
Parent offspring conflict - who to feed? since parents have limited resources, they often need to decide which offspring they should invest into. this decision is often based in the basis of the behaviour or signalling displayed by the offspring such as begging signals. •Chick begging (what it means) –two hypotheses Signalling of: a)Need to be fed (hunger) or b) Strength (reproductive value) Do parents feed hungry or strong chicks?
Both of the two hypothesis are correct, but under different conditions.
•In good conditions begging signals need (hunger) because they can feed all the chicks
•In poor conditions begging signals physical quality because they cant feed all the chicks
The honesty of begging can also be influenced by the number by the presence or number of siblings in the nest:
•Honest signalling of need occurs in species where there is no sibling rivalry for food.
•Honest signalling of body condition occurs in species with sibling rivalry for food (especially when food is scarce).
(Caro et al., 2015)
Parent –offspring conflict: care for how long?
Weaning: when parent-offspring conflict escalates; the infant is demanding more than the mother is prepared to give (seen in chimpanzees and other primates).
It benefits the infant, but not the parent, to prolong its dependency from the parent beyond the crucial dependency period.
Parental investment in relation to offspring sex
•Prenatal sex-biased investment
Refers to manipulation of birth sex ratio –evidence mixed
•Postnatal sex-biased investment Predictive factors: -Which sex costs more to grow -Which sex has higher value in terms of RS (reproductive success) -Which sex is at greater risk
this relates to the Trivers & Willard hypothesis:
We should expect sex-biased investment when:
–one sex has greater variance in Lifetime Reproductive Success (LRS) than the other,
–parents (usually the mother) vary in physical condition (or resource base)
Also to consider
-Male is the more costly (to grow) and fragile (e.g. compromised immune system) sex
Prenatal biased investment in relation to offspring sex at birth –evidence from mammals
•Low condition, late season breeders in coypus reabsorb male foetuses producing female-biased litters.
•High condition red deer hinds give birth to sons earlier in the birth season, low condition hinds give birth to females late in the season.
•High status female macaques give birth to daughters earlier in birth season.
Manipulating Inter-Birth Intervals (delay to next conception)
•Longer after sons in most species (i.e. females invest longer into male infants)
•Sometimes mediated by mother’s social status (IBI after sons longer for high-ranking females, e.g. chimpanzees)
•Sons are more likely to die –more costly in many species, especially when mothers are in poor condition
Human reproductive strategies
•Family planning = reproductive decisions aimed at enhancing fitness.
•PI strategies= enhance fitness once actually parents (e.g. investing more in one sex).
•Complex human societies = strategies to enhance lineage survival more important than maximising number of surviving offspring.
•Infanticide
–Paternity uncertainty
–Poor offspring quality
–Lack of parental resources
Can mothers manage alone? The importance of allomothering.
Allomothering–care beyond biological parents
•Humans: Grandmothers contribute 10% extra food to households (Hadza) increasing infant growth rates, lowering costs to mothers and contributing to infant survival. Grandmothers contribute to family income (L.A.), especially maternal grandmothers.
•Vervetmonkeys: Infants are more likely to survive when there is a grandmother present in their group.(protection of mother & infant from harassment)
•Elephants: Infants more likely to survive when grandmothers are present in their family unit. (transmission of knowledge across generations, protection from predators, some suckling of babies) Lee (1987).
Investing into females is less of a gamble.
Infanticide in humans –a special case of parental decisions
•Widespread
•But not normative (not accepted)
•Perplexing behaviour considering robust psychological mechanisms ensuring strong mother-infant bonds
•At the same time, psychological mechanisms in place facilitating infanticide:
e.g. newborn not human until gives first breath or first cry, or until named
•Last resort strategy, typically abandoning
•Abandoning rare in Africa (infants fostered to extended kin to reduce investment).
•Hunter-gatherers in South America (abandoning rate –12-38%): fostering not feasible since everyone faces similar demands to parents.
Parental investment and paternal uncertainty:
•Males expected to invest only in offspring with high Paternal Certainty
•There are between-sex asymmetries in parental certainty
•Males demand assurances
–brides = virgins at marriage
–claustration of females, “modest” dress restrictions; foot-binding; chaperones (Dickemann1979)
•Honour & shame systems – to ensure chastity of daughters (better political future for family)
•Perceptual reassurance (e.g. Maternity ward comments e.g. “Ditto father”-Daly & Wilson 1982)
Children living with step parents are more likely to suffer fatal abuse
Parental investment and poor infant quality:
•Severe physical deformity, especially in societies that lack institutional care, predicts infanticide (Daly & Wilson 1984; 1988)
Low evolutionary pay-off; unlikely to reproduce.
•In modern societies, congenitally handicapped children more likely victims of abuse
Parental investment and lack of parental resources:
•Even healthy children, under unfavourable environmental conditions, can be victims of infanticide (save energy for future)
•Krummhorn (19th century): primiparous widows tended to terminate investment if resources disappear and to improve “attractiveness” as marriage partner (Voland1988)
•Western societies: termination of pregnancies by young single women (Lycett& Dunbar 1999)
Manipulating sex ratio in humans:
Selective infanticide/child neglect in humans is influenced by social status and marriage prospects of offspring
•In the historic Rajputs caste (India), the high status castes performed female-biased infanticide since marrying down the hierarchy was not acceptable for women (in a highly hypergenic society) (Dickemann 1979)
•Among the relatively poor pastoralist tribe of Mukogudo (Kenya)
male infants were more likely than female infants to become a victim of parental neglect, likely because women had better marriage prospects than man (Cronk 1989)
