Predicting family dynamics in social

vertebrates鄭先祐 (Ayo)

國立台南大學 環境生態研究所

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Family dynamics in social vertebrates 10.1 The changing scope of cooperative breeding

research 10.2 Ecological constraints and the formation of

family groups 10.3 Kinship and the tendency to cooperate 10.4 Conflicts with changing family composition 10.5 Conflict over who reproduces 10.6 The myth of the stable family? 10.7 Toward a unified evolutionary social theory Appendix Evolutionary predictions of living within

family kin groups

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Family definition ( 家庭的定義 ) The definition of families is where offspring continue to interact, into adulthood, with their parents.

Simple family, only a single and female group members breed, while Extended families, two or more group members of one ore both sexes reproduce. The presence of a breeding male is not essential to the definition of a family. (biparental, matrilineal 母系的 ) Intact families, the original breeders are still the reproductive, and Replacement (or step-) families, a breeder has been replaced.

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10.1 The changing scope of cooperative breeding research 合作養育的行為 occur in roughly 3% of bird and mammal species. 倘若野外研究愈多，這個比率可以再加倍。 於 1978 年的問題，

1. Did ‘helpers’ really contribute significant assistance to the breeders that they attended?2. If they did, 這樣的行為是如何演化出來的？

於 1984 年的問題，1. The most common form of group found was that of grown offspring helping their parents to rear younger siblings.2. Why did offspring remain with their parents?

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第一個問題，可用 ecological constraints 解釋。 Offspring stayed home when opportunities for successful dispersal and independent breeding were ‘constrained’.

第二個問題， it turned out that helpers themselves benefited in several additive ways by their helping actions. They increased their own probability of becoming breeders in the future. They increased their inclusive fitness by helping to rear close genetic relatives.

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The early 1980s, reproductive skew theory In the cooperative societies genetic

conflicts of interest are inevitable. Competition will exist between group

members over who breeds and who does not.

A general theory of such conflict and its resolution, now termed reproductive skew theory.

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By 1991 The original paradox of cooperative breeding largely disappeared with the widespread confirmation that

Helpers frequently do improve their chances of becoming breeders by staying at home and helping temporarily They frequently do obtain large indirect genetic benefits by helping to rear collateral kin (Emlen, 1991)

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目前的研究 The vast majority of birds and mammals that

exhibit cooperative breeding do live in multigenerational family groups.

Families provide an excellent arena for developing and testing evolutionary social theory.

By focusing on families one seeks similarities, rather than differences, among cooperative species.

Focusing on the family structure of many animal societies highlights parallels with the early social organization of our own species.

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10.2 Ecological constraints and the formation of family groups In most organisms, young disperse from their area of birth well before or at least by the age of sexual maturity.

If offspring and parent come into contact later in life, they show no signs of recognition or preferential interaction with one another. 於 multigenerational families, offspring remain in association with their parent(s) beyond the age of sexual maturity and, commonly, throughout their lifetimes.

The key to understanding the evolution of families is understanding delayed dispersal.

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Why should a growing offspring postpone its dispersal? Most mature birds or mammals that

remain in their natal group do not reproduce; their breeding is suppressed by their more dominant parents. This is an automatic cost to saying home

Some researchers have emphasized the importance of the constraints on leaving; others the benefits of staying home. There are very few ‘vacancies’ for dispersing

individuals, or that the vacancies are of poor quality.

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Economic model of family formation1. Delayed breeding occurs when the production of

mature offspring exceeds the availability of acceptable opportunities for their independent reproduction

2. Under such circumstances, some offspring must postpone personal reproduction until acceptable breeding opportunities arise and they are able to successfully compete to obtain them

3. Families will form when such waiting is best done at home.

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Acorn woodpeckers, Melanerpes formicivorus Live in groups of two to 12 individuals and typically occupy permanent, year-round territories in the American Southwest. High-quality territories are in short supply, and most offspring become reproductives by weiting for an established breeder on an existing territory to die, and then successfully competing to fill the breeding vacancy. (Fig. 10.1)

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Fig. 10.1 (a) Ecological constraints and family formation in acorn woodpeckers (a) the proportion of yearlings that remain at home, plotted as a function of the severity of territorial shortages.

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Fig. 10.1 (b) the likelihood that yearlings stay at home, plotted as a function of the quality of their natal territories.

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Superb fairy wren( 鷦鷯 ), Malurus cyaneus Live in southeastern Australia in families

consisting predominantly of parents and grown sons.

Shortages of both territories and mats (females) have been suggested as possible constraints to independent breeding.

By removing breeding males from nearby territories they created breeding vacancies, Thirty-one of 33 mature sons left home to fill the newly created breeding vacancies.

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The Seychelles warbler, Acrocephalus sechellensis Is a formerly endangered species whose range is

restricted to a few small islands north of Madagascar.

In 1960, when the population consisted of just 26 individuals, habitat restoration programmes were implemented.

Over the following 30 years, the population grew impressively.

No family groups were reported until 1973, roughly the time at which all suitable breeding habitat became occupied (Fig. 10.2a)

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Fig. 10.2 Ecological constraints and family formation in Seychelles warblers (a) the number of individuals and occupied territories on Cousin island between 1959 and 1990.

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Fig. 10.2 (b) the likelihood that yearlings stay at home, plotted as as function of the quality of their natal territories.

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10.3 Kinship and the tendency to cooperate Families are fundamentally different form other forms of social groupings because they form by the retention of grown young with their parents. Families are comprised primarily of close genetic relatives.

Inclusive fitness theory Kin selection has long been hypothesized to be a selective factor favouring the evolution of cooperative breeding.

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合作養育 和 multigenerational family group There 112 species of birds and 63 species of mammals (excluding primates) reported to live in multigenerational family groups.

Of these, fully 96% of the birds and 90% of the mammals exhibit cooperative breeding. Fully 88% of the birds and 95% of the mammals that breed cooperatively live in multigenerational family groups (Emlen, 1995) 絕大多數，都是有明顯的 kin favouritism 已知只有一種例外， Maxican jay, Aphelocoma ultramarina, (Brown and Brown, 1990)

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White-fronted bee-eaters (birds) Live in extended family groups in which up to four

pairs may breed simultaneously. Helpers are non-breeding individuals that join one

of the active nests and aid in incubation as well as nestling and fledgling care.

Only 50% of the non-breeders become helpers; the rest sit out the season as non-participants.

Kinship proved to be a strong predictor (Fig. 10.3) They preferentially help the breeding pair to whom they

are most closely related (94% of 115 cases)

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Fig. 10.3 Data are presented as dyadic comparisons of nest choices plotted according to the helper’s relatedness to the recipient nestlings.

親緣相關係數

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Incest avoidance Incestuous matings between close kin have deleterious genetic consequences in most normally outbreed species. 因此於 Family group, 應該有 inbreeding avoidance mechanisms. Family dwelling species provide an excellent testing ground for incest avoidance predictions because mature offspring remain in close social contact with their parents and siblings throughout subsequent reproductive episodes. Incestuous matings within families are statistically rare.

但是 incest avoidance is not universal among vertebrates.

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10.4 conflicts with changing family composition The death, divorce or departure of a breeding parent, and its replacement from outside the group, will alter the basic genetic and dominance structure of the family unit.

As a result, 家庭組成改變 (replacement families) ，情況將會不同。 A replacement mate will typically be unrelated to extant family member. 如此就沒有 incest restrictions. Son could mate with its stepmother, daughter could mate with its stepfather.

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Stripe-backed wrens, Camphylorhynchus nuchalis Live in nuclear families of two to seven individuals in the savannas of Venezuela. Piper and Slater (1993) contrasted the behaviour of sons in intact versus replacement families.

Sons displayed no sexual interest in their mothers, but they frequently engaged in courtship activities with their stepmothers. When sons attempted to consort sexually with their stepmothers, fathers upped their mate-guarding behaviours and became increasingly aggressive toward their sons. (Fig. 10.4)

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Fig. 10.4 Differences in sexually related behaviours within intact (open bars) and replacement (shaded bars) families of stripe-backed wrens.

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White-fronted bee-eaters extended family Male offspring pair with unrelated females

but bring their mates home to breed within their natal family groups.

Sons show no sexual interest in their mothers but brothers do attempt sexual activity with their sisters-in-law and fathers occasionally copulate with their daughters-in-law.

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Infanticide Replacement mates are not expected to invest substantially in dependent young remaining from previous breedings because they are genetically unrelated to such offspring. 倘若照顧這些小孩的代價過高， infanticide 的情況就有可能發生。 This point has been repeatedly confirmed in studies of family-dwelling rodents, carnivores and primates, as well as in many species of birds.

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Replacement families Offspring in replacement families also suffer a reduction in the indirect fitness benefit. The offspring will share only 25% of their genes with future half-siblings, in contrast to the 50% shared with full-siblings. 因此， percentage of helper 應該會下降。

Florida scrub jay and Seychelles warbler (Fig. 10.5)

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Fig. 10.5 The likelihood that non-breeding family members will serve as helpers, plotted as a function of family type. (a) Florida scrub jay

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Fig. 10.5 The likelihood that non0breeding family members will serve as helpers, plotted as a function of family type. (b) Seychelles warbler

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Replacement families Red-cockaded woodpeckers and stripe-

backed wrens, in contrast, did not alter their feeding rates when provisioning full-versus half-siblings.

Replacement families are predicted to be less stable than intact biological families.

Instability, in terms of increased dispersal tendencies of mature offspring or in terms of increased separation rates of re-paired breeders.

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Fig. 10.6 Generalized genealogical diagrams of two forms of replacement family (a) A stepfamily formed from a formerly intact nuclear family when, following the death of the original breeding female the male parent takes a new mate and reproduce again.(b) A ‘blended’ family in which two formerly nuclear families, each with offspring, join when the widowed male parent from one pairs with the widowed female of the other.

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10.5 Conflict over who reproduces The dominant pair monopolizes breeding Subordinates would often increase their

fitness if they could become reproductive themselves.

They are two ways a subordinate may become a breeder within its natal group.

1. It can simply replace the dominant breeder2. It can share reproduction alongside the

dominant

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10.5 Conflict over who reproduces 10.5.1 The benefit of group-living 10.5.2 The probable success of attempted

independent reproduction 10.5.3 kinship 10.5.4 social dominance and fighting

ability

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10.5.1 the benefit of group-living This provides the adaptive explanation for

why dominant individuals share breeding at all.

The greater the magnitude of the group-living benefit realized by the dominant, the greater the potential leverage wielded by the subordinate.

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10.5.2 The probable success of attempted independent reproduction Consider the case where the chance of

successful independent reproduction is near zero

In such instances, parents have extreme leverage because an offspring has little option but to remain at home.

The situation changes when opportunities for independent reproduction improve.

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10.5.3 kinship Genetic relatedness between group members is the third critical parameter. A dominant breeder 會與其近親分享 breeding?

No. 通常是基因型差異較大的，才需要分享。 This prediction has been confirmed in numerous species.

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Kinship consideration also predict that reproduction will be shared more equitably in sibling-sibling associations than in parent-offspring groupings. Reproductive sharing generally will not be favoured between mothers and daughters (or fathers and sons).

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10.5.4 social dominance and fighting ability Dominance hierarchies exist in most

vertebrate families. Dominant individuals are always at risk of

losing their top position. The costs of challenges can be high for

both participants. When the risk is sufficiently great, it will

become advantageous for the dominant to share reproduction as a ‘peace incentive’ with its potential challenger.

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10.6 the myth of the stable family? Family structure is dynamic. Predictable changes should occur:

1. As the benefits of large group size wane;2. As ecological opportunities for independent

breeding increase or decrease;3. As breeder deaths and replacements, as well as

immigrations alter family composition;4. As the social dominance of individual changes

with age and experience.

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10.7 toward a unified evolutionary social theory The building blocks of a unified science, as described in this chapter, consist of ecological constraints theory, kin-selection theory, social dominance theory and reproductive skew theory. Cooperatively breeding species have played, and will continue to play, a pivotal ( 樞軸的 ) role in the development of evolutionary social theory.

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Appendix Evolutionary predictions of living within family kin groups1. Family groupings will be inherently unstable.2. Families that control high-quality resources will

be more stable than those with lower quality resources.

3. Assistance in rearing offspring will be more prevalent in family groups than in otherwise comparable groups comprised of non-relatives.

4. Assistance in rearing offspring will be expressed to the greatest extent between those family members that are the closest genetic relatives.

5. Sexually-related aggression will be less prevalent in family groups than in otherwise comparable groups comprised of non-relatives.

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6. Breeding males will invest less in offspring as their certainty of paternity decreases.

7. The loss of a breeder will result in family conflict over the filling of the resulting reproductive vacancy.

8. Sexually-related aggression will increase after the re-pairing of a parent.

9. Replacement breeders (step-parents) will invest less in existing offspring then will biological parents.

10. Non-reproductive family members will reduce their investment in future offspring following the replacement of a closely related breeder by a more distantly or unrelated individual.

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11. Replacement families will be inherently less stable than biologically intact families.12. Reproduction within a family will become increasingly shared as the severity of ecological constraints decreases.13. Reproduction within a family will become increasingly shared as asymmetry in social dominance between potential cobreeders decreases.14. Reproduction within a family will be shared more equitably when the potential cobreeders consist of siblings than when they consist of parent and grown offspring.15. Reproduction will be shared most with those family members to whom the dominant breeders are least closely related.

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