Life History and Paternal Care - Medium

• Life history fundamentals encompass key traits and their evolutionary significance

Key Life History Traits

• Age at first reproduction is a key life history trait
• The duration and schedule of reproduction is a key life history trait
• Number and size of offspring is a key life history trait
• Reproductive effort (pattern of investment in reproduction) is a key life history trait
• Lifespan is a key life history trait

Evolutionary Significance of Life History Traits

• Life history strategies (the pattern of investment in growth and reproduction) are subject to evolutionary forces
• Evolution of these traits involves trade-offs as organisms have limited energy to invest in survival, maintenance, and reproduction
• Investing in one trait often means less investment in another
• Natural selection optimizes these trade-offs to maximize the number of offspring surviving to maturity, which depends on the likelihood of survival to different age classes
• Environmental factors, such as predation risk, can drive the rapid evolution of life history traits
• Guppies in predator-rich streams evolve different life history traits compared to those in predator-free streams as an example
• Selection may favor mutations that are beneficial early in life, even if they are detrimental later in life
• Investment in early reproduction can often reduce reproduction later in life

Resource Allocation Trade-offs

• Life history strategies involve trade-offs because organisms have limited energy to invest in survival, maintenance, and reproduction
• Investment in one trait often results in lower investment in another
• Investment in reproduction can be at the expense of growth or maintenance
• Selection optimizes these trade-offs to maximize the number of offspring surviving to maturity, determined by the likelihood of survival to different age classes
• Trade-offs can occur between early and late reproduction, where investment in early reproduction may reduce reproduction later in life
• Senescence (deterioration with age) can be seen as a trade-off between investing in reproduction early in life and investing in body maintenance for longevity
• Environmental factors, such as predation risk, can influence the evolution of these trade-offs

Predation Impact via Opossum and Guppy Case Studies

• On the mainland of Georgia, predation is high, with predators killing 80% of opossums
• Opossums in this high-predation environment show rapid declines in age and survival
• Many mainland opossums develop cataracts and arthritis by 1.5 years old and most die by 2 years
• Sapelo Island has had no predators for 5000 years
• This difference in predation pressure likely leads to different life history strategies compared to mainland opossums
• Predation risk is a driving force in the evolution of life history traits in guppies
• Rapid evolution of life history traits has been observed in guppies in response to different levels of predation
• These traits include age at male maturity, female age at first offspring, male and female size at first offspring, number of offspring, and embryo mass
• The document mentions guppies from predator-rich streams and guppies that were moved to a predator-free stream for 11 years
• The guppy study indicates how life history traits change in response to altered predation pressure

Parental Investment and Mating Behaviors

• Females are generally more likely to provide parental care
• Males typically invest less in reproduction
• Uncertain paternity in males is a factor contributing to their lower investment
• Some species exhibit role-reversal in parental care
• The operational sex ratio (OSR) refers to the ratio of sexually active males to sexually receptive females
• A male-biased OSR is likely to produce extreme variance in reproductive success of males
• In role-reversed pipefish, typical sex roles are reversed
• Females usually benefit more from providing parental care than males
• Role reversals offer opportunities to test predictions of sexual selection

Offspring Number and Sex Ratio Regulation

• Organisms may regulate offspring number to maximize fitness, which can occur through miscarriage and cannibalism
• Sand gobies are mentioned as an example of a species that cannibalises eggs
• Organisms may also regulate offspring sex ratio to maximize fitness
• Mothers can alter the sex ratio of their offspring depending on their condition
• Mothers in poor condition may produce more daughters, as daughters are likely to have offspring even if in poor condition
• Mothers in good condition may produce more sons, as larger males tend to attract more mates
• The Trivers-Willard hypothesis predicts this pattern of sex ratio adjustment based on maternal condition
• Sex ratio adjustment has been observed in Seychelles warblers, where high resources favor females (helpful daughters), and low resources favor males (dispersing from poor habitat)
• Red junglefowl also present opportunities to 'alter' sex ratio

Trivers-Willard Hypothesis

• The Trivers-Willard hypothesis states that mothers alter the sex ratio of their offspring based on their condition
• Mothers in poor condition tend to produce more daughters because daughters are still likely to have offspring even if they are also in poor condition
• Mothers in good condition tend to produce more sons because larger males are more successful at attracting mates
• This pattern of sex ratio adjustment has been observed in Seychelles warblers
• With high resources, females are favoured, and having up to three helping daughters is beneficial
• With low resources, males are favoured, as they will disperse away from the poor habitat

Intersexual and Parental Conflicts

• Strategies to maximise offspring produced over a lifetime can differ for the sexes
• Males benefit when mothers invest more in current offspring
• Females benefit by saving resources for future offspring
• Parental conflict can lead to battles of gene expression in offspring

Parental-Offspring Conflict

• Parents and offspring disagree with regard to investment
• The optimal allocation of parental investment differs depending on the perspective of the parent versus the offspring
• What is most beneficial to the mother is not always the same as what is most beneficial to the offspring
• Parental care creates opportunities for conflict between parents and offspring

Sibling Rivalry

• Parental care creates opportunities for conflict among siblings
• Sibling rivalry influences resource competition among offspring
• Offspring conflict in barn swallow chicks is centred around a specific trait
• American coots exhibit sibling rivalry

Genomic Imprinting

• Genomic imprinting is an epigenetic effect
• It also has a parent-of-origin effect
• Methylation is one epigenetic mechanism involved
• Genomic imprinting occurs due to parental conflict
• Parental conflict can lead to battles of gene expression in offspring
• David Haig from Harvard University is associated with the study of genomic imprinting and parental conflict

Senescence and Reproductive Aging

• Senescence is the deterioration with age
• It results from a trade-off between investing in reproduction early in life and investing in body maintenance for longevity
• Caloric restriction can slow the aging process
• This may involve trade-offs, as C. elegans mutants that age more slowly have lower fitness
• Genes involved in repair are switched on under stress, which might be related to caloric restriction's effect
• Postponing senescence is a key concept
• Reproduction late in life can be risky, according to the mother hypothesis
• Menopause may have evolved after humans split from other apes
• It may be an adaptive trait
• The grandmother hypothesis suggests that the loss of fertility associated with menopause leads to a shift in investment towards grandchildren
• Maintaining strength (by not reproducing) allows grandmothers to assist with grandchildren

Integration of Life History Traits and Conflicts

• Life history traits include age at first reproduction, duration and schedule of reproduction, number and size of offspring, reproductive effort, and lifespan
• Life history strategies are subject to evolutionary forces
• Evolution of these traits involves trade-offs due to limited energy for survival, maintenance, and reproduction
• Selection optimises these trade-offs to maximise the number of offspring surviving to maturity, depending on the likelihood of survival at different ages
• Changes in the environment can lead to rapid evolution of life history traits
• Predation risk drives life history evolution
• In mainland Georgia, high predation (80% killed) leads to opossums with rapidly declining age and survival, developing conditions like cataracts and arthritis early, and most dying by 2 years
• In contrast, Sapelo Island has no predators, influencing life history strategies in those species
• Rapid evolution of life history traits has been observed in guppies in response to different predation levels, affecting traits like age and size at maturity, number of offspring, and embryo mass
• Parental conflict consists of strategies to maximise offspring produced over a lifetime, which can differ between sexes Males benefit from mothers investing more in current offspring, while females benefit by saving resources for future offspring
• This conflict can lead to battles of gene expression in offspring
• Parent-offspring conflict notes that parents and offspring disagree on the optimal level of parental investment and what benefits the mother most isn't always what benefits the offspring most
• Parental care creates opportunities for this conflict
• Sibling rivalry notes that parental care also creates opportunities for conflict among siblings, influencing resource competition
• Offspring conflict has been observed in barn swallow chicks around a specific trait, while American coots also exhibit sibling rivalry
• Genomic imprinting is an epigenetic effect with a parent-of-origin basis (involving methylation) that occurs due to parental conflict
• Parental conflict can lead to these 'battles' of gene expression, and David Haig is associated with this area of study