Life Histories and Parental Care - Easy

Life History Fundamentals

• Key traits include age at first reproduction, duration and schedule of reproduction, number and size of offspring, reproductive effort (pattern of investment in reproduction), and lifespan
• Life history strategies, the pattern of investment in growth and reproduction, are subject to evolutionary forces
• Evolution of the traits involves trade-offs because 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 relies on likelihood of survival to different age classes
• Environmental factors, such as predation risk, can drive the rapid evolution of life history traits
• 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 and Optimization

• 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, for example, investment in reproduction can be at the expense of growth or maintenance
• Optimizing these trade-offs maximizes the number of offspring surviving to maturity and depends on the likelihood of survival to different age classes
• Trade-offs can occur between early and late reproduction, where investment in early reproduction reduces later reproduction
• Senescence (deterioration with age) can be seen as a trade-off between investing in early reproduction and investing in body maintenance for longevity
• Environmental factors, such as predation risk, can influence the evolution of these trade-offs

Case Studies: Predation Influence

• Environmental factors like predation influence life history strategies in species such as opossums and guppies
• On the mainland of Georgia, predation is high, with predators killing 80% of opossums
• Opossums in this environment show rapid declines in age and survival.
• Many develop cataracts and arthritis by 1.5 years old and most die by 2 years
• In contrast, Sapelo Island has had no opossum predators for 5000 years, which affects life history strategies
• 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
• Guppies from predator-rich streams and guppies that were moved to a predator-free stream for 11 years indicates a study of how life history traits change in response to altered predation pressure

Parental Investment and Mating Behaviors

• Study parental investment patterns, including sex differences, roles, and operational sex ratio effects on 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, the 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 regulate offspring number to maximize fitness, possibly through miscarriage and cannibalism
• Sand gobies are an example of a species that cannibalizes eggs
• Organisms 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, since 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

• Predicts 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
• In Seychelles warblers, this pattern of sex ratio adjustment has been observed
• With high resources, females are favored, and having up to three helping daughters is beneficial
• With low resources, males are favored, as they will disperse away from the poor habitat

Conflicts Over Investment

• Strategies to maximize 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 leads to battles of gene expression in offspring
• 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
• Parental care creates opportunities for conflict among siblings
• Sibling rivalry influences resource competition among offspring
• Offspring conflict in barn swallow chicks is centered around a specific trait
• American coots exhibit sibling rivalry

Genomic Imprinting

• Genomic imprinting is an epigenetic effect and has a parent-of-origin effect
• Methylation is one epigenetic mechanism involved and 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 Aging

• Senescence is the deterioration with age resulting from a trade-off between investing in early reproduction 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 mentioned as 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
• Menopause 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

Life History Traits and Conflicts

• Life history traits and reproductive strategies evolve in response to ecological pressures and conflicts among parents, offspring, and siblings
• 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 optimizes these trade-offs to maximize 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, which influences their life history strategies
• 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
• Strategies to maximize offspring produced over a lifetime 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
• Parents and offspring disagree on the optimal level of parental investment, and parental care creates opportunities for this conflict
• 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
• American coots also exhibit sibling rivalry
• Genomic imprinting with a parent-of-origin basis involving methylation occurs due to parental conflict
• Parental conflict can lead to battles of gene expression
• David Haig is associated with this area of study