10.4 Sibling rivalry

Sibling rivalry is a common phenomenon in the animal kingdom, driven by competition for limited parental resources. This evolutionary strategy has led to various behaviors and adaptations across species, from begging and physical contests to siblicide in extreme cases.

Parental investment, birth order, and resource availability all play crucial roles in shaping sibling interactions. While rivalry is prevalent, cooperative behaviors among siblings can also evolve, influenced by factors like inclusive fitness and kin selection.

Evolutionary origins of sibling rivalry

• Sibling rivalry has evolved as a result of competition for limited parental resources and investment
• Natural selection favors offspring that can effectively compete for resources, leading to the development of various strategies to outcompete siblings
• The intensity of sibling rivalry varies across species depending on factors such as parental care system, resource availability, and life history traits

Resource competition among siblings

Parental investment limitations

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• Parents have limited resources (time, energy, and nutrients) to invest in their offspring
• As the number of offspring increases, the amount of resources available to each individual decreases
• Resource limitation leads to competition among siblings for access to food, parental care, and other essential resources

Sibling conflict over food

• Siblings often compete for access to food resources provided by parents
• Competition can involve begging behaviors, physical contests, or scramble competition (first-come, first-served)
• Examples of sibling conflict over food include:
  • Begging calls in nestling birds to attract parental feeding
  • Pushing and shoving among puppies during nursing

Sibling conflict over parental care

• Siblings may compete for parental attention, protection, and other forms of care
• Behaviors such as crying, vocalizations, or physical contact are used to solicit parental care
• Examples of sibling conflict over parental care include:
  • Piglets jostling for access to the most productive teats during nursing
  • Nestling birds jockeying for the best position to receive food from parents

Sibling aggression

Siblicide in birds and mammals

• In some species, sibling aggression can escalate to lethal levels, resulting in siblicide (the killing of a sibling)
• Siblicide is often associated with extreme resource limitation or large size differences among siblings
• Examples of siblicide include:
  • Obligate siblicide in some bird species (black eagles) where the older chick always kills the younger
  • Facultative siblicide in spotted hyenas, where the dominant cub may kill its subordinate sibling if resources are scarce

Non-lethal aggressive behaviors

• Many species exhibit non-lethal forms of sibling aggression, such as fighting, biting, or displacing siblings
• These behaviors serve to establish dominance hierarchies or secure access to resources without causing direct mortality
• Examples of non-lethal aggressive behaviors include:
  • Play fighting among lion cubs to establish social ranks
  • Feather-pulling and wing-slapping in nestling birds to assert dominance

Birth order effects on sibling rivalry

Advantages of being firstborn

• In many species, firstborn offspring have a competitive advantage over their younger siblings
• Firstborns often have a size and strength advantage, allowing them to monopolize resources and parental care
• Examples of firstborn advantages include:
  • In some bird species (great tits), firstborn chicks receive more food and have higher survival rates
  • Firstborn elephant calves are more likely to survive and reproduce than their younger siblings

Disadvantages of being lastborn

• Lastborn offspring often face disadvantages in terms of resource access and parental investment
• Lastborns may be smaller, weaker, or less developed than their older siblings, making it harder to compete for resources
• Examples of lastborn disadvantages include:
  • In some bird species (blue-footed boobies), lastborn chicks have lower survival rates due to resource limitation
  • Lastborn meerkats are more likely to be evicted from the group during times of resource scarcity

Parental strategies to reduce sibling rivalry

Asynchronous hatching in birds

• Some bird species have evolved asynchronous hatching, where eggs hatch at different times
• Asynchronous hatching creates a size hierarchy among siblings, reducing the intensity of sibling rivalry
• Examples of asynchronous hatching include:
  • In some raptor species (peregrine falcons), asynchronous hatching ensures that at least some chicks survive if food is limited
  • Asynchronous hatching in some penguin species (emperor penguins) reduces sibling aggression and improves overall brood survival

Differential provisioning by parents

• Parents may allocate resources differently among their offspring based on factors such as sex, size, or condition
• Differential provisioning can help reduce sibling rivalry by ensuring that each offspring receives an appropriate amount of resources
• Examples of differential provisioning include:
  • In some primate species (rhesus macaques), mothers invest more in sons than daughters due to their higher reproductive potential
  • In some bird species (American robins), parents preferentially feed smaller chicks to compensate for their competitive disadvantage

Cooperative behaviors among siblings

Inclusive fitness benefits

• Despite the prevalence of sibling rivalry, cooperative behaviors among siblings can also evolve through inclusive fitness benefits
• Inclusive fitness considers the shared genes among related individuals, so helping siblings can indirectly increase an individual's own fitness
• Examples of inclusive fitness benefits include:
  • In some social insect colonies (honey bees), workers cooperate to raise their siblings, increasing the overall success of the colony
  • In some mammal species (meerkats), older siblings help raise younger siblings, improving the survival and reproduction of the group

Kin selection and altruism

• Kin selection theory explains how altruistic behaviors can evolve among related individuals
• Altruistic behaviors, such as helping siblings, can be favored by natural selection if the benefits to the recipient's fitness outweigh the costs to the actor's fitness
• Examples of kin selection and altruism include:
  • In some bird species (white-fronted bee-eaters), older siblings help feed and protect younger siblings, increasing the overall fitness of the family
  • In some mammal species (African elephants), older sisters help care for and defend younger siblings, improving their survival and future reproductive success

Long-term consequences of sibling rivalry

Effects on survival and reproduction

• The outcomes of sibling rivalry can have long-lasting effects on an individual's survival and reproductive success
• Individuals that successfully compete for resources during development may have higher survival rates and greater reproductive output later in life
• Examples of long-term effects on survival and reproduction include:
  • In some bird species (barn swallows), nestlings that receive more food during development have higher adult survival rates and produce more offspring
  • In some mammal species (red deer), individuals born in years with less sibling competition have higher lifetime reproductive success

Influence on dispersal patterns

• Sibling rivalry can also influence dispersal patterns, as individuals may leave their natal group to avoid competition with siblings
• Dispersal can have important consequences for population dynamics, gene flow, and the colonization of new habitats
• Examples of sibling rivalry influencing dispersal patterns include:
  • In some bird species (western bluebirds), subordinate siblings are more likely to disperse to avoid competition with dominant siblings
  • In some mammal species (spotted hyenas), younger siblings may disperse from their natal clan to avoid aggression from older siblings

Sibling rivalry across animal taxa

Sibling rivalry in invertebrates

• Sibling rivalry is not limited to vertebrates and can also be observed in various invertebrate species
• Invertebrate sibling rivalry often involves competition for food resources or access to mates
• Examples of sibling rivalry in invertebrates include:
  • In some parasitoid wasp species (Copidosoma floridanum), larvae compete for host resources, with larger larvae often killing smaller siblings
  • In some spider species (Stegodyphus lineatus), siblings compete for prey caught in the communal web, with larger individuals monopolizing resources

Sibling rivalry in non-avian reptiles

• Sibling rivalry has been documented in various non-avian reptile species, including lizards, snakes, and crocodilians
• Reptilian sibling rivalry often involves competition for food, basking sites, or other essential resources
• Examples of sibling rivalry in non-avian reptiles include:
  • In some lizard species (Yarrow's spiny lizard), siblings compete for basking sites, with dominant individuals displacing subordinates
  • In some snake species (timber rattlesnakes), siblings compete for prey, with larger individuals often consuming more food

Methods for studying sibling rivalry

Observational studies in the wild

• Observational studies in natural habitats provide valuable insights into the dynamics of sibling rivalry under realistic conditions
• Researchers can collect data on behaviors, resource allocation, and fitness outcomes to understand the causes and consequences of sibling rivalry
• Examples of observational studies in the wild include:
  • Monitoring begging behaviors and parental feeding rates in nestling birds to assess sibling competition for food
  • Tracking the survival and reproduction of individually marked mammals to determine the long-term effects of sibling rivalry

Experimental manipulations in captivity

• Controlled experiments in captive settings allow researchers to manipulate variables and test specific hypotheses about sibling rivalry
• Common experimental manipulations include altering brood size, resource availability, or sibling size hierarchies
• Examples of experimental manipulations in captivity include:
  • Manipulating the number of nestlings in a brood to study the effects of sibling competition on growth and survival
  • Providing different food quantities or qualities to sibling groups to investigate the role of resource limitation in sibling rivalry