Sibling Rivalry
PSYCH 3T03
Lecture 17
Animal Behavior – Chapters 10 & 11
1. What is sibling rivalry?
2. Why is there conflict between
siblings?
3. Facultative vs obligate strategies
4. Lethal & non-lethal competition
5. What behaviours have evolved as
a consequence?
Blue footed booby © Derek Keats
Reproductive Conflicts & Trade-offs
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Each individual ‘wants’ to maximise its own
lifetime reproductive fitness
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Optimal strategies will differ between males –
females – offspring
Uneven parental investment creates conflict:
One parent vs the other
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Sibli
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One breeding season vs another
One sibling vs another
Benefits of Parental Care
Increases the survival and fitness of offspring
That could mean:
Greater survival to adult
Faster growth
Healthier / better able to attract mates
Social development & learning
Indirect benefits to parents
Paper wasp
© Alvesgaspar
Trade offs in Parental Care
When should a parent stop parental care and invest in further reproduction?
Greater investment = greater costs to
parents
But for offspring greater investment =
greater benefits
Potential conflict over:
1. Duration of parental care
2. Distribution of parental care
© Alex Proimmos
Examples of Parent-Offspring Conflict
1. Resource provision
Offspring want more food
2. Defence
Risky behaviours for parents benefit offspring
3. Clutch size
More offspring good for parents but more
siblings may be bad for offspring
4. Sex ratios
Optimal sex ratio of offspring may be different
to optimal sex ratio of siblings
Parents want to maximise reproductive output…
Examples of Sibling-Sibling Conflict
… offspring want to maximise individual investment
1. Resource provision
Offspring want more food from parents
2. Inheritance
Offspring want access to natal territory
3. Future rivals
Siblings are competition for future territory/food
4. Sex ratios
Future competition with same sex siblings for mates
Direct and Indirect Competition
Offspring may compete for attention or actively interact with each other
1. Begging
2. Scrambling
3. Sublethal dominance
4. Siblicide
© Katie McVey, USFWS
Competition for resources in cichlid fish
Territorial fish found in Lake Tanganyika
Variabilichromis moorii
Biparental care – defence but no provisioning
1.
© JBenoit
Offspring compete for access to food
within natal territory
Competition for resources in cichlid fish
Territorial fish found in Lake Tanganyika
Satoh et al. 2021. Behav Ecol.
Biparental care – defence but no provisioning
1.
Offspring compete for access to food
within natal territory
2. Larger juveniles monopolise food and
grow more quickly than subordinates
Competition for resources in cichlid fish
Territorial fish found in Lake Tanganyika
Satoh et al. 2021. Behav Ecol.
Biparental care – defence but no provisioning
1.
Offspring compete for access to food
within natal territory
2. Larger juveniles monopolise food and
grow more quickly than subordinates
3. Subordinates leave nest earlier to find food
Dominant juveniles get more access to food
And receive parental care for longer
Competition for future territory in jays
Socially breeding bird found in northern Eurasia
Siberian Jay - Perisoreus infaustus
Biparental care with previous offspring helping
1.
Offspring can either disperse after
fledging or remain in the natal territory
2. Dominant offspring have better access to
food within the territory
© Bouke ten Cate
Competition for future territory in jays
Socially breeding bird found in northern Eurasia
Ekman et al. 2002. Anim Behav.
Biparental care with previous offspring helping
1.
Offspring can either disperse after
fledging or remain in the natal territory
2. Dominant offspring have better access to
food within the territory
3. Dominant offspring (larger size) more
likely to remain in natal territory
Competition for future territory in jays
Socially breeding bird found in northern Eurasia
Ekman et al. 1999. PRSB.
Biparental care with previous offspring helping
1.
Offspring can either disperse after
fledging or remain in the natal territory
2. Dominant offspring have better access to
food within the territory
3. Dominant offspring (larger size) more
likely to remain in natal territory
4. Remaining offspring have more breeding
success over their lifetimes
Parental insurance policies
Parents may produce more offspring than they can look after
Not all offspring will survive
Back up option just in case
Not all offspring are equally fit
Competition allows for selection of strongest
Conditions change
Often only a short period of time in which to breed
© Diego Cue
Overproduce early and reduce numbers is a better
strategy than waiting to catch up later
Siblicide
Offspring may kill their siblings
Reduce competition for parental care
Remove future rivals
Directly acquire resources
(cannibalism)
Maybe direct (killing) or
indirect (out competing)
Maybe obligate or facultative
Earwig
© Photo by Tom Oates, 2010
Siblicide provides extra resources
Earwigs raise offspring in communal nests
Forficula auricularia
Nymphs do not disperse until after they molt
1.
© Tom Oates, 2010
Nymphs will kill and cannibalize siblings
Siblicide provides extra resources
Earwigs raise offspring in communal nests
Dobler & Kölliker. 2009. Behav Ecol.
Nymphs do not disperse until after they molt
1.
Nymphs will kill and cannibalize siblings
2. Survivors benefit from
Reduced competition
Nutritional gain from eating sibling
3. Cannibalism more common among halfsiblings compared to full-siblings
© Tom Oates, 2010
Siblicide provides extra resources
Intrauterine cannibalistic siblicide in sand tiger sharks
Carcharias taurus
Sand tiger sharks give birth to live young
1.
© Shankar S
Female will produce many embryos that
start development
Siblicide provides extra resources
Intrauterine cannibalistic siblicide in sand tiger sharks
Chapan et al. 2013. Biol Lett.
Sand tiger sharks give birth to live young
1.
Female will produce many embryos that
start development
2. Eggs develop and hatch within the mother
3. First hatchling to hatch eats all of its
siblings and undeveloped eggs
4. Growth rate increases massively
Siblicide skews the sex ratio
Future reproductive success can depend on the sex ratio
Copidosoma floridanum
Parasitic wasp with haplodiploidy system
Two types of offspring
Metamorphosing reproducing offspring
Non-metamorphosing offspring with large jaws
© John Rosenfeld, Bugguide.net
Siblicide skews the sex ratio
Future reproductive success can depend on the sex ratio
Grbić et al. 1992. Nature.
Parasitic wasp with haplodiploidy system
Two types of offspring
Metamorphosing reproducing offspring
Non-metamorphosing offspring with large jaws
Clonal non-metamorphosing offspring kill males
One male can fertilize all females
Killing brothers gives more resources to sisters
© John Rosenfeld, Bugguide.net
Competition can lead specialised adaptations
Competition between siblings can drive the evolution of morphological weapons
Domestic pigs
Sows typically produce 20% more offspring than
they can raise
Competition among piglets for access to milk
Despite not needing teeth to drink milk
Piglets born with fully erupted canines
Teeth used to fight siblings
Often removed by farmers to protect their livestock
Competition can lead specialised adaptations
Competition between siblings can drive the evolution of morphological weapons
Fraser & Thompson. 1991. Behav Ecol Sociobiol.
Experimentally removed the teeth of the
heaviest piglets from multiple litters
Compared growth rate between piglets
with and without canine teeth
Piglets with canines grew faster
Effect greatest in large litters
Where competition was most intense
Resource availability
Facultative siblicide in hyena
Spotted hyena frequently give birth to twins
Crocuta crocuta
Twins born regardless of environmental conditions
1.
Competition between siblings will depend
on how much food the group can provide
Facultative siblicide in hyena
Spotted hyena frequently give birth to twins
Hofer & East. 2008. Behav Ecol Sociobiol.
Twins born regardless of environmental conditions
1.
Competition between siblings will depend
on how much food the group can provide
If cubs are growing slowly rate of siblicide
increases
Facultative siblicide in hyena
Spotted hyena frequently give birth to twins
Hofer & East. 2008. Behav Ecol Sociobiol.
Twins born regardless of environmental conditions
1.
Competition between siblings will depend
on how much food the group can provide
If cubs are growing slowly rate of siblicide
increases
Total food provision does not decrease after siblicide
Growth rate of surviving cub increases after
siblicide
Facultative vs Obligate Siblicide
Blue Footed Booby
Masked Booby
© MC VC
© pjd1
Facultative vs Obligate Siblicide
Blue Footed Booby
Both species produce two eggs and incubate immediately
Asynchronous hatching = alpha and beta chicks differ in size
Blue footed booby is facultative
© MC VC
Only kills sibling when food is scarce
Masked Booby
Masked booby is obligate
Always kills sibling immediately
© pjd1
When conditions are favourable Blue Footed can raise
two chicks, but Masked cannot
Control of Facultative vs Obligate Siblicide
Great Blue Heron
Great Egret
© Russ
© Mike Baird
Control of Facultative vs Obligate Siblicide
Great Blue Heron
Both species have similar breeding behaviour
Great Blue Heron siblicide is rare
© Russ
Great White Egret
Great White Egret siblicide is common
Up to 85% of 4 egg clutches will have siblicide
Chicks attack each other
Why are there differences in the rate of siblicide?
© Mike Baird
Control of Facultative vs Obligate Siblicide
Great Blue Heron
Cross fostering experiment (Mock 1984. Science)
Herons raised by egrets and vice versa
Egret chicks in heron nests could not monopolise food
© Russ
Great White Egret
High competition and siblicide but all chicks small and weak
Heron chicks in egret nests increased aggression
Increased siblicide among heron chicks
Largest chicks monopolized food
Important role of parent behaviour
© Mike Baird
The role of parents – competition a benefit?
Parents can encourage or discourage sibling competition
1. Selectively feed particular offspring
2. Preventing / ignoring aggression
3. Synchronous / asynchronous
incubation – control age difference
4. Control the number of offspring
produced / infanticide
© Chanel City Camera Club
Any Questions?
PSYCH 3T03
Lecture 16
1. For offspring increased allocation to the
detriment of parents and/or siblings can be
an individual advantage
2. Siblings will compete for access to
immediate benefits (e.g., food) and future
opportunities (e.g., mating access)
3. Parents often produce more offspring than
they can raise
4. Siblicide is common and can be beneficial
for both parents and surviving offspring
Great egret © Mike Baird