Editing Behavioral ecology (section)

====Sibling–sibling conflict====
[[File:Galápagos fur seals (4229111296).jpg|thumb|Galápagos fur seals]]
The lifetime [[parental investment]] is the fixed amount of parental resources available for all of a parent's young, and an offspring wants as much of it as possible. Siblings in a brood often compete for parental resources by trying to gain more than their fair share of what their parents can offer. Nature provides numerous examples in which sibling rivalry escalates to such an extreme that one sibling tries to kill off broodmates to maximize parental investment (''See [[Siblicide]]''). In the [[Galápagos fur seal]], the second pup of a female is usually born when the first pup is still suckling. This competition for the mother's milk is especially fierce during periods of food shortage such as an [[El Niño–Southern Oscillation|El Niño]] year, and this usually results in the older pup directly attacking and killing the younger one.<ref>{{cite journal |last=Trillmitch|first=F.|author2=Wolf, J.B.W.|title=Parent–offspring and sibling conflict in Galapagos fur seals and sea lions|journal=Behavioral Ecology and Sociobiology|year=2008|volume=62|pages=363–375 |doi=10.1007/s00265-007-0423-1 |issue=3|s2cid=41834534}}</ref>

In some bird species, sibling rivalry is also abetted by the [[wikt:asynchronous|asynchronous]] hatching of eggs. In the [[blue-footed booby]], for example, the first egg in a nest is hatched four days before the second one, resulting in the elder chick having a four-day head start in growth. When the elder chick falls 20-25% below its expected weight threshold, it attacks its younger sibling and drives it from the nest.<ref name="Drummond">{{cite journal|last=Drummond|first=H.|author2=Chavelas, C.G.|title=Food shortage influences sibling sggression in the Blue-footed Booby|journal=Animal Behaviour|year=1989|volume=37|pages=806–819|doi=10.1016/0003-3472(89)90065-1|s2cid=53165189}}</ref>

Sibling relatedness in a brood also influences the level of sibling–sibling conflict. In a study on [[passerine]] birds, it was found that chicks begged more loudly in species with higher levels of [[Promiscuity|extra-pair paternity]].<ref>{{cite journal |last1=Briskie |first1=James V. |last2=Naugler |first2=Christopher T. |last3=Leech |first3=Susan M. |title=Begging intensity of nestling birds varies with sibling relatedness |journal=Proceedings of the Royal Society B: Biological Sciences |year=1994 |volume=258 |pages=73–78 |bibcode=1994RSPSB.258...73B |doi=10.1098/rspb.1994.0144 |issue=1351|s2cid=85105883 }}</ref>