Sibling rivalry may extend to breastfeeding infants who keep their mothers up at night to prevent them getting pregnant, an expert has claimed.
Infants who constantly wake up demanding to be fed are trying to delay the birth of a brother or sister, according to evolutionary biologist Professor David Haig.
Breastfeeding widens the gap between births by undermining fertility.The act of suckling blocks hormonal signals that lead to ovulation, and the longer a woman breastfeeds, the longer she has to wait to get pregnant again.
Over millions of years, human infants have evolved the feeding strategy to reduce competition and improve their chances of survival, Prof Haig believes.
He points out that smaller periods of time between the birth of siblings are associated with higher death rates of infants and toddlers.
"Natural selection will have preserved suckling and sleeping behaviours of infants that suppress ovarian function in mothers because infants have benefited from delay of the next birth," said Prof Haig, from Harvard University in the US.
"Maximal night waking can be conjectured to overlap with the greatest benefits of contraceptive suckling."
It may be fathers' genes that are responsible, he added. Evidence from babies with Angelman syndrome (AS), a rare developmental disorder marked by extreme restlessness, indicated that paternal genes promote suckling and waking.
Instructing parents not to respond to night waking by children with AS resulted in dramatic improvements in sleep quality.
Writing in the journal Evolution, Medicine And Public Health, Prof Haig said while the selective forces that led to frequent infant night waking had been reduced in the modern world, their effects remained "part of our biological heritage".