“Predictably philandering females prompt poor paternal provisioning”

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Julia Schroeder, Yu-Hsun Hsu, Isabel Winney, Mirre Simons, Shinichi Nakagawa, and Terry Burke

Male sparrows can judge if a spouse is prone to infidelity, providing less food for their brood if their partner is unfaithful

Sparrows with unfaithful ‘wives’ care less for their young

Male house sparrow (Passer domesticus) on the island of Lundy in the Bristol Channel.
(Credit: Alfredo Sánchez Tójar)

Sparrows form pair bonds that are normally monogamous, but many females are unfaithful to their partner and have offspring with other males. Biologists believe that the male birds are unfaithful to ensure they father as many chicks as they can, while females are unfaithful with males of better ‘genetic quality’ – ones that are fitter and could produce stronger offspring.

However, cheating comes with a cost – the cheating female’s partner will provide less food for their nest of young. It has long been suspected that males know that not all the chicks in their nest are theirs, and so make a decision to provide less. But an alternative explanation is that cheating females and lazy males tend to pair up naturally.

Researchers from the UK, Germany, and Australia have now revealed that males make the decision of how much to provide for their chicks based on the tendency of their partner to cheat. The study, appearing in The American Naturalist, followed the entire sparrow population of the island of Lundy in the Bristol Channel for 12 years.

Lead researcher Dr. Julia Schroeder of the Department of Life Sciences at Imperial College London says: “Males changed their behavior based on their partner. When they switched from a faithful partner to one prone to infidelity, they provided less food for their brood.” Females might also change their behavior when paired with a less lazy male, cheating less with a more attentive father.

The research shows that males cannot actually identify whether all the chicks in their nest are theirs or not, and instead base their feeding decisions on decisions on who their female partner is. “If chicks were switched into a nest where the female was faithful, then the father at that nest kept up his hard work of provisioning the chicks, suggesting they have no mechanism, such as smell, to determine which chicks are theirs,” says Dr. Schroeder. “Instead, the males may use cues from the female’s behavior during her fertile period – for example how long she spends away from the nest.”

The study followed 200 males and 194 females as they formed 313 unique monogamous pairs and hatched 863 broods on Lundy. Some sparrow ‘divorces’ occurred – but most changes of life partner were due to a death.

The team DNA genotyped every sparrow, allowing them to build up precise family trees, and find out which females were most unfaithful and which males they cheated with. “Lundy is a unique natural laboratory because it is almost a closed system – very few birds leave the island or arrive from the mainland. In the entire 12 years only four birds immigrated to Lundy, possibly carried by boat.”

Dr. Schroeder and her team are continuing to study the Lundy sparrows to uncover how and why social behaviors like monogamy arose. Being unfaithful may be a costly behavior for females because they only lay a limited number of eggs, and it may be a holdover from when their ancestors were not monogamous, rather than a useful strategy for getting the strongest offspring. Read the Article