Sex and genetics: Why birds are unfaithful to their partners

October 10, 2002

Matings between relatives have negative consequences for the offspring, a phenomenon known as inbreeding depression. But what if you end up with a related partner? Initiated by a scientist at the Max Planck Research Centre for Ornithology, a study by an international team of scientists showed that social mates that are genetically similar use alternative reproductive behaviors to avoid paying the price of inbreeding. Combining field observations on free-living populations of three shorebirds with molecular methods to determine parentage and relatedness between the partners they found that extra-pair parentage occurred when mates were more closely related (Nature, Oct. 10th, 2002).

Over the past decade, the use of molecular techniques to determine parentage has led to the realization that monogamy is rare in nature. Although most bird species are socially monogamous, broods often contain young that are not related to one of the parents tending the nest. This can be the result of two alternative reproductive behaviors. Extra-pair paternity occurs when females copulate with males other than their social partner, and these copulations lead to fertilizations. Extra-pair paternity is common in songbirds, but much less common in other birds. Quasi-parasitism occurs when a male copulates with another female, who then lays one or more eggs that he fathered in his nest. This is a rare and little understood phenomenon. The reasons why birds are unfaithful to their social partner, with whom they raise the offspring, has been the focus of much debate. A scientist at the Max Planck Research Centre for Ornithology initiated a study on parentage in shorebirds, and in collaboration with researchers at the Konrad Lorenz Institute for Comparative Ethology in Vienna, combined data on three species from three continents. An international team of scientists studied populations of marked individuals in the field and closely monitored their breeding behavior. Based on blood samples from parents and offspring, and using DNA-fingerprinting techniques, they found that extra-pair paternity and quasi-parasitism occurred at low frequencies. More importantly, however, in each of the three species, extra-pair parentage occurred when the parents were more related to each other.

In this way, the study links the occurrence of alternative reproductive strategies with the phenomenon of inbreeding depression. It is well established that if close relatives mate, they suffer reduced breeding success, probably because deleterious recessive alleles are expressed. This suggests that natural selection will favour the avoidance of matings between genetically similar individuals. However, in practice this is not always possible, for example because there are no alternative mates available. The study on the three shorebird species now suggests that females seek extra-pair copulations to avoid inbreeding depression or other negative effects of genetic similarity. Moreover, it provides a similar explanation for the evolution of quasi-parasitism, suggesting it is a male-driven strategy.

The study leaves us with an intriguing implication: males and females must be able to assess how genetically similar they are to their social mate and to potential extra-pair partners. There is evidence from a study on peacocks that males can assess the relatedness to other males, even if they could not use social learning or other environmental cues (e.g. if they would have been raised in the same nest). The challenge of future research will be to determine which cues birds use to assess relatedness with a potential mate or copulation partner.

Another intriguing possibility that can explain why extra-pair paternity occurs when mates are more related has to do with the mechanism of sperm competition. If most females copulate occasionally with other males, then sperm of the social mate and the extra-pair males will be mixed in the female reproductive tract and compete for fertilization of the eggs. If the genetic similarity with the female (or the egg) reduces the competitiveness of sperm, then an extra-pair male might have a much higher chance of fertilizing an egg when pair members are closely related.
Research on the evolutionary origin and consequences of male and female promiscuity are the focus of the Junior Research Group at the Max Planck Research Centre for Ornithology.


Related Birds Articles from Brightsurf:

In a warming climate, can birds take the heat?
We don't know precisely how hot things will get as climate change marches on, but animals in the tropics may not fare as well as their temperate relatives.

Dull-colored birds don't see the world like colorful birds do
Bengalese finches -- also called the Society finch -- are a species of brown, black and white birds that don't rely on colorful signals when choosing a mate.

Some dinosaurs could fly before they were birds
New research using the most comprehensive study of feathered dinosaurs and early birds has revised the evolutionary relationships of dinosaurs at the origin of birds.

If it's big enough and leafy enough the birds will come
A new study from the Cornell Lab of Ornithology highlights specific features of urban green spaces that support the greatest diversity of bird species.

How do birds understand 'foreign' calls?
New research from Kyoto University show that the coal tit (Periparus ater) can eavesdrop and react to the predatory warning calls of the Japanese tit (Parus minor) and evokes a visual image of the predator in their mind

Microelectronics for birds
Ornithologists and physicists from St Petersburg University have conducted an interdisciplinary study together with colleagues from Sechenov Institute of Evolutionary Physiology and Biochemistry of the Russian Academy of Sciences and the Biological Station Rybachy of the Zoological Institute of the Russian Academy of Sciences.

Birds of a feather better not together
A new study of North American birds from Washington University in St.

Not-so-dirty birds? Not enough evidence to link wild birds to food-borne illness
Despite the perception that wild birds in farm fields can cause food-borne illness, a WSU study has found little evidence linking birds to E. coli, Salmonella and Campylobacter outbreaks.

Birds are shrinking as the climate warms
After 40 years of collecting birds that ran into Chicago buildings, scientists have been able to show that the birds have been shrinking as the climate's warmed up.

Diving birds follow each other when fishing
Diving seabirds watch each other to work out when to dive, new research shows.

Read More: Birds News and Birds Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to