Birds ‘paint’ own eggs with bacteria to protect the embryo

Researchers from the University of Granada and the Higher Council of Scientific Research (CSIC) have found that hoophoes cover their eggs with a secretion produced by themselves, loaded with mutualistic bacteria, which is then retained by a specializad structure in the eggshell and which increases successful hatching. So far this sort of behaviour has only been detected in this species of birds, and it is a mechanism to protect their eggs from infections by pathogens.

Through an experiment published in the Journal of Animal Ecology, scientists from several research groups precluded several female hoophoes from impregnating their eggs with this substance, which they produce themselves inside the so-called uropygial gland. The research groups involved in this project were the following: Animal Behaviour and Ecology, Microorganism-Produced Antagonistic Substances, both from the UGR, and Evolutive Ecology and the Behaviour and Conservation groups from the Dry Areas Experimental Station (Almería, CSIC)

By doing so they confirmed that the amount of pathogen bacteria that could be found inside the eggs which failed to hatch was higher in those nests in which they had experimentally precluded the females from using their secretion than in those where they were allowed to use this substance. They concluded that this secretion provides a barrier for the entry of pathogens towards the interior of the egg.

Presence of enterococci

On the other hand, not just the secretion as a whole, but particularly the bacteria that did produce bacteriocins (small antimicrobial proteins) in that secretion, the enterococci, are beneficial for the developing embryos, since successful hatches were directly related to the amount of these enterococci in the egg shells and in the secretions of the females. The more enterococci they had, the higher the rate in their successful hatching.

As UGR zoology professor, Manuel Martín-Vivaldi, one of the authors of this research underlines, during the last few years the field of evolutive ecology has acknowledged “the important role played by bacteria, not just as infectious agents capable of producing diseases, but also as allies of animals and other living creatures in their struggle against disease, due to their extraordinary capacity to synthesise compounds with antimicrobial properties”

In the case of the hoophoe’s uropygical gland, scientists have confirmed that its components are very different from those of other birds. This is to a large extent due to the action of the bacteria present in this particular gland.

This research has also revealed that hoophoes have developed an exceptional property in their eggs–which has not so far been found in any other species of bird. This consists in the presence in the surface of many small depressions that do not completely penetrate the shell, and whose function appears to be the retention of this bacteria-carrying secretion that covers the egg.

Bacteria in the eggshell

“With this experiment, we have been able to establish that if the females can use their secretion, towards the end of the incubation period, those tiny craters are full of a substance saturated with bacteria. If we preclude the use of this secretion, these tiny craters appear empty towards the end of the hatching process”, said professors Martín-Vivaldi.

These results prove that in this particular species of bird, “its reproductive strategy has evolved hand in hand with the use of bacteria which may be beneficial for the production of antimicrobial substances, which they cultivate in their gland and then apply upon eggs which are particularly endowed to retain them”

These scientists are currently working to determine the specific composition of the bacterial community within the gland, how these symbionts are acquired, and the types of antimicrobial compounds which synthesize these bacteria, capable of protecting the embryos which are undergoing development.

Further research along these lines will facilitate a better understanding of the way in which mutualistic interactions function between animals and beneficial bacteria, and also to detect new antimicrobial substances with a potential to be used in medicine of for food preservation.

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