Pigeons produce "milk" with antioxidants - Genomics of the lactating crop

in biology

Deakin University scientists have revealed some of the secrets behind the pigeon’s rare ability to produce ‘milk’ to feed its young by studying the genes behind pigeon ‘milk’ production. They found that, like mammalian milk, it contains antioxidants and immune-enhancing proteins important for the growth and development of the young.

Producing milk to feed babies is normally the domain of mammals, including humans. However, the pigeon is one of only three bird species (the others being flamingos and male emperor penguins) to produce a milk-like substance to feed their young.  Dr Crowley, a co-author, explained:  “we looked at the genes involved in the production of pigeon ‘milk’ and found that it contains antioxidants and immune-enhancing factors. This suggests that, like mammalian milk, it plays a key role in enhancing the immune system of the developing baby.”

pigeon baby lactation crop genetics
A pigeon baby. Image provided to ScienceDebate.com by
Mandi O'Garretty,Deakin University.

Both female and male pigeons produce a nutrient rich substance in their crop to feed their young (squabs). This substance has been likened to lactation in mammals and is referred to as pigeon ‘milk’. This ‘milk’ is essential for the growth and development of the pigeon squab, and without it they fail to thrive.

The paper was published recently in the peer-reviewed open access journal BMC Bioinformatics. Deakin PhD student Meagan Gillespie and research fellow Dr Tamsyn Crowley, along with colleagues from the University’s Institute for Technology Research and Innovation and CSIRO Livestock Industries were among the authors.

“Bird crops are normally used to store food. However, in the pigeon the crop changes prior to ‘lactation’ in response to hormones and returns to its ‘non-lactating’ state at the end of the lactation period, a bit like the mammary gland,” Ms Gillespie explained.

During ‘lactation’, a curd-like substance is created from fat-filled cells that line the crop and regurgitated to feed the squab. This ‘milk’ is made up of protein (around 60 per cent), fat (up to 36 per cent), a small amount of carbohydrate (up to three per cent), a range of minerals and antibodies.

While studies have investigated the nutritional value of pigeon ‘milk’, very little is known about what it is or how it is produced. The Deakin researchers studied the genes expressed in the ‘lactating’ crop

Birds are different to other animals in that they don’t have sweat glands, but they do have the ability to accumulate fat in their outer skin cells (keratinocytes) which act like sweat glands. We found that the evolution of pigeon ‘milk’ appears to have developed from the ability of these outer skin cells to accumulate fat. The way pigeon ‘milk’ is produced is an interesting example of the evolution of a system with similarities to mammalian lactation, with pigeon ‘milk’ fulfilling a similar function to mammalian milk but produced in a different way.

The crop in most species of birds is normally used as a food storage area. It is located between the oesophagus and the top of a bird’s stomach where food is moistened before further breakdown and digestion through the gastrointestinal tract.  In pigeons the milk starts to be produced in the crop of the parent birds two days before eggs hatch.

The global gene expression profile comparison between 'lactating' crop and non-'lactating' crop showed that 542 genes are up-regulated in the 'lactating' crop, and 639 genes are down-regulated. Pathway analysis revealed that genes up-regulated in 'lactating' crop were involved in the proliferation of melanocytes, extracellular matrix-receptor interaction, the adherens junction and the wingless signalling pathway.

Based upon the gene expression study, the authors conclude that, "there is a hyperplastic response in the pigeon crop epithelium during 'lactation' that leads to localised cellular stress and expression of antioxidant protein-encoding genes."

Squabs are fed the ‘milk’ until they are around 10 days old. Once the young are weaned the ‘milk’ stops being produced.

Source Article: Histological and global gene expression analysis of the 'lactating' pigeon crop.  Meagan J Gillespie , Volker R Haring , Kenneth A McColl , Paul Monaghan , John A Donald , Kevin R Nicholas , Robert J Moore  and Tamsyn M Crowley.  BMC Genomics 2011,12:452 doi:10.1186/1471-2164-12-452. Published: 19 September 2011.
Additional Source: Deakin University, Australia

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