Bird milk, the fluid that's produced in the crop of pigeons and a few other species of birds, nourishes their chicks. And now, scientists have found the gene expression involved, and may be able to extract milk from pigeons in the future. But why would you want to?
Although only mammals produce what a biologist would rigorously define as 'milk,' we're not the only type of animals that feed their young with secretions produced from their bodies. Some birds produce what's called 'crop milk.' The crop is a little pouch either somewhere just off the bird's throat or more towards the stomach. Primarily, the crop stores food before it is passed down for digestion. In some birds, it also produces a secretion that the parents regurgitate into the young bird's mouths. Flamingos produce this milk, and male Emperor penguins do after a long winter, before the females return to take over child-rearing. But both sexes of the common pigeon do as well — and for the first time we understand the genetics of how they do it.
The idea of drinking pigeon milk may bring a shudder to every sane and rational person in the world, but it's actually quite nutritious. Although it's high in fat, to help the young squabs develop fast, it's also packed full of antioxidants and immune-system-boosting proteins. (Considering pigeons spend their entire lives in urban squalor, they must have incredible immune systems.) The pigeon genome has never been sequenced, so scientists were flying blind when they tried to figure out exactly what genes caused pigeon 'lactation' for a paper to be published in BMC Genomics.
They looked at gene expression in pigeons that were and were not producing crop milk, and compared them to chickens' DNA. (Chickens, thank goodness, do not lactate or we'd all be drinking "milken" already.) They found that the lactating pigeons had over-expression of genes involved with immune response and antioxidant production - no surprise there. They also found genes involved in the production of triglycerides. These fats are stored energy, from any extra calories consumed during a meal, and circulate in small levels in the blood stream. They are produced by the liver, which suggests that the pigeon's liver helps the lactation process.
Knowing the genes involved may make scientists able to control the production of crop milk in birds. What they do with that can only be the stuff of nightmares. And, possibly, school lunches.
Image: Arnstein Ronning
Via BioMed Central.