Our Wisdom Of Birds (Or What Happens In Their Female Parts)

I came across one of my favourite stories about the history of scientific discovery when I was reading Tim Birkhead's wonderful book, The Wisdom of Birds. Birkhead is a zoologist and professor of behavior and evolution at the University of Sheffield, UK, and something he's particularly interested in is how we came to know what we know about birds.

One of the greatest examples of the incremental nature of scientific discovery in regards to birds is the story of how we've come to understand what goes on inside an egg, or ovum-the origin of new life. A really peculiar aspect of this in birds is what's known as prolonged fertilisation. Since the time of Aristotle, people have known that a hen can lay fertile eggs for several weeks after just one sexual encounter with a rooster, but it took centuries of scientific experimentation to explain how this occurs.

During the 17th Century, Italian anatomist Heironymus Fabricius de Aquapendente tried to explain prolonged fertilisation through a series of dissections, and came up with two explanations : either the eggs were all being simultaneously fertilised at the moment of insemination, or they were being sequentially fertilised, in which case there must be some kind of semen storage going on. He wasn't able to prove anything, and from this point onward, many began looking to the contents of the egg to discover the role of semen in the process of fertilisation, and at the reproductive tract of the females to determine when this fertilisation occurred.

Fabricius' student, William Harvey, of blood circulation fame, continued his master's work after his death, and this time did what Fabricius had not. He dissected hens right after they'd copulated. Each time, Harvey failed to find any visible semen stored anywhere, but he did manage to overturn the theory of Albertus Magnus, or Albert the Great, who proposed 300 years earlier that the chalazae-the two twisted, gelatinous strands that connect the egg white, or albumen, to the yolk-were the male's semen.

Despite being truly invested in unlocking the secrets of avian reproduction, Harvey and many of his peers, such as highly influential English naturalist John Ray, ignored the opportunities offered by the microscope. Microscopes had been available since the 1590s, but it wasn't until 1660s and 1670s that scientists were using them extensively. Discoveries in the field of fowl reproduction were ramped up during this time, and it was discovered that the egg's germinal disc-a visible 2-3mm in diameter white spot on the yolk-contained a nucleus, which stored some of the makings of a new organism, and that semen contained sperm cells, or spermatozoa. Antony van Leeuwenhoek, who referred to the sperm cells as animalcules, proposed that they created an embryo by combining with an egg.

With the passing of time came better quality microscopes with smarter designs, and with this it became widely accepted that the semen of all animals and birds contained millions of sperm, and that, like Leeuwenhoek had suggested, played a crucial role in fertilisation. This was contrary to what Ray believed, because being deeply religious, he couldn't bear the idea that God would be so wasteful in the design of semen. In 1875, German biologist Oskar Hertwig came up with the idea to study the transparent eggs of sea urchins to observe for the first time the journey of a sperm as it penetrates an egg to fuse with the nucleus within.

A student named Eugene Harper, who worked with American zoologist Charles Otis Whitman in the 1890s, managed to observe fertilisation occurring in birds, not sea urchins, by taking a different approach. Whitman lived with several hundred birds, and particularly valuable to him were his pigeons. Unlike other birds, pigeons are known to punctuate the laying of their eggs in a predictable fashion-the first in the late afternoon and the second in the early afternoon two days following.

Harper recognised that by dissecting these birds at different stages throughout the day, he could observe for the first time that just a few hours after the first egg is laid, the second is released from the oviduct, through which it will travel, gathering coatings and membranes to form its hard shell. "By focussing on the second ovum, removing the germinal disc and examining it under the microscope, Harper could see that fertilisation had occurred," writes Birkhead. "Finding that the second ovum was fertilised almost before it was fully released from the ovary, Harper was the first to demonstrate that fertilisation in birds occurred in the uppermost region of the oviduct."

So Harper had confirmed that fertilised eggs were being released from the ovary in neat succession, but how did this occur for days and weeks after insemination? During the 1900s, scientists were still tossing up between Fabricius' simultaneous and sequential fertilisation theories. In 1920, an experiment was conducted that saw recently inseminated hens have their oviducts flushed with spermicide. This was to determine whether the eggs were fertilised simultaneously as soon as they were inseminated, in which case they'd continue to lay eggs despite the spermicide, or if they were storing sperm in their oviduct, in which case there would be no fertilised eggs in the coming days.

While the hens continued to lay fertilised eggs, spontaneous fertilisation was not the answer. Centuries after Fabricius first proposed the idea that hens had semen stores or 'nests', biologist Peter Lake, of the Poultry Research Centre in Edinburgh, discovered a crevice lower down in the hen's oviduct, near the junction of the vagina and the shell gland that was packed with sperm. Over the next 20 years, scientists discovered similar crevices in a number of wild bird species too. Incredibly, these crevices kept the sperm safe from being flushed out with the spermicide.

It's common knowledge now in avian biology that between 24 and 48 hours before they're laid, all female birds release each ovum from the ovary to be fertilised at the top of the oviduct. The ovum has just 15 minutes to be fertilised as it sits here, which makes sperm storage the perfect solution (as opposed to mating with a male at that exact moment). Muscle contractions help move the sperm up from their crevices to meet the ovum and fertilise it. It's an incredibly sophisticated system, so as humble as a hen or a pigeon might be, it's fitting that humans spent literally centuries trying to figure it out.

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Image credit: Top: Biodiversity Heritage Library (from Zur Fortpflanzungsgeschichte der gesammten Vögel by Friedrich August Ludwig Thienemann); Bottom: University of Wisconsin-Madison.