For about 30 years, I practiced as a clinical neurologist in a Sydney teaching hospital. Neurology is a visual discipline and, when video cameras became available in the early 1980s, I began to video my patients, particularly those with interesting physical signs or those where there might be doubt about the diagnosis. The videos are published online by Oxford University Press as a Manual of Neurological Signs, with my colleague Paddy Grattan-Smith.
On retiring 12 years ago, my interest turned to birds, in particular the way they blink, and later to other animals. I have tried to bring the same discipline I followed in neurological practice, of careful observation coupled with a review of the literature, to the study of blinking. At this stage, it is not easy to explain why blinking has evolved differently across the species. To quote the eminent neurologist William Gowers, 'in the present state of my ignorance it seems more useful to gather facts than to formulate hypotheses'.
I have been able to set up this website with the help of my daughter Rebecca. My cousin Donald Morris, a zoologist, has been a wonderful source of knowledge and interpretation of the findings.
WHY WINKS & BLINKS?
Anyone who has observed birds at close quarters cannot fail to be struck at how frequently they blink. Why do they do this? I started to video birds to see if I could shed light on this curious phenomenon. Then I moved onto other creatures and more questions arose. Why do some creatures blink with their upper lids, others with their lower? Why do some have nictitating membranes? Why do some retract their eyeballs? So many questions! I started to read what had been written about on the subject but could find very little. There were tomes on vision and on eyes, but eyelids tended to be relegated to afterthoughts on 'adnexae' (the parts adjoining the organ of main interest). Much of the anatomical work had been done not in the last century but the one before!
This site catalogues the myriad ways in which blinking has evolved since fish left water behind and established themselves on land, and provides information on what is known about the anatomical basis for the different blinks. It has the advantage over standard methods of communicating this type of information, such as scientific journals, of being able to integrate videos freely with the text. The disadvantage is that there is no panel of referees to validate the information presented. I hope that readers may send me feedback and comments.
Blinking is something that we all do, an unconscious action of little apparent significance or importance. Yet blinking is not only essential for the health of our eyes, it played a key role in the emergence of our ancestors from the sea and in their adoption of a terrestrial life style.
The cornea and lens focus light on the photoreceptors of the retina. To do this, they must be transparent. This means they can have no direct blood supply, for blood vessels would impair their transparency. In fish, oxygen dissolved in water diffuses into the cornea and then across the aqueous fluid in the anterior chamber of the eye, to the lens.
Once fish began to venture onto land, they faced the problem that the surface of the cornea tended to dry out. Oxygen in the air cannot diffuse into the cornea unless it is wet. And so anatomical structures, eyelids, evolved which allowed the cornea to remain moist at all times. These structures also assumed other functions like protection of the eyes, which were now more vulnerable to injury as particles travel faster in air than in water. Eyelids being soft structures might shut out dust or wind but provide little protection from mechanical pressure or blows. Blinking in many species thus came to involve retraction of the eyeball into the relative safety of the orbit, where the bony skull could take the impact. In other species, prominent eyebrow ridges evolved under which the eyes could shelter.
Blinking involves transient eye closure. Sustained eye closure occurring during sleep poses a problem as the cornea, moist or not, no longer has access to atmospheric oxygen. This problem has been dealt with by providing both (the conjunctival surface of) the eyelids and nictitating membranes with a rich blood supply from which oxygen can diffuse into the cornea.
The ways in which blinking diversified is the subject of this site. Upper eyelids, lower eyelids, nictitating membranes, globe retraction – these are the available components of blinks. There are also fast blinks and slow blinks which are discussed in the section entitled 'Tonic and phasic blinks'. The role of blinking varies from species to species. Particular attention is given to birds, of which there are about 10,000 species, as these vary markedly in the way that they blink.
Most of the blinks demonstrated are spontaneous - recorded while observing animals without in any way interfering with them. Reflex blinking, in response to the cornea being touched was observed in one bird. In some birds, blinking also occurs during pecking.
This study borrows heavily from the classic monograph of Gordon Lynn Walls, The vertebrate eye and its adaptive radiation (1963). Blinking in a wide range of birds and other animals is illustrated using photographs taken from video stills and videos. The time scale is provided by the milliseconds after onset of the blink shown on each frame. Initially, videos were done at 25 frames per second, each frame having a duration of 40ms. Later, some videos were 100fps. The speed at which the film is played differs from the recording speed.