A meerkat and kestrel
A preliminary study comparing the nictitating membrane blink of a mammal with a bird
Abstract: As part of a larger study of the blinking which occurs during head turns in birds, the duration and timing of nictitating membrane blinks with head turns were measured in a nankeen kestrel (Falco cenchroides) using 100fps video. The onset of blinks coincided with or preceded the onset of head turns. The longer the head turn took, the longer was the blink. Similar studies were performed on a meerkat (Suricata suricatta), a mammal which also blinks with its nictitating membrane on head turns. Here the onset of head turn preceded the onset of the blink and the duration of the blink did not increase with the duration of the head turn. These findings suggest that blinking in the kestrel fulfills a different or additional function to blinking in the meerkat.
Introduction and methods: On a visit to the cliffs overlooking Long Reef on the coast of Sydney, an opportunity arose to observe at close quarters a semi-wild nankeen kestrel which was being fed by one of the locals. Like most birds, it blinked with its nictitating membrane when it moved its head. It was video-ed at 100 frames per second using a Panasonic Lumix DMC-FZ200 Digital Camera. Videos were analysed frame by frame using Cyberlink PowerDirector and VLC (VideoLan Client) Media Player video editing software. This was used to provide a measure of the duration and timing of blinks and head turns, each video frame representing 10ms. Not long afterwards, on a visit to Taronga Zoo, a meerkat on sentinel duty was also video-ed while blinking with its nictitating membrane on head turns.
Results and discussion:
The onset of the blinks in the kestrel largely fell within the same 10ms video frame as the onset of the head turns; a few began in the previous frame ie 10ms before the onset of the head turn (Figure 1). More striking was the finding that the longer the head turn took, the longer was the blink (Figure 2). In effect, vision was partially occluded by the semi-transparent nictitating membrane for most of the head turn. Blinks also occurred with the head stationary, and some head turns were not accompanied by blinks.
Figure 1. Durations (ms) of head turns, intervals (between head turns and blinks) and blinks in meerkat (38 blink/head turn pairs) and kestrel (28 blink/head turn pairs). In the meerkat, the blink onset follows the onset of the head turn. In the kestrel, the blink and head turn onsets largely coincide. The blink duration of the meerkat and kestrel are, respectively, 40 and 87.5% of the head turn duration.
Figure 2. Head duration as a function of blink duration in a kestrel. This shows a strong correlation (R² = 0.8372; p<0.00001). Observation duration: 82 seconds. Head turns with no blinks: 1. Blinks with no head turn: 0. Blinks with head turn: 28.
Analysis of the video taken of the meerkat showed some differences. Here, the onset of the head turn preceded that of the blink (Figure 1) by a median value of 50ms. The median duration of the blink was 100ms compared with a median duration of 250ms for the head turn (Figure 3) with no correlation between the two. Head turns without accompanying blinks were common but no blinks were observed while the head was stationary.
Sequential 10ms frames from the video sequences of the meerkat and kestrel are shown in Figure 4.
Figure 3. Head duration as a function of blink duration in a meerkat. This shows no correlation. Blink durations cluster around 100ms with two outliers. Observation duration: 126 seconds. Head turns with no blinks: 26. Blinks with no head turn: 0. Blinks with head turn: 38.
Figure 4a. Sequential (left to right) 10ms frames of meerkat head turn and blink. The onset of the head turn precedes the onset of the blink
Figure 4b. Sequential (left to right) 10ms frames of kestrel blink and head turn. The blink begins before the head turn.
So what conclusions can be drawn from these observations? The meerkat evolved in the extreme heat and dry dust of the Kalahari Desert. Clearing dust and moistening the cornea with each head movement using the nictitating membranes would seem to be a beneficial adaptation. The trigger for the blink appears to be the head turn, as blinks were not seen with the head stationary.
But what of the kestrel? Here the blink largely coincided with and lasted almost as long as the head turn. These features suggest that blinking in the bird fulfills a different or additional function than in the mammal. Perhaps it prevents images that are blurred by head movement from reaching the retina. This was first proposed by Mowrer and colleagues (Dunlap and Mowrer 1930; Mowrer 1933) following Dodge’s seminal observation (1900) that vision is suppressed during human saccades. Perhaps it is time to revisit Mowrer’s hypothesis.
Dodge, R. (1900). Visual perception during eye movement. Psychological Review 7, 454-465
Dunlap, K. and Mowrer O.H. (1930). Head movements and eye functions of birds. Journal of Comparative Psychology 11, 99-113.
Mowrer, O.H. (1933). A comparison of the visual function of nictitation and blinking. Journal of Comparative Psychology 15, 75-94
I thank Taronga Zoo for giving permission to use the photographs of the meerkat