Sceloporus
occidentalis lizards, also known as Western fence lizards, are a frequent sight at
Harvey Mudd College and the surrounding Claremont area, including the nearby
Bernard Field Station. As a research assistant working with them, I’m used to
finding them camouflaged against some tree or resting on a flat surface. I’m even
more used to them running away from my partner and me when we’re trying to nab
them with a capture noose for the experiment. We have both often observed that
the lizards varied in how close they allow us to get before they start fleeing.
A lightly colored adult fence lizard on a tree in the Claremont Colleges, taken in September, 2013.
A darker-colored lizard, though not as dark as many others we've seen.
2/2, 11:00-11:05 – sun and clouds, 77o F –
Scripps – A Western fence lizard resting on an elevated part of the sidewalk
near the Scripps music buildings, about one foot above the ground. It was an
adult that was very dark in color, in contrast with the grey-white of the
sidewalk, but it was right next to some vegetation cover to duck under. The
lizard had a FID of approximately 2 feet and scuttled right under the shrubs.
2/4, 1:25-1:30 – sun, 77o F – Bernard
Field Station – A juvenile was perched sideways in the recovering coastal sage
scrub area, around two feet up on a tree, enjoying some sun. The body was not
very dark in coloration and so it was camouflaged pretty well. I approached it
at a moderate pace and it ran around the tree at around 2 feet. Slowly edging to
the other side, I found the same lizard on the back of the tree in the shade
but away from other forms of possible cover. I reached out with a hand instead at
a slightly slower speed, and managed to close the distance to perhaps 2-3
inches from it before the juvenile fled.
The first juvenile at the Bernard Field Station, with nice cryptic coloration going on.
There was a juvenile perched on a tree branch about
2-3 feet up with both sun and shade. It fled at a distance of around 1 meter
with my decently paced approach, but had quite the way to go to find cover.
A dark adult fence lizard was on a fallen branch in
a setting with both sun and shade. That fallen branch was surrounded by brush
and cover. The Sceloporus occidentalis lizard
had a flight distance of about 1 foot.
2/10 – 12:10-12:15 PM – 76o degrees –
Harvey Mudd College – A light-colored adult was in pushup (really more like
downward dog) position against the sunny part of a tree near the LAC. It had a
flight initiation distance of 1 meter.
The FIDs varied quite a bit even among only five
lizards, ranging from inches to meters, and I wondered about what might be
causing this variation. As it turns out, there are several possible reasons. Physical
reasons include cryptic coloration and size (Johnson 1970). The more
camouflaged and the smaller a lizard is, the smaller their FID is. The
dark-colored adult lizard from Scripps and the one in the field station were
much less camouflaged than the lighter adult at Harvey Mudd, yet both had smaller
FIDs. Small size and cryptic coloration make it easier for lizards to hide and
movements like running would alert predators to their location, so the
well-hidden fence lizards tend to run later than lizards that are bigger and
not camouflaged as well. With that in mind, the FIDs of these three lizards
seem rather strange. The Mudd lizard should have had a smaller FID than the
lizards with less cryptic coloration. However, there are also environmental
factors that influence the flight initiation distance. These include degree of vegetation
cover, distance to refuge, and height of perch (Samia et al. 2015). The Scripps
lizard, the field station adult, and the first juvenile were much closer to
plants to run under than the Mudd adult and the second juvenile. The first
three were also in areas with greater amounts of cover in general.The more cover there is, and the shorter the distance to some kind of refuge, the lower the FID. The observations do seem in accord with this trend.
A very big adult fence lizard behind the Case dorms.
One difference between the lizards at the Bernard Field Station and the lizards in the Claremont Colleges, particularly the ones around Harvey Mudd College, is that the lizards at the Bernard Field Station cannot be removed from the area without permission and so are less likely to have been noosed. As Lizard Lab takes place each year in the fall, the western fences at Harvey Mudd tend to be caught more often. The research that goes on in any given semester, along with the occasional thermoregulatory lab for the introductory biology lab class, adds onto the number of people noosing lizards, the number of times a lizard can possibly get caught, and the number of lizards that experience being noosed. These noose captures may have become a factor themselves since the density of predators and the repetition of “attacks” can also influence FIDs by causing the lizards to flee earlier (Samia et al. 2015). The prevalence of captures in Mudd could be lengthening the population’s flight initiation distances. This suggests two related experiments. The first experiment would see if there was actually a statistically significant difference in flight initiation distance between lizards in a population that is exposed to nooses frequently and lizards in a population where nooses are used infrequently. The second would test whether it was the nooses or the humans that accounted for any differences. In both cases, controlling for the other factors that affect FIDs may be difficult, but I think these experiments could lead to some rather interesting endeavors and results.
Further Reading:
Johnson, C. R. 1970. Escape Behavior
and Camouflage in Two Subspecies of Sceloporus
occidentalis. American Midland Naturalist 84. 1: 280-282. http://www.jstor.org/stable/2423755
Samia,
D. S. M., Blumstein, D. T., Stankowich, T. and Cooper, W. E. 2015. Fifty years
of chasing lizards: new insights advance optimal escape theory. Biological Reviews. 1: 2-19. http://onlinelibrary.wiley.com/doi/10.1111/brv.12173/pdf
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