|Phrynosoma modestum, JCM|
The Phrynosomatidae, is a group of iguanian lizards that range
from Canada to Panama and contains some of the most familiar saurians in North
America, including: the spiny lizards (Sceloporus),
horned lizards (Phrynosoma), and
side-blotched lizards (Uta). The
family contains nine genera (ten genera if the genus Sator is recognized separately from Sceloporus) and more than 136 species. Weins et al. (2010)
used molecular techniques to show that phrynosomatids are divided into two
major clades the Phrynosomatinae and Sceloporinae. Phrynosomatinae contains the
horned lizard clade Phrynosoma and
the sand lizard clade (Callisaurus, Cophosaurus, Holbrookia, and Uma).
|Skull of a Phrynosoma with spiny processes.||JCM|
of the genus Phrynosoma are remarkably
cryptic. Their dorso-laterally flattened bodies, tan and brown coloration and
spine covered heads and bodies make them exceptionally difficult to find. The
spine covered heads and bodies of these lizards undoubtedly serve as a
deterrent to predators, but horned lizards also eat ants and store the noxious
formic acid from the ants in their blood. Yet some predators are able to deal
with the spines and chemicals. O’Connor, et al. (2010) report finding an adult female Great Basin Nightsnake (Hypsiglena
chlorophaea deserticola) in Kittitas
County, Washington that regurgitated a half-digested adult male Pygmy
Short-horned Lizard (Phrynosoma douglasii) estimated to be about 41 mm
SVL and 6 grams. The snake's mass was estimated to be just over 12 g.
so the predator-prey mass ratio was about 0.50.
Lahti, et al.
(2010) found Phrynosoma (Tapaja) douglasii
inhabiting 3 distinct microhabitat types (lithosol, loamy, and ecotone) within
the shrub-steppe of central Washington’s Quilomene Wildlife Area. The study
site had been used for grazing until 1979, and fires were minimal in the last
30 years. June and July were the peak activity months for lizards. Most
lizards were encountered in lithosol (61%), a habitat with sparse vegetative
cover and weathered fragments of rock; followed by ecotone (31%); and loamy
(8%) microhabitats. Lizards, particularly those inhabiting lithosol
microhabitats, did not usually retreat to shrub cover until approached within
1 m. While horned lizards are considered low-density species relatively
high population densities have been reported for Phrynosoma (Tapaja) douglasii (14.3
to 14.6 lizards/ha in eastern Idaho). However, the authors report a density of about
2 lizards per hectare at their study site, a density that is more
characteristic of that reported for other Phrynosoma
species. Neonates were almost always encountered closest to Thymeleaf Buckwheat
and would often retreat toward the plant when approached. Thymeleaf Buckwheat
has the smallest and most compact growth form of any perennial plant at
Quilomene Wildlife Area. In contrast, adults would usually retreat to either
Stiff Sage or Rock Buckwheat, both of which are larger. The authors conclude
that Phrynosoma (Tapaja) douglasii occurs at low densities
in the shrub-steppe of Washington where females are larger and more abundant
than males, neonates are rare, and reproductive output is low. Younger lizards
maintain activity into hotter periods and remain active later in the activity
season than do adults, a trait likely related to the importance of garnering
sufficient energy to emerge in good condition after a long winter. While this species is most commonly
encountered in shrub-steppe habitats, it shows considerable spatial and
seasonal variation in the use of microhabitats.
et al. (2010) describe
the effects of rotational livestock grazing and prescribed winter burning on the
resources and survival of the Texas Horned Lizard (Phrynosoma cornutum)
in southern Texas. Winter burning provided an increase in food resources and
led to increased survival rate in the second growing season after fire.
However, grazing-induced changes in vegetation cover reduced survival, probably
by increasing lizard vulnerability to predation. Fire and grazing reduced
litter, increasing open ground and forb cover but did not alter woody
vegetation. Ant activity was greater in burned sites and varied with grazing
level, season, and year. Higher survival observed on burned sites in the second
year after burning. Survival rates were ordered from highest in un-grazed sites
to lowest in heavily grazed sites.
In three papers Cooper
and Sherbrooke (2010a,b,c) investigated anti-predator behavior in the
Round-tailed Horned Lizard (Phrynosoma modestum) and the Texas horned lizard (Phrynosoma cornutum). They (Cooper and Sherbrooke, 2010a)
investigated the effects of repeated attacks by a predator on the Texas Horned
Lizard, P. cornutum, and the opportunity cost of fleeing during a
social encounter in P. modestum. The results suggest flight
initiation distance was greater the second time a predator approached and
probability of fleeing decreased as the distance between the predator and prey
increased, but was greater when the predator turned toward than away from a
lizard. The flight initiation distance was shorter during social encounters
than when lizards were solitary. It appears that risk assessment by horned
lizards conforms to the predictions of escape theory and is similar to that in
other prey despite their specialized defenses. The results suggest that escape
theory based on costs and benefits applies very generally, even to highly
cryptic prey with specialized defense mechanisms.
In a second paper,
Cooper and Sherbrooke (2010b) note that Phrynosoma
is eucryptic in that it resembles small stones and the authors predicted that
flight initiation distance by P. modestum is shorter among stones than
on uniform sand and that flight initiation distance is greater after movement
and when standing than when still and lying on the ground. Movement and upright posture disrupt crypsis
in this lizard. The authors measured running speed and flight initiation
distance to determine relationships among body temperature, speed, and escape
decisions. Running speed and flight initiation distance were reduced at lower
body temperature, suggesting that crypsis reinforced by immobility is more
advantageous than longer flight initiation distance for cool, slow lizards.
Thus, the Round-tailed Horned lizard adjusts its escape decisions to the current
effectiveness of crypsis and escape ability.
Sherbrooke (2010c) found that Texas horned lizards (Phrynosoma cornutum)
would take flight sooner when approached rapidly rather than slowly and when
approached directly rather than indirectly. They also found P.
cornutum were much more likely to move and jump when a model predatory bird
passed overhead and cast a direct shadow on them as opposed to casting a shadow
near the lizard. They suggest P. cornutum assess themselves as being in
immediate peril when suddenly covered by a shadow. So, while the Texas Horned
Lizard relies heavily on crypsis, they make escape decisions based on the degree
of predation risk.
Cooper, W. E.
and W. C. Sherbrooke. 2010a. Plesiomorphic Escape Decisions in Cryptic Horned
Lizards (Phrynosoma) Having Highly Derived Antipredatory Defenses. Ethology, 116: 920–928.
Cooper, W. E.
and W. C. Sherbrooke. 2010b. Crypsis influences escape decisions in the
Round-tailed Horned Lizard (Phrynosoma modestum). Canadian
Journal of Zoology,
Cooper, W. E.
and W. C. Sherbrooke. 2010c. Initiation of Escape Behavior by the Texas Horned
Lizard (Phrynosoma cornutum). Herpetologica 66:23-30.
Hellgren, E. C.,
A. L. Burrow, R. T. Kazmaier, and D. C. Ruthven. 2010. The Effects of Winter
Burning and Grazing on Resources and Survival of Texas Horned Lizards in a
Thornscrub Ecosystem. Journal of Wildlife Management 74(2):300-309.
Lahti, M. E., D.
D. Beck, and T. R. Cottrell. 2010. Ecology of the Pygmy Short-Horned Lizard [Phrynosoma
(Tapaja) douglasii] in Washington. Northwestern Naturalist 91(2):134-144.
2004. Geographic variation in Phrynosoma
coronatum (Lacertilia, Phrynosomatidae): further evidence for a peninsular
archipelago. Herpetologica 60 (1): 117-139
O'Connor, A. P.,
J. L. Wallace, R. E. Weaver, and M. P. Hayes. 2010. Pygmy Short-Horned Lizard (Phrynosoma
douglasii): Unrecorded Prey for the Great Basin Nightsnake (Hypsiglena
chlorophaea deserticola). Northwestern
Wiens , J. A.,
C. A. Kuczynski, S. Arif , and T. W. Reeder. 2010. Phylogenetic relationships
of phrynosomatid lizards based on nuclear and mitochondrial data, and a revised
phylogeny for Sceloporus. Molecular Phylogenetics and Evolution
Labels: community ecology, defensive behavior, evolution, Phrynosoma, Phrynosomatidae