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Abstract

Many descriptions of evolutionary adaptations are criticized as “just-so stories” ([ 1 ][1]) that are based more on intuition than on direct tests of adaptive hypotheses. The elaborate crowns of horns possessed by many species of horned lizards (genus Phrynosoma ) are classic examples of
How the Horned Lizard
Got Its Horns
Kevin V. Young,
1
Edmund D. Brodie Jr.,
1
Edmund D. Brodie III
2
*
Many descriptions of evolutionary adapta-
tions are criticized as “just-so stories” (1) that
are based more on intuition than on direct
tests of adaptive hypotheses. The elaborate
crowns of horns possessed by many species
of horned lizards (genus Phrynosoma) are
classic examples of intuitively adaptive fea-
tures that lack direct tests of function. The
bony horns that give horned lizards their
name are presumed to function as a defense
against predators (Fig. 1B). Here we present
data from the wild showing that natural se-
lection by loggerhead shrikes favors longer
horns (fig. S1) in the flat-tailed horned lizard
(Phrynosoma mcalli).
Predation is difficult to document in the
wild. Some predators, however, leave behind
explicit records of individual predation events
that can be exploited to assay natural selection.
Loggerhead shrikes (Lanius ludovicianus) often
impale their prey onto thorns, twigs, and even
barbed wire as a means of subduing their quarry
(2). When shrikes attack horned lizards, they
typically spear the lizard through the neck and
pull off the soft tissue. What remains is a record
of the successful shrike predation attempts
marked by desiccated skulls of horned lizards
hanging in trees and bushes (Fig. 1A).
We quantified selection (3, 4) on relative
horn lengths of flat-tailed horned lizards by
comparing the skulls (n 29) of shrike-killed
lizards with the heads of live lizards (n 155).
Our results showed predation by loggerhead
shrikes generated selection that favored longer
parietal and squamosal horns (Fig. 1, C and D).
The average parietal horn length of live horned
lizards was 10.0% longer (x SE : 9.65 0.01
mm) than that of shrike-killed lizards (8.77
0.21 mm), and the average squamosal horn
length was 10.4% greater in live lizards
(24.28 0.21 mm) than in those killed by
shrikes (21.99 0.49 mm). Visualization of the
selection function indicates that both traits ex-
perience positive directional selection with
threshold lengths above which predation is rare
or absent. Standardized selection gradients
[measured in standard deviation units (3)] sug-
gest that selection is stronger on the length of
squamosal (␤⬘ 0.0945; P 0.007) than on
the length of parietal horns (␤⬘ 0.0549; P
0.055). These magnitudes of selection are less
than the median observed in most selection stud-
ies (␤⬘ 0.15) (5) but nonetheless indicate that
constant selection with moderate heritability (0.5)
of horn length would change squamosal and pa-
rietal horn lengths a full standard deviation in 21
and 36 generations, respectively.
Modern methods for analyzing natural se-
lection have increased our understanding of
which traits experience selection (6). These
methods, however, typically cannot identify
agents of selection or reveal the functional re-
lations that result in natural selection (3). Even
most classic data sets demonstrating selection
in the wild, including Bumpus’s sparrows (7)
and Lande and Arnold’s pentatomid bugs (8),
did not reveal the agents responsible for the
observed patterns of survival. Our results
present a rare opportunity to link the statistical
form of selection to an identifiable agent, in this
case predation by shrikes. Our study does not
show that other agents and forms of selection
do not play a role in the evolution of horn
size, but clearly illustrates that defense
against shrike predation is one factor driving
the radical elongation of horns in some spe-
cies of horned lizards.
References and Notes
1. R. Kipling, Just So Stories (Doubleday, New York,
1902).
2. R. Yosef, Evol. Ecol. 6, 527 (1992).
3. E. D. Brodie III, A. J. Moore, F. J. Janzen, Trends Ecol.
Evol. 10, 313 (1995).
4. Materials and methods are available as supplemental
material on Science Online.
5. J. M. Hoekstra et al., Proc. Natl. Acad. Sci. U.S.A. 98,
9157 (2001).
6. J. G. Kingsolver et al., Am. Nat. 157, 245 (2001).
7. H. C. Bumpus, Biol. Lect. Woods Hole Mar. Biol. Sta.
6, 209 (1899).
8. R. Lande, S. J. Arnold, Evolution 37, 1210 (1983).
9. Funded by the Department of Defense Legacy Re-
source Management Program through the U.S. Ma-
rine Corps, Marine Corps Air Station, Yuma, and
administered by the Southwest Division Naval Facil-
ities Engineering Command, Natural Resources
Branch, and by the Bureau of Reclamation. Fieldwork
was facilitated by P. Cutler, W. Fisher, B. Morrill, R.
Palmer, R. Pearce, and A. Young.
Supporting Online Material
www.sciencemag.org/cgi/content/full/304/5667/65/DC1
Materials and Methods
SOM Text
Fig. S1
References
17 December 2003; accepted 11 February 2004
1
Department of Biology, Utah State University, Logan,
UT 84322–5305, USA.
2
Department of Biology, Indi-
ana University, Bloomington, IN 47405–3700, USA
*To whom correspondence should be addressed. E-
mail: edb3@bio.indiana.edu
Fig. 1. (A) Flat-tailed horned lizard skull and dorsal skin impaled on a branch. [Photo, E. D. Brodie
Jr.] (B) Live flat-tailed horned lizard in defensive posture. [Photo, K. V. Young] The live lizard in this
photo had unhealed wounds anterior to the rear legs, consistent with an unsuccessful attack by a
predator. Selection surfaces showing relations between survival probability and (C) relative parietal
horn length and (D) relative squamosal horn length. Bars show means and 95% confidence intervals
for shrike-killed and live lizards.
BREVIA
www.sciencemag.org SCIENCE VOL 304 2 APRIL 2004 65
... As the failure to avoid predation is definitive, i.e. death, evolution has produced a plethora of antipredator traits, ranging from physiology and morphology to specific behaviours (e.g. Cott 1940;Brodie Jr 1977;Ydenberg & Dill 1986;Brönmark & Miner 1992;Young, Brodie Jr & Brodie III 2004;Price, Friedman & Wainwright 2015;Hodge et al. 2018). Due to its importance, predator-prey interactions have for long been a central pillar in evolutionary biology and ecology and are in many ways still a vibrant research area. ...
... mimicry or warning signals (Cott 1940;Endler 1978). If detected, then secondary defences such as fleeing (Ydenberg & Dill 1986), freezing (Höglund et al. 2005) or to use toxic chemicals (Brodie Jr 1977;Bakus 1981) or a defensive morphology (Brönmark & Miner 1992;Young, Brodie Jr & Brodie III 2004;Hodge et al. 2018) can increase the chance of surviving the prey-capturing process. In this thesis, I have mainly focused on the latter, i.e. morphological anti-predator traits. ...
... In this thesis, I have mainly focused on the latter, i.e. morphological anti-predator traits. Such defence strategies may evolve over generations and become constitutively expressed in the phenotype irrespectively of the current risk situation, as for example body armour in the nine-spined stickleback (Välimaki, Herczeg & Merila 2012), the modified stiff and spiny hairs of hedgehogs (Brodie Jr 1977) or the bony horns in horned lizards (Young, Brodie Jr & Brodie III 2004). However, during the last decades it has become increasingly evident that some prey organisms also respond to predation risk by phenotypic plasticity in key defence traits, i.e. they have the ability to tune various aspects of the phenotype to the prevailing risk of predation (Tollrian & Harvell 1999). ...
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... Concerning a second line of defence, for example, Young et al. [14] quantified the selection of a bird predator on relative horn lengths of a lizard species. They demonstrated convincingly that defence against bird predation drove the elongation of horns in the investigated lizards. ...
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... These results suggest avian predators are a significant threat to Texas horned lizards, which is similar to findings in other studies that show that avian predators like shrikes consume large numbers of horned lizards and may even be responsible for the evolution of increased cranial horn lengths in flat-tailed horned lizards (Phrynosoma mcallii) (Munger, 1986;Young & Brodie, 2004). ...
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... The models with missing limbs may also be the result of a typical greater roadrunner "centrifugal-slam" attack in which the bird grabs a lizard by any limb or tail and smashes it on the ground (Sherbrooke 1990). These results suggest avian predators are a significant threat to Texas horned lizards, which is similar to findings in other studies that show that avian predators like shrikes consume large numbers of horned lizards and may even be responsible for the evolution of increased cranial horn lengths in flat-tailed horned lizards (Phrynosoma mcallii ) (Munger 1986, Young et al. 2004). ...
... The models with missing limbs may also be the result of a typical greater roadrunner "centrifugal-slam" attack in which the bird grabs a lizard by any limb or tail and smashes it on the ground (Sherbrooke 1990). These results suggest avian predators are a significant threat to Texas horned lizards, which is similar to findings in other studies that show that avian predators like shrikes consume large numbers of horned lizards and may even be responsible for the evolution of increased cranial horn lengths in flat-tailed horned lizards (Phrynosoma mcallii ) (Munger 1986, Young et al. 2004). ...
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Texas horned lizards (Phrynosoma cornutum) have a number of ways to avoid predation, including camouflage, sharp cranial horns, flattening of the body, and the ability to squirt blood from the eyes. These characteristics and their relatively low survival rates in the wild suggests these lizards are under high predation pressure. These lizards have been declining in much of their eastern range due to increased urbanization, agriculture, and loss of prey species. However, they can be still be found in some small south Texas towns where they can reach densities that are much higher (˜50 lizards/ha) than in natural areas (˜4-10 lizards/ha). We hypothesized that one reason for the high densities observed in these towns may be due to reduced predation pressure. We used model Texas horned lizards to test whether predation levels were lower in two south Texas towns than on a nearby ranch. We constructed models from urethane foam, a material that is ideal for preserving marks left behind by predators. Models (n = 126) and control pieces of foam (n = 21) were left in the field for 9 days in each location in early and late summer and subsequent predation marks were categorized by predator taxa. We observed significantly more predation attempts on the models than on controls and significantly fewer attempts in town (n = 1) compared to the ranch (n = 60). On the ranch, avian predation attempts appear to be common especially when the models did not match the color of the soil. Our results suggest that human modified environments that have suitable habitat and food resources may provide a refuge for some prey species like horned lizards from predators. Abstract: Texas horned lizards (Phrynosoma cornutum) have a number of ways to avoid predation, including camouflage, sharp cranial horns, flattening of the body, and the ability to squirt blood from the eyes. These characteristics and their relatively low survival rates in the wild suggests these lizards are under high predation pressure. These lizards have been declining in much of their eastern range due to increased urbanization, agriculture, and loss of prey species. However, they can be still be found in some small south Texas towns where they can reach densities that are much higher (˜50 lizards/ha) than in natural areas (˜4-10 lizards/ha). We hypothesized that one reason for the high densities observed in these towns may be due to reduced predation pressure. We used model Texas horned lizards to test whether predation levels were lower in two south Texas towns than on a nearby ranch. We constructed models from urethane foam, a material that is ideal for preserving marks left behind by predators. Models (n = 126) and control pieces of foam (n = 21) were left in the field for 9 days in each location in early and late summer and subsequent predation marks were categorized by predator taxa. We observed significantly more predation attempts on the models than on controls and significantly fewer attempts in town (n = 1) compared to the ranch (n = 60).
... For prey living in a world full of natural enemies, it is essential to perceive and accurately respond to impending danger. Predation is a primary force driving adaptation in prey, involved in the evolution of a broad suite of prey traits that reduce the risk of predation, including morphological (Brönmark & Miner, 1992;Hodge et al., 2018;Young, Brodie Jr., & Brodie III, 2004), physiological (Clinchy, Sheriff, & Zanette, 2013;Furtbauer et al., 2015;Sapolsky, Romero, & Munck, 2000) and behavioural adaptations (Creel, 2018;Lima & Dill, 1990;Neill & Cullen, 1974;Sih, 1980;Ydenberg & Dill, 1986). ...
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  • J M Hoekstra
J. M. Hoekstra et al., Proc. Natl. Acad. Sci. U.S.A. 98, 9157 (2001).
  • J G Kingsolver
J. G. Kingsolver et al., Am. Nat. 157, 245 (2001).
  • R Kipling
R. Kipling, Just So Stories (Doubleday, New York, 1902).
  • H C Bumpus
H. C. Bumpus, Biol. Lect. Woods Hole Mar. Biol. Sta. 6, 209 (1899).