Editorial Type:
Article Category: Research Article
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Online Publication Date: 01 Dec 2001

Blood-Squirting Variability in Horned Lizards (Phrynosoma)

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Page Range: 1114 – 1122
DOI: 10.1643/0045-8511(2001)001[1114:BSVIHL]2.0.CO;2
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Abstract

Variability within the genus Phrynosoma in the occurrence of ocular-sinus blood-squirting, reportedly a defense used in canid encounters, is reviewed from the literature. Six species have been reported to squirt blood, and seven species remain unreported. Five of the latter species were tested in dog trials; one exhibited blood-squirting (Phrynosoma hernandesi), one exhibited precursor behaviors but failed to squirt blood (Phrynosoma ditmarsi), and three yielded negative results (Phrynosoma mcallii, Phrynosoma modestum, and Phrynosoma platyrhinos). Instances of blood-squirting in response to human encounters were collected and largely support the negative results for the three species P. mcallii, P. modestum, and P. platyrhinos. A phylogeny of blood-squirting and nonblood-squirting species is presented with blood-squirting being plesiomorphic in the genus and the synapomorphic condition of nonsquirting species being restricted to a single clade of P. mcallii-modestum-platyrhinos. The possibility of P. douglasii independently evolving an autapomorphic condition remains unresolved. Dog trials with 40 adult Phrynosoma cornutum were conducted to determine influences of body size and sex on squirt frequency and blood mass expelled, as well as to examine aspects of the potential physiological cost of the defense. In 153 trials, 85% of all lizards squirted in at least one trial, 82% squirted in more than one trial, and two lizards squirted daily over the seven-day trial period. Initial body mass positively correlated with the total number of squirts/individual (r2 = 0.28; P < 0.001) and the number of days a lizard continued squirting (r2 = 0.63; P < 0.01). Number of squirts/individual/day declined over the seven-day trial period (r2 = 0.20; P < 0.05). Cumulative mass loss for individual lizards attributable to blood-squirting averaged 0.7 ± 0.8 g (2.0 ± 2.0% body mass), with a high of 2.8 g (6.8% body mass). In addition, juvenile P. cornutum and P. hernandesi were shown to squirt blood in dog trials, illustrating the early developmental onset of the behavior.

Copyright: The American Society of Ichthyologists and Herpetologists
 Fig. 1. 
 Fig. 1. 

Regression of lizard (Phrynosoma cornutum) body mass on trial day (r2 = 0.07, P = 0.11 as illustrated; r2 = 0.23, P = 0.004 with outlier omitted)

 Fig. 2. 
 Fig. 2. 

Influence of lizard (Phrynosoma cornutum) body mass on the total number of blood squirts recorded (r2 = 0.09, P ;cf 0.08 as illustrated; r2 = 0.28, P = 0.001 with outlier omitted)

 Fig. 3. 
 Fig. 3. 

Regression of number of lizard (Phrynosoma cornutum) blood squirts per trial (open circles, dashed line; r2 = 0.20, P < 0.05) and loss of mass (g) (solid triangles, solid line; r2 = 0.81, P < 0.001) on trial day

 Fig. 4. 
 Fig. 4. 

Mass of squirted blood (from the eye orbits of Phrynosoma cornutum) as a function of number of blood squirts elicited during trials in which blood was squirted (r2 = 0.66; P < 0.001)

 Fig. 5. 
 Fig. 5. 

Blood-squirting and nonblood-squirting species of Phrynosoma adapted to a cladogram of the genus (Zamudio and Parra-Olea, 2000, as adapted from Montanucci, 1987, and Zamudio et al., 1997). The Phrynosoma mcallii-modestum-platyrhinos clade, in gray, is hypothesized to have lost blood-squirting as an antipredator defense. The blood-squirting status of two species remains unresolved: Phrynosoma douglasii* may or may not have lost this mechanism independently, and Phrynosoma braconnieri* is predicted to be a blood-squirting species. All other species in the genus are known to squirt blood, and no outgroup members squirt blood from the ocular sinus

Accepted: 04 May 2001
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