For my dissertation, I am exploring the role of male
color phenotypes in the evolution of an endemic Mexican lizard, Sceloporus minor. I will be testing
hypotheses for the role of sexual signal evolution as a mechanism of speciation
in this group using a combination of behavioral, morphological and
genetic approaches, both in the lab and in the field. Please contact me
if you're interested in learning more!
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I. Color Traits in Sceloporus
Lizards of the genus Sceloporus are excellent model systems for investigating the function of sexually dimorphic traits.
Sexual dichromatism in particular is a common phenomenon in this
genus, males usually exhibiting striking color patterns on the
throat and/or venter (and less often, the dorsum). For example, in most
species only males possess paired ventral patches (usually bright blue); these patches are ordinarily hidden from view,
but are revealed to conspecifics through stereotyped displays.
Sceloporus torquatus (male)
Sceloporus variabilis (male)
Restriction of ventral and throat coloration to the underside of
the male may be an
adaptation to selection from diurnal predators. Supporting this view,
dorsal skin in both sexes is usually relatively cryptic and sexually
monomorphic.
Ventral and/or throat patches may provide conspecifics with many
different kinds of information, including signaller sexual identity,
species identity, and reproductive status. In
a few species, females have secondarily evolved conspicuous patches.
Agonistic interactions between females seem to be more intense than in species where
females lack these
patches. In some other species, males appear to have secondarily
lost ventral color,
although postural displays are retained. In at least one such
species replacement of ventral color induced a flight response
in opponents (Quinn
and Hews 2000), suggesting a retained response to the lost signal, as well as an explicit role for ventral coloration in male contest outcome.
Females do not seem to exhibit preference for
conspecific males with patches over those without, in taxa with and
without male patches (e.g., Quinn 2001). Taken together, these observations
support the idea that the evolution of male sexually dimorphic color
traits in
lizards seems to have been guided more by male contest competition
than female choice.
II. Study Species: Sceloporus minor
Although sexually dichromatic features are normally restricted to the ventral and/or throat regions in Sceloporus, there are exceptions. For example, in the S. formosus
species group of the rainforests of Central America, males are bright
green dorsally, whereas females are typically a dull green or
green-brown. However, these dorsal phenotypes appear to be more or less
consistent within a species.
This is not the case in the central
Mexican endemic Sceloporus minor (sensu Wiens, et al. 1999). Across its range, this species exhibits tremendous dorsal phenotypic variation in males, ranging from
yellow-brown and brown-grey to a striking cobalt blue or
blue-orange pattern. In general, males in populations in desert
habitats exhibit a more dull brown or grey dorsal color, and tend to
resemble females in this regard. Conversely, males in more closed,
mesic habitats at higher elevations exhibit the blue or blue-orange
phenotypes (both occur in the same populations), whereas females
exhibit a brown or steel grey phenotype that strongly resembles that of females in desert habitats. In addition, there is evidence that the latter blue phenotypes have independently evolved
at least twice from a monomorphic brown state (Wiens and Penkrot
2002), suggesting that the blue or blue-orange combination may be
especially effective in sexual signalling, at least in certain habitats.
Sceloporus
minor (male)
Female Sceloporus minor (female)
III. Study Sites
In 2005, I initiated research in the beautiful pine-oak forests of Los
Mármoles National Park in NE Hidalgo state, about 150 mi NE of
Mexico City. This small park includes some of the best remaining high-elevation conifer forests in central Mexico. In addition to supporting
healthy populations of S. minor, no fewer than 15 other reptile and amphibian species (and probably at least twice that number) occur in the immediate
study region. I will provide photographs and an expanded checklist of all herpetofauna documented in
the park during my study in the near future. In addition, I intend to
spend several weeks this year at a site located just outside of San
Luis Potosí City, where a population of the cryptic, dorsally
monomorphic form exists.
IV. Field Methods
In
2005, I marked
and released some 55 adults on three study plots. For marking, I used a unique
sequence of
small colored beads to identify released individuals. I also collected
morphometric data, and used Munsell color chips to categorize male
dorsal and ventral color. After release, my assistant and I spent about 7-8 hours/day following lizards on each plot. We
collected information on movement,
behavioral displays, feeding, and interactions with conspecifics and
heterospecifics. Largely
due to an
extended period of poor weather in August, the number of observations
per lizards was low, inhibiting extensive analysis. However, available
data strongly suggest that fundamental aspects of male and female
behavior correspond to established paradigms for territorial
lizards: males display at higher rates than females, tend to have
larger home ranges, associate with multiple females, etc. It is still
unclear
at this time whether variation in blue-orange coloration actually
predicts male mating success in this population. However, I will be
continuing this research in 2006, incorporating a paternity analysis
with additional behavioral data to better address this question. I will
also be using a spectrophotometer to collect quantitative data on color
reflectance in males and females.
