Behavioral Ecology/Evolution III
Sunday March 13:
10:00am-11:30am
Back to Concurrent Session
III
ABSTRACTS |
ENDOCRINE-IMMUNE INTERACTIONS IN MALE Sceloporus
LIZARDS: TESTING AN ASSUMPTION OF THE IMMUNOCOMPETENCE
HANDICAP HYPOTHESIS
Chakraborty, M. and D. K. Hews
Department of Ecology and Organismal Biology, Indiana
State University,
Terre Haute, IN 47809
Variation in male secondary sexual
traits contributes to explaining variation in reproductive
success. Regulation of secondary sex traits is often
mediated by the endocrine system, especially testosterone.
Although high testosterone can enhance sexual characters
and competitive ability, it may also prejudice immune
function, a key assumption of the “immunocompetence
handicap hypothesis”. Little work in reptiles
has tested these assumptions. We experimentally assessed
the testosterone immunosuppression assumption in adult
male Sceloporus undulatus lizards captured during
the breeding season. We manipulated circulating androgen
levels and then measured two aspects of immune function
that reflect humoral and cellular responses, respectively.
We surgically castrated males and/or implanted them
with testosterone capsules to alter plasma androgen
levels. Following primary and secondary immunizations
with a standard immunogen (phthalate-conjugated Keyhole
Limpet Hemocyanin, pt-KLH), we measured humoral antibody
responses with a competitive ELISA. We compared this
to humoral responses produced using the sheep RBC
hemagglutination protocol, to assess the efficacy
of each protocol. To estimate cell-mediated immune
responses, we used a delayed-type hypersensitivity
test involving subcutaneous injections of antigen
(pt-KLH) versus saline and measuring differences in
swelling 24-hrs post-injection. Swelling primarily
reflects T-cell migration into the injection site.
Using radioimmunoassay, we measured plasma levels
of testosterone and corticosterone to validate the
hormone manipulations and to assess any immunosuppressive
effects of high corticosterone, which could increase
due to immune challenges and/or surgery treatments.
Keywords: Testosterone, immunocompetence,
lizards, pt-KLH
|
|
|
EFFECTS OF TESTOSTERONE AND IMMUNE CHALLENGE ON WHITE
BLOOD CELL COUNTS IN MALE Sceloporus undulatus
LIZARDS
Zaina S. K. and Hews, D.K.
Department of Life Sciences, Indiana State University,
Terre Haute, IN 47809
Testosterone affects behavior and morphological
traits important in sexual selection. Testosterone
also plays a role in the development and functioning
of immune system components. The immunocompetence
handicap hypothesis (Folstad and Karter, 1992) proposes
that tradeoffs, mediated by plasma levels of testosterone,
exist between the immune system and the elaboration
of secondary sexual traits. Endocrine-immune interactions
are well documented in many species, and steroid hormones
can be immunoenhancing or immunosuppressive. Relatively
little work has examined this in reptiles. Using testosterone
implants and/or castration, we asked if circulating
levels of testosterone cause changes in immune parameters
in adult male Sceloporus undulatus lizards. We assessed
how an antigenic challenge (phthalate-conjugated Keyhole
Limpet Hemocyanin) affects these measures in males
with and without altered testosterone. We calculated
the average proportion of each WBC type, total WBC
counts, hematocrit, and heterophil to lymphocyte ratio
(H:L ratio). The ratio of these two most abundant
circulating leukocytes is used in many studies as
a diagnostic indicator of stress (Gross, 1981). We
found that total WBC counts, the proportion of heterophils,
and the H:L ratio differed among treatment groups
after testosterone manipulation and immune challenge.
|
|
SIZE
MATTERS, BUT BIGGER IS NOT ALWAYS BETTER:
FEMALES, PREDATORS AND GENITAL EVOLUTION
(Honorable mention!)
Langerhans, R.B.1, C.A. Layman2, and
T.J. DeWitt3
1 Department of Biology, Washington University, St.
Louis, MO 63130
2 Department of Ecology & Evolutionary Biology,
Yale University, New Haven, CT 06520
3 Department of Wildlife & Fisheries Sciences, Texas
A&M University,
College Station, TX 77843
Male genitalia may experience more rapid,
divergent evolution than any other animal character—but
why? Research during the past several decades has culminated
in the view that genital diversification primarily results
from postmating sexual selection (e.g. sperm competition,
cryptic female choice). However, the potential roles
of premating sexual selection (e.g. mate choice) and
natural selection have received little attention. We
examined the possible importance of these mechanisms
by investigating divergence in male genitalia between
populations differing in predator regime for two species
of livebearing fish (Gambusia affinis in Texas, USA
and G. hubbsi in the Bahamas). Controlling for body
size, males exhibited a larger gonopodium (sperm-transfer
organ) in predator-free environments than in predatory
environments—a trend that persisted across space
(multiple populations), time (multiple years), and species.
By conducting laboratory experiments with G. affinis,
we found that premating sexual selection appears to
favor larger male genitalia (females exhibited mating
preference for males having larger gonopodia), but natural
selection in the presence of predatory fishes seems
to favor reduced genital size (larger gonopodium size
was associated with reduced burst-swimming performance,
an important antipredator behavior). These findings
contrast with the prevailing views regarding genital
evolution, and suggest a need to reconsider the current
paradigm for genital diversification.
Keywords: phenotypic differentiation,
natural selection, predation, sexual selection |
|
ALL
THE RIGHT MOVES: PHYLOGENETICALLY
WIDESPREAD NATRICINE PROMISCUITY
Wusterbarth, T.L.1, R.B. King1, M.R.
Duvall1, W.S. Grayburn1, and G.M. Burghardt2
1 Department of Biological Sciences, Northern Illinois
University, DeKalb, IL 60115
2 Department of Psychology, University of Tennessee,
Knoxville, TN 37996
Promiscuity and the occurrence of multiple
paternity within litters of offspring are characteristic
of many species within the animal kingdom, and are analogous
to the mating strategies of cross-pollinating plants.
Specific knowledge of reptile mating systems, however,
is lacking. Previous independent investigations determined
multiple paternity to be characteristic of four disjunct
natricine snake species, including Thamnophis sirtalis,
Nerodia sipedon, Thamnopis butleri, and Thamnophis elegans.
We investigated the various untested clades and subclades
of North American natricine snakes to determine the
phylogenetic extent of multiple paternity within this
subfamily. Representative species tested included members
of the semifossorial clade (Storeria dekayi, Storeria
occipitomaculata), watersnake clade (Nerodia rhombifer,
Regina septemvittata) and gartersnake clade (Thamnophis
radix – widespread subclade; Thamnophis sauritus
– ribbon and common gartersnake subclade; Thamnophis
melanogaster – Mexican subclade). Using primers
originally developed for Thamnophis and Nerodia, DNA
from litters with known mothers was amplified at highly
variable microsatellite loci. After identifying maternal
alleles in offspring genotypes, the number of paternal
alleles per litter was counted. The presence of more
than two paternal alleles (the maximum when a sire is
heterozygous) within any single litter indicated that
multiple paternity is widespread throughout Natricinae,
with the exception of the Mexican subclade, represented
by T. melanogaster. This study is the first to examine
multiple paternity across a widespread, related group
of vertebrates.
Keywords: multiple paternity, snake,
natricine, microsatellite DNA. |
|
|
Back to Concurrent Session
III
|
