[updated September 17, 2014]
|Vikram Kode Iyengar|
|Department of Biology||
|800 Lancaster Avenue|
|Villanova, PA 19085|
|office: (610) 519-8081|
|fax: (610) 519-7863|
|Behavioral Ecology, Chemical Ecology, Entomology. My research involves studying the behavioral ecology of arthropods, with an emphasis on sexual selection in insects. Arthropods are the most abundant and diverse group in the animal kingdom, and they occupy nearly every ecological niche in marine, freshwater and terrestrial habitats. The extraordinary evolutionary success of arthropods can be partly attributed to the remarkable diversity of mating systems, and these fascinating creatures provide many wonderful opportunities to do both field and laboratory studies. My research is driven by my interests in sexual selection and its consequences, and I study many different organisms that communicate through chemistry including moths, beetles and crustaceans. Specifically, I examine how the costs and benefits of mate choice and differences in parental investment shape the evolution of arthropod mating systems.|
|Utetheisa ornatrix (Lepidoptera: Arctiidae): The Rattlebox Moth|
|Sexual selection is an important area of behavioral ecology that explains phenomena including exaggerated male traits, female mating preferences, precopulatory courtship signals, and postcopulatory sperm selection. A great deal of my research is focused on the reproductive biology of Utetheisa ornatrix, commonly known as the rattlebox moth. Tom Eisner introduced me this fascinating lepidopteran during graduate school at Cornell University, and I have been working to unravel its mysteries ever since.|
|The details of this moth's reproductive strategy are complex. Utetheisa, as a larva, feeds on plants of the genus Crotalaria (family Fabaceae), containing poisonous pyrrolizidine alkaloids. Utetheisa is insensitive to the alkaloids, and the larva stores the chemicals systemically, retaining them through metamorphosis into the adult stage. At mating, the male transfers a substantial fraction of his alkaloidal load to the female with the sperm package (spermatophore). The gift is transmitted by the female in part to the eggs, together with a supplement of her own alkaloidal supply. All developmental stages of Utetheisa are protected by the alkaloid, the larvae and adults against spiders, and the eggs are avoided by ants and coccinellid beetles. The spermatophore is of substantial size, amounting on average to over 10% of male body mass. It also contains nutrient, which the female invests in egg production. Females mate on average with four to five males over their lifespan of 3 to 4 weeks. With each mating, the female is able to increase her fecundity by 15%. Fecundity in Utetheisa is also a function of intrinsic female body mass: large females lay greater numbers of eggs.|
|Female Utetheisa do not mate randomly with males but do so selectively with males of higher alkaloid content. The female does not gauge male alkaloid content directly but does so indirectly, on the basis of a pheromone (hydroxydanaidal) that the male produces from alkaloid, in proportion to his alkaloid load, and airs during close-range precopulatory interaction with the female. Males richest in alkaloid, having the strongest pheromonal scent, are also largest, and apt to bestow the largest alkaloidal (and presumably nutritive) gifts. In essence, by selecting males of high alkaloid content, the female is selecting males of large size.|
|My doctoral research established that body size is heritable in Utetheisa. This finding indicated that by favoring larger males, females obtain not only larger nuptial gifts but also larger sons and daughters. The offspring, as a consequence, could receive direct phenotypic benefits (from the nuptial gifts) and indirect genetic benefits (from the expression of largeness in both sons and daughters). Further investigation revealed that the offspring of preferred males do indeed fare better than the offspring of non-preferred males. Specifically, I showed that (i) eggs sired by preferred males are less vulnerable to predation; (ii) sons of preferred males are more successful in courtship; and (iii) daughters of preferred males are more fecund. In Utetheisa, multiple benefits appear to have contributed to the female mating preference for large males, and I am interested in how direct and indirect benefits may interact in the evolution of female choice.|
|As a postdoctoral associate, I continued my research in the areas of behavioral and chemical ecology. My main projects involved testing sexual selection models by quantifying the strength, heritability, and sex-linkage of the female mating preference (for large males) in Utetheisa. In Utetheisa, as in all lepidopterans and birds, males are homogametic (ZZ) whereas females are heterogametic (ZW). Interestingly, the female mating preference gene(s) are Z-linked, which means that they are strictly paternally inherited. In addition to providing support for the protected invasion theory, which links exaggerated male traits with genetic architecture, this result highlights the importance of indirect genetic benefits in the evolution and maintenance of sexual selection in this moth.|
I am currently working on two major projects regarding sexual selection in Utetheisa ornatrix. The first project involves the use of microsatellites to determine whether female polyandry may driven selection for male sperm competitiveness - this represents one of the few empirical test of the sexy-sperm hypothesis to explain multiple mating by females. The second project focuses on larval competition, which is likely to be the heritable trait that drives sexual selection in adults. Utetheisa has been studied for over three decades, and previous research on all aspects of its life history have made it a model system for studying the chemical basis of reproductive behavior. With these projects, I hope further our understanding of fundamental principles of sexual selection as I lay the foundation for future experiments that will advance the expanding, interdisciplinary field of chemical ecology.
|Anisolabis maritima (Dermaptera: Anisolabididae): The Seaside or Maritime Earwig|
|We are currently conducting experiments in which we examine
sexual selection in the seaside or maritime earwig Anisolabis maritima
(Order Dermaptera). This insect, found in aggregations under beach debris in
both tropical and temperate regions around the world (including both coasts
of North America), is sexually dimorphic regarding its most distinctive
feature in that females have straight posterior forceps/pinchers whereas
males have asymmetrical, curved forceps (see picture on right - female
above, male below).
Males and females also differ fundamentally in their aggression during agonistic encounters. Females lay eggs and violently guard their nest, often killing conspecifics by using their straight forceps like scissors to cut opponents into pieces. Males, on the other hand, usually resolve their disputes non-lethally by squeezing each other, perhaps a means to assess strength and fighting ability (see picture below).
Two males fighting (from Munoz & Zink 2012)
|Animal aggregations can occur for a variety of abiotic factors, such as resource limitation, or biotic factors including sexual selection and predator-prey interactions. The maritime earwig is often found in clusters among members of its own species as well as other beach insects, and our research is primarily done over the summer at Friday Harbor Marine Laboratories in Washington. We are continuing our research on sexual selection in the maritime earwig by examining the roles of mate choice and competition, as there is potential for multiple selective forces to contribute to the mating system in species where both sexes possess weaponry. We will be presenting our results in January 2015 in West Palm Beach, FL at the annual meeting for the Society for Integrative and Comparative Biology (SICB), and submitting a manuscript for publication soon thereafter.|
|Calopteryx aequabilis (Odonata: Caloperygidae): The River Jewelwing|
|Sexual selection is defined as the differential reproductive success due to competition over access to mates, and it can be divided into categories of intersexual selection (female choice) and intrasexual selection (male competition). Within species, sexual selection can lead to exaggerated male traits used in attracting females or intrasexual combat. Between species, it can maintain separate species through reproductive isolation even in areas where both species occur. We are currently interested in precopulatory forces that prevent interspecific matings - specifically, we studied intrasexual aggression by territorial males to gain insight into reproductive isolation between the damselfly species Calopteryx aequabilis (The River Jewelwing) and Calopteryx maculata (The Ebony Jewelwing). The importance of intrasexual selection is supported by male wing spot variation in both damselflies - the wings look similar in allopatric parts of their ranges (places where only one species occurs), whereas C. maculata wings are darker and C. aequabilis wing spots are smaller in areas of sympatry (places where the species co-occur). This character displacement (divergent shift in character associated with species recognition) is likely due to selection on one or both species to reduce courting errors or misplace intrasexual aggression.||
Calopteryx aequabilis (The River Jewelwing)
Calopteryx maculata (The Ebony Jewelwing)
Tom Castle (senior thesis student) measuring aggression by using individuals tether to a fishing pole
|We recently found two field sites near Ithaca,
NY that were similar in appearance and abiotic factors, differing only
in relative abundance of C. aequabilis and C. maculata. We
caught, marked, measured (spot size), and released males of both species to
be used as focal territorial individuals in trials where we tested levels of
intrasexual aggression toward conspecifics and heterospecifics. We found the
C. maculata males were more aggressive towards large-spotted (vs.
small-spotted) C. aequabilis males,
providing evidence that elevated intrasexual aggression from C. maculata
males toward melanized males is driving character displacement in C.
aequabilis in areas of sympatry.
We presented our results a the Animal Behavior Society meeting in 2013, and our results were published in Behavioral Ecology and Sociobiology in 2014. We are currently interested in looking at intraspecific factors that may affect spot size in C. aequabilis, as well as abiotic factors that may affect the relative abundance and distribution of both Calopteryx species.
|Megalorchestia californiana (Amphipoda: Talitridae): The California Beach Flea|
|Currently, I am also investigating the mating system and reproductive strategies of the California Beach Flea (Megalorchestia californiana), an amphipod found on sandy beaches from California to Washington. Adults, which reach over 1" in size, are often found roaming the beaches in the evening scavenging washed up animals or eating seaweed. Females spend most of the daytime in burrows that are guarded by males. Competition over females can lead to territorial fights between males looking to control access to the group of females (harem) found inside the burrow. Victorious males get the reward of mating with the females, who brood their eggs in an internal pouch and release fully active juveniles.|
|I was initially interested in M. californiana because there is a sexual dimorphism that is unusual among arthropods in that males are larger than females. Furthermore, males have larger, redder antennae and an enlarged second pair of gnathopods - both of these characters are used by the male to hold the female and guard her prior to mating. Individuals of both sexes are found in burrows near or beneath beach wrack, and they are scavengers that feed on decaying plant or animal tissue. Competition for burrows can be fierce, and burrows appear to be a valuable resource by offering protection from predators, prevention of desiccation and a safe place to copulate and breed. Previous work indicated that these organisms have a harem mating system in which a single male may guard and mate with many females, and our experiences in the field confirmed that multiple females are often found in a burrow with a single male. Our initial observations revealed that males use their enlarged antennae to prevent other males from removing them from their burrow or to evict smaller males from a burrow. Although it had been suggested that larger males are more successful in mating with females, we wanted to empirically test aspects of sexual selection including male competition and female choice. Specifically, we looked at interactions between two males alone and in the presence of a female.|
|We found that both sexes preferred to be in separate burrows rather than together in one burrow, regardless of the relative sizes of the individuals. We also observed that males were more aggressive towards one another, whereas females demonstrated very little intrasexual aggression. The remainder of our experiments investigated the interactions between the sexes when 2 males and 1 female were placed in an arena together. Overall, the female ended up in a burrow with the larger male in the majority of the trials. Although it is difficult at this time to disentangle the mechanism by which this occurs, it appears that male competition may play a larger role than female choice. We found that, when given a choice between empty burrow and one occupied by a member of the opposite sex, males will seek the burrow with the female whereas the female chooses her burrowing site randomly. Regardless of whether male competition and/or female choice is occurring, larger males do appear able to secure more matings with females.|
|Although a great deal was accomplished last field season, further study is needed to fully understand all of the facets of sexual selection in Megalorchestia californiana. I plan to return to Friday Harbor Labs next summer ton continue my research, and hopefully bring back some organisms to perform some laboratory experiments as well. Many behavioral aspects of this mating system remain unknown, so there are many exciting avenues of investigation to explore.|