RESEARCH
Interaction between pre- and post- copulatory selection
Promiscuity in many lizard species implies that males face competition not only in accessing female mates but also face competition within a female’s reproductive tract to fertilize her eggs. This can potentially result in strong selection on both primary and secondary sexual characteristics in males.
While informative, such trait-based correlations across species are often inconsistent with correlations found within a species. This may be due to the fact that traits are correlated in complex ways and negative or positive correlations may be found depending on the choice of traits measured. Further, it is not clearly known how social and ecological factors such as density, sex-ratio etc. change the extent and direction of correlation between these components of sexual selection. My thesis is focused on examining the interaction between pre- and post-copulatory selection using a variance partitioning analysis, and further assessing how this correlation changes across generations and with natural variation in density and sex ratio in a wild population of the sexually dimorphic brown anole lizard, Anolis sagrei.
Measuring pre- and post-copulatory fitness components in the wild
​
In the Cox lab, I have worked with other members of the lab to collect lifetime fitness data (survival and reproductive success) of brown anoles on a small island along the Matanzas River, Florida. We have mark-recapture data on adults and juveniles on this island from 2015-2019. We have also successfully developed a panel of 215 markers to assign genotypes to individuals and assign parentage to individuals until 2018 using the GTSeq method (Campbell et al. 2015, Seears et al. in prep). Figure below demonstrates sampling scheme and variance in lifetime reproductive fitness between males and females. Our data suggests that brown anoles have a promiscuous mating system with males and females both mating multiple times. However, they also follow conventional sex roles in which males are under much stronger sexual selection as seen below given the higher variance in reproductive fitness in males.
Using this lifetime fitness data, I am using a variance partitioning method to estimate components of male and female fitness (mating success, female fecundity, siring success and offspring survival) and how its additive and interactive components determine opportunity for sexual selection. With the same fitness data, I will examine how fine scale variation in density, sex-ratio and female rate of multiple mating affect the interaction between pre- and post-copulatory selection in males across multiple generations. More exciting results await...
​
Implications for trait evolution
Given the degree of promiscuous mating observed in our population and the fact that males are still under stronger sexual selection made us wonder how exactly do traits like larger body size help increase a male's reproductive fitness in the wild. Understanding pre-copulatory sexual selection in a wild population is often hindered by our ability to document copulations in real time. While studies often demonstrate that large males sire more offspring, higher mating success is inferred indirectly through parentage or often assumed, with few empirical tests regarding the same.
We are using transfer of fluorescent powders from males to females during copulations to assess mating rates for different size classes in a wild population of the sexually-dimorphic brown anole, Anolis sagrei (Top panel of figure demonstrates method and colors in middle panel indicate the colors used in this study). We have collected this data from March-July 2019 and hope to complete another round of data collection in 2021.
Data collected from july suggests that females copulate more frequently with larger males (above median size of males - S3 and S4 size classes). Across seasons, large males had significantly higher number of copulations (57.6%) compared to small males (42.3%) providing direct evidence for pre-copulatory sexual selection on larger body size in males.
​
In the future, we will estimate the lifetime reproductive fitness and test if pre- and post-copulatory selection act synergistically or antagonistically on male body size . Research awards from The Explorers Club and GSPA will support completion of fieldwork in 2021 and genotyping for individuals collected from 2019-2020.
Evolution of a female - specific polymorphism in brown anoles
​
While polymorphisms within males have been studied extensively particularly in the context of alternative reproductive strategies, relatively less attention has been paid to polymorphisms specific to females. Although examples of female polymorphisms are rare in the animal kingdom, the presence of a back-pattern polymorphism specific to females of the brown anole lizard, Anolis sagrei provided an enticing opportunity to examine the evolution of correlated suites of traits in females. In most brown anole populations, females have a distinctive back pattern comprised of a longitudinal bar, diamonds, or diamond-bars. However, in some populations, females can also have a chevron back pattern similar to that of males.
Using data from an ongoing mark-recapture study of an island population of brown anoles in Florida, I showed that ML and FL morphs do not differ in lifetime survival and reproductive fitness, indicating that these morphs may not be under selection. Undergraduate researcher Katie Larsen(Class of 2020) and I showed that the male-like chevron back pattern follows a mendelian mode of inheritance consistent with HW equilibrium. I also conducted behavioral trials in the wild, with teacher fellows from the Evolution Education program to assess if aggression displays differed between the morphs and we showed that there were no behavioral differences between the morphs. Currently, undergraduate researcher Carrie Smith is investigating if selection on morphological traits is morph-specific.
Overall our work shows so far points to the idea that these polymorphisms are not under selection and that differing frequencies of these morphs may be an outcome of genetic drift. I am excited to write these results up.
Sex-specific personality in Indian Rock agamids
Sexual dimorphism represents the most astounding example of correlated suites of traits being maintained in a species. For my master’s research at IISER Pune, I worked with Dr Kavita Isvaran at the Indian Institue of Science(IISc) to study the nature of predator-escape response in the sexually dimorphic Indian rock lizard, Psammophilus dorsalis and examined sex-differences in the repeatability of this response in a wild population. We found that contrary to expectations, males were warier of an approaching predator when compared with females. Interestingly, sex and body condition were relatively more important than habitat in determining their escape response. Check out the paper which describes these results
Both sexes were equally and highly repeatable in their escape response. Interestingly, we found that there were sex-specific causes and consequences of repeatability in this behavior. This charismatic species was my first introduction to the world of reptiles, and I hope to work with them again in the future to understand the reproductive and social system of these lizards and its behavioral causes and consequences. Check out some exciting work going on in this study system at the Center for Ecological Sciences (CES) at IISc.
A male rock agama in a breeding color to attract female attention
