I am broadly interested in female traits that are not expected to occur under the classic narrative of sexual selection. I am currently researching the form and function of two traits- female-female aggression and female song- in a Michigan population of house wrens. Below you can find the questions I'm addressing.
Why it matters
Does aggression help females defend resources?
Does female aggression help or hurt reproductive performance?
Do steroid hormones control female aggression and female song?
Do aggressive females have less healthy immune systems?
Why do female house wrens sing?
Does female song contain features that provoke aggression?
Does female song communicate something about female condition or quality? Past research
Since the time of Darwin, biologists studying elaborate and competitive traits like intrasexual aggression, colors, and song have focused overwhelming on males. Because of the higher potential reproductive rate of males in most breeding systems, these characteristics are expected to evolve in males under the action of sexual selection. While these behaviors and morphological traits are traditionally considered characteristics of males, there is now growing appreciation that these traits are frequently displayed by females in a wide variety of mating systems, particularly for traits that are used in social contexts beyond mating competiton (see Rosvall 2011, Clutton-Brock 2007, 2009). The standard sexual selection narrative is no longer exclusively sufficient for understanding present differences between male and female phenotypes, how sexual dimorphism evolves, or why these traits occur in female organisms for three reasons: 1. These female traits are more widespread than initially predicted (see Rosvall 2011) occuring in breeding systems where female sexual competition isn't expected. 2. Female traits may more likely be under selection in social contexts outside of strict mating competition. 3. The relative influences of sexual, natural, and social selection in shaping female traits are likely different from the corresponding male case, resulting in interactions between ecology, life history, and competitive traits that may look quite different from males.
To understand why these traits occur in some females but not others we first need to understand what these traits are doing for females. We understand the male case, but females are vastly understudied. This is what I'm trying to address using species- the house wren (Troglodytes aedon).
In males, aggression is beneficial for defending resources like territories or mates. Both male and female house wrens will readily respond to house wren playback as though it were an intruding bird, and both males and females vary in how intensely they respond. One reason aggression may be beneficial for females is that it helps them defend resources.
House wrens need to have a cavity to breed. Both males and females will compete intensely over nest boxes. I limit the number of available boxes and then ask if more aggressive females do a better job competing.
House wrens are notorius in birding circles for their ovicide behavior. Both males and females will kill house wren eggs if they can find them. Female aggression may also help females protect their eggs from conspecific intruders.
above: "intruder wren" used to elicit aggression, right: nest box and house wren nest
One reason why female aggression may be less common or less intense than male aggression is that parental or reproductive demands constrain the further elaboration of female aggressiveness. On the other hand, if more aggressive females make more successful moms, females aggression may be under direct selection via natural selection.
To test these possibilities in house wrens, I take a variety of reproductive measurements.
Right: 10 day old baby house wren
might evolve. I take blood samples from females to test for steroid hormones to determine if circulating testosterone or estrogen correlate with aggression or song.
above: lab mate Jean Johnson taking a small blood sample from a female wren
One potential cost to female aggression may be health. In some species, more aggressive high tesosterone males have depressed immune systems (Grieves et al. 2006). To test for this possibility in female house wrens, I will be making blood slides with the help of a post doc in our lab. These slides can be used to measure the level of infection by various blood parasites.
Bird song is traditionally seen as a male behavior. Most of the early work on bird song was done on males in the temperate zone where female song is considered quite rare (Catchpole & Slater 2008). Perhaps because of this early history, bird song is considered a classic sexually selected trait in males that has then evolved in some females of tropical duetting species.
Recent work turns this paradigm on its head. Odom et al. (2014) looked at the entire family of songbirds (oscine Passerines) and found strong evidence that the common ancestor of all songbirds was a singing female. Female song has existed for millions of years!
In my temperate zone population of house wrens, I've found that around 66% of the females are observed singing at some point during the breeding season. Furthermore, females show an incredible amount of variation in their songs- much more than the males. Females have an average of 2 song types, but some can sing up to seven!
Right: Female C4, the first female I recorded singing in 2011
There are several reasons why females might sing. Song might be used to coordinate activities with males, use to solicit copulations, or used to defend territories or nests. I've found evidence that females use their songs to defend nests against other wrens. Females sing most frequently during the time in the breeding cycle when other wrens are the biggest threat. Females also respond with elevated singing rates after they hear male or female house wren songs (Krieg & Getty in prep). At least one of the functions of female song appears to be nest defense.
When I started running aggression tests, I noticed that females responded quite differently to different female songs. In fact, one song stimulus worked so well I've started using
it to mistnet adult females. Detailed analyses of the structure of these playback songs shows certain acoustic features correlate with stronger aggressive responses. Songs with a greater diveristy of syllable types, songs with more harsh broadband noise, and songs containing a particular syllable (named "HI") tend to provoke stronger responses. I am currently examining whether females change their song structure when engaging with intruders or whether these aggression provoking songs come from females with particular intrinsic characteristics (size, age, etc.).
Bird song is a complicated behavior that involves many different brain regions that must form the necessary connections at the appropriate time during development. Females use male song to judge potential mates (Catchpole & Slater 2008). We don't know why female house wren song shows so much variation, but it's possible song communicates something about the singing female. This summer an undergrad working with me will be testing whether certain song features correlate with female physical condition, potentially explaining the pattern above. We'll also be examining whether song features correlate with parental quality or immune function, which would lay the foundation for male choice in this species.
aggressive females in 2012
Right: 10 day old baby house wren
During the summer of 2009, I worked as an NSF funded REU at the Cary Institute of Ecosystem Studies alongside Dr. Kara Belinsky and Dr. Ken Schmidt. I worked with the veery (Cartharus fuscescens), a migratory song bird with an acoustically complex song (see above). Using a robotic veery and playback, we investigated whether vocal signals predicted aggressive actions in territorial males. Our results suggest that songs lacking introductions signal descalation and that songs and "whisper calls" signal escalation.
- Catchpole, C.K. & Slater, P.J. 2008. The Study of Bird Song: Biological Themes and Variations, 2nd edn. Cambridge: Cambridge University Press.
- Clutton-Brock, T. 2009. Sexual selection in females. Anim Behav 77:3-11.
- Clutton-Brock, T. 2007. Sexual selection in males and females. Science 318:1882-1885.
- Grieves, T.J., McGlothlin, J.W., Jawor, J.M., Demas, G.E. & Ketterson, E.D. 2006. Testosterone and innate immune function inversely covary in a wild population of breeding Dark-Eyed Juncos (Junco hyemalis). Funct Ecol 20:812-818.
- Ketterson, E.D., Nolan, V. & Sandell, M. 2005. Testosterone in females: mediator of adaptive traits, constraint on sexual dimorphism, or both? Amer Nat 166:S87-S98.
- Odom, K.J., Hall, M.L., Riebel, K., Omland, K.E. & Langmore, E. 2014. Female song is widespread and ancestral in songbirds. Nature Comm 5:4379.
- Rosvall, K.A. 2011. Intrasexual competition in females: evidence for sexual selection? Behav Ecol 22:1131-1140.