Next steps for understanding the selective relevance of female-female competition

After decades of neglect, recent empirical research on exaggerated female traits (e.g., ornaments, armaments, aggression, acoustic signals, etc.) has revived interest in this widespread but poorly understood phenomenon, and shown that these traits often function in the context of female-female competition (West-Eberhard, 1983; Amundsen, 2000; Clutton-Brock, 2009; Rosvall, 2011a; Stockley and Bro-Jørgensen, 2011; Rubenstein, 2012 [Theme issue]; Stockley and Campbell, 2013 [Theme issue]). However, recent reviews have emphasized the applicability of sexual vs. social selection, rather than rigorously examining the role of different ecological contexts in shaping the evolution of traits used in competitive contexts (hereafter, “competitive traits”) in females. Thus, we still lack a solid understanding of the ecological and evolutionary mechanisms driving the evolution of female trait expression, in particular whether, how, and why these mechanisms vary among species, and between the sexes

Life history trade-offs and behavioral sensitivity to testosterone: an experimental test when female aggression and maternal care co-occur.

Research on male animals suggests that the hormone testosterone plays a central role in mediating the trade-off between mating effort and parental effort. However, the direct links between testosterone, intrasexual aggression and parental care are remarkably mixed across species. Previous attempts to reconcile these patterns suggest that selection favors behavioral insensitivity to testosterone when paternal care is essential to reproductive success and when breeding seasons are especially short. Females also secrete testosterone, though the degree to which similar testosterone-mediated trade-offs occur in females is much less clear. Here, I ask whether testosterone mediates trade-offs between aggression and incubation in females, and whether patterns of female sensitivity to testosterone relate to female life history, as is often the case in males. I experimentally elevated testosterone in free-living, incubating female tree swallows (Tachycineta bicolor), a songbird with a short breeding season during which female incubation and intrasexual aggression are both essential to female reproductive success. Testosterone-treated females showed significantly elevated aggression, reduced incubation temperatures, and reduced hatching success, relative to controls. Thus, prolonged testosterone elevation during incubation was detrimental to reproductive success, but females nonetheless showed behavioral sensitivity to testosterone. These findings suggest that the relative importance of both mating effort and parental effort may be central to understanding patterns of behavioral sensitivity in both sexes

Do males offset the cost of female aggression? An experimental test in a biparental songbird

Aggressive behavior in females is thought to be costly due to a trade-off between aggression and parental care. In biparental systems, resolution of this trade-off may depend on the extent to which males mitigate the cost of female aggression. Using a population of tree swallows (Tachycineta bicolor) in which intrasexual aggression has been shown to be beneficial to females in acquiring a nesting cavity, but costly in terms of offspring quality, I asked if the cost of female aggression is offset by her partner. First, I determined if pairs mate disassortatively by aggressiveness and whether the degree of dissimilarity of aggressiveness correlates with parameters of reproductive success. I then experimentally handicapped males to test whether female aggressiveness becomes more costly when males provision young less. I found no evidence of disassortative mating, although pairs differing more in aggressiveness laid more and larger eggs. When male provisioning was reduced, offspring were no worse in quality, but nestling mortality increased. Aggressive behavior was only associated with a fitness cost in control nests. Therefore, males may mitigate the cost of aggression for their female partners indirectly, not by compensating for poor parenting by aggressive females, but instead by females investing more heavily in reproduction when mated to a male that is more different from her own phenotype. To the extent this differential allocation outweighs the cost of aggressiveness, male phenotype may play a key role in understanding the selective pressures shaping the evolution of aggressive behavior in females. Copyright 2009, Oxford University Press.

Aggression.

<p>Testosterone-implanted females (shaded bar) were significantly more aggressive than control females (open bar). Sample sizes are shown in parentheses; error bars represent the standard error of the mean.</p

Plasma Testosterone.

<p>Experimental females (shaded bars) had significantly higher testosterone after implantation than before implantation. Experimental females also had significantly higher testosterone levels than control females (open bars). Sample sizes are shown in parentheses. These data are back-transformed from the natural log of plasma testosterone; error bars represent the standard error of these back-transformed means.</p

Incubation.

<p>Testosterone-implanted females’ nests (shaded bar) were significantly cooler than control females’ nests (open bar). Sample sizes are shown in parentheses; error bars represent the standard error of the mean.</p

Daily temperature data.

<p>Exemplars of one full day (24 hours) of temperature data in (a) control and (b) experimental nests, beginning just before first light. Drops in temperature correspond to the female leaving the nest between bouts of incubation. Ambient temperature shown in gray line. Control and experimental nests were paired for location and date.</p

Plasticity in female timing may explain earlier breeding in a North American songbird

Many species have shifted their breeding phenology in response to climate change. Identifying the magnitude of phenological shifts and whether climate-mediated selection drives these shifts is key for determining species’ resilience to climate change. Birds are a strong model for studying phenological shifts due to numerous long-term research studies; however, generalities pertaining to drivers of phenological shifts will emerge only as we add study species that differ in life history and geography.We investigated 32 years of reproductive timing in a non-migratory population of dark-eyed juncos Junco hyemalis. We predicted that plasticity in reproductive timing would allow females to breed earlier in warmer springs. We also predicted that selection would favour earlier breeding and asked whether the temperatures throughout the breeding season would predict the strength of selection.To test these predictions, we examined temporal changes in the annual median date for reproductive onset (i.e. first egg date) and we used a sliding window analysis to identify spring temperatures driving these patterns. Next, we explored plasticity in reproductive timing and asked whether selection favoured earlier breeding. Lastly, we used a sliding window analysis to identify the time during the breeding season that temperature was most associated with selection favouring earlier breeding.First egg dates occurred earlier over time and strongly covaried with April temperatures. Furthermore, individual females that bred in at least 3 years typically bred earlier in warmer Aprils, exhibiting plastic responses to April temperature. We also found significant overall selection favouring earlier breeding (i.e. higher relative fitness with earlier first egg dates) and variation in selection for earlier breeding over time. However, temperature across diverse climatic windows did not predict the strength of selection.Our findings provide further evidence for the role of phenotypic plasticity in shifting phenology in response to earlier springs. We also provide evidence for the role of selection favouring earlier breeding, regardless of temperature, thus setting the stage for adaptive changes in female breeding phenology. We suggest for multi-brooded birds that advancing first egg dates likely increase the length of the breeding season, and therefore, reproductive success.Evidence for the role of plasticity in driving earlier breeding in warmer springs and for the role of selection favouring earlier breeding, regardless of temperature, thus setting the stage for adaptive changes in female breeding phenology. The authors draw conclusions about multi-brooded birds’ resilience to climate change.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/175116/1/jane13772.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/175116/2/jane13772-sup-0001-Supinfo.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/175116/3/jane13772_am.pd