Temporal and genetic variation in female aggression after mating

Abstract

Aggression between individuals of the same sex is almost ubiquitous across the animal kingdom. Winners of intrasexual contests often garner considerable fitness benefits, through greater access to mates, food, or social dominance. In females, aggression is often tightly linked to reproduction, with females displaying increases in aggressive behavior when mated, gestating or lactating, or when protecting dependent offspring. In the fruit fly, Drosophila melanogaster, females spend twice as long fighting over food after mating as when they are virgins. However, it is unknown when this increase in aggression begins or whether it is consistent across genotypes. Here we show that aggression in females increases between 2 to 4 hours after mating and remains elevated for at least a week after a single mating. In addition, this increase in aggression 24 hours after mating is consistent across three diverse genotypes, suggesting this may be a universal response to mating in the species. We also report here the first use of automated tracking and classification software to study female aggression in Drosophila and assess its accuracy for this behavior. Dissecting the genetic diversity and temporal patterns of female aggression assists us in better understanding its generality and adaptive function, and will facilitate the identification of its underlying mechanisms.

Fig 1. Mated females fought for longer than virgin females in all genotypes and w¹¹¹⁸ females fought for less time than Canton-S and Dahomey females.

Blue points represent mated females while yellow points represent virgin females in each genotype. Each point represents the contest duration for one pair of females. The black bars represent the treatment means ± 1 standard error. Sample sizes are recorded in S1 Table.

Fig 2. Mated females start to fight for longer than virgins around four hours after mating.

a. Manual scoring of headbutt duration. b. Automated tracking and classification of headbutt duration. Blue points represent mated females, while yellow points represent virgin females. Each point represents the contest duration for one pair of females. The black bars represent the treatment means ± 1 standard error. The points in a and b represent the same videos scored manually (in a) and by the automated system (in b). We place them side by side here for comparison of the different methods.

Fig 3. Manual and tracking data are positively correlated for mated females but not for virgin females.

Blue points represent mated females from all treatments, while yellow points represent virgin females from all treatments. Each point represents the contest duration for one pair of females. The lines indicate the linear model fit between the manual scoring and tracking data for mated (blue) and virgin (yellow) females separately. The highlighted areas around the line indicate the 95% confidence interval.

Fig 4. Schematic of timing of various processes in mated females for the first 24 hours after mating.

A. The top portion of the figure represents aggression in mated females over the first 24 hours after mating as contest duration in seconds. This matches Fig 1 in this paper. B. The middle part of the figure demonstrates the patterns of sperm storage (in the seminal receptacle only), egg laying (oviposition), and ovulation in mated females. These lines represent general patterns and are not to scale with each other, but merely to give an indication of when and how these processes change over time. Sources: sperm storage [27], ovulation [26], egg laying [26]. C. The bottom portion of the figure demonstrates the timing of the presence of various seminal fluid proteins in different parts of the female–e.g. the top line represents when ovulin has been detected in the female hemolymph after mating. Sources: ovulin in hemolymph [53,54], ovulin in ovaries [26], mating plug and sperm ejection [23,55], sex peptide bound to sperm [56], sex peptide in hemolymph [57].