The driving forces behind female-female aggression and its fitness consequence in an Asian agamid lizard

Abstract

Abstract: Female-female aggression and its effect on fitness are poorly understood compared to male-male aggression. We quantified female-female aggression in the viviparous toad-headed agama, Phrynocephalus vlangalii, and tested the mate limitation and resource competition hypotheses, to explain the driving forces behind heightened female-female aggression. We established populations in outdoor enclosures and separately manipulated mate and food availability while quantifying aggressive interactions between known females. Female mass and offspring quantity and quality were monitored for 2 years and used as an index of female fitness. Interestingly, female-female aggression was highest outside the mating season, when females were pregnant. Also, male availability did not significantly influence female-female aggression. Consequently, we found no evidence in support of the mate limitation hypothesis. Female-female aggression, however, was greatest during periods of low-food availability. Greater food availability did have positive fitness consequence for females: high-food treatment females were more likely to gain mass and give birth than females from the low-food treatment, and offspring from high-food treatment females were more likely to survive the first winter. However, we did have an effect of year: female mass was negatively associated with female-female aggression in year 1 but positively associated in year 2, although the association was not significant. The same pattern appeared in the association between offspring sprint speed and female-female aggression: higher levels of female-female aggression in year 1 were associated with slower offspring, while in year 2, higher female-female aggression was associated with faster offspring. Overall, our results demonstrate a link between female-female aggression and competition for resource (food) and highlight the potential importance of female aggressive behavior as a mechanism that influences female fitness. Significance statement: Male-male contest competition has been a cornerstone of sexual selection theory for decades because dominant males can control more space and access to females. Conversely, female aggression is poorly understood and sometimes considered a paradox, especially in species lacking parental care. We tested whether female-female aggression in the viviparous lizard P. vlangalii is best explained by competition for males (mate limitation hypothesis) or competition over food (food limitation hypothesis). Interestingly, female aggression peaks after mating, while females are still pregnant. Heightened aggression at this time supports the food limitation hypothesis because any advantage females have in terms of access to resources will benefit offspring born into her home range. Likewise, access to additional food resources may improve female survival and future reproductive investment through greater energy storage.