The driving forces behind female-female aggression and its fitness consequence in an Asian agamid lizard
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.
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.
KeywordsFemale-female aggression Fitness Mate limitation Resource competition Reptile Phrynocephalus vlangalii
We are grateful to Er Ga and Cuo ke for logistical assistance in Zoige. Thanks to Xintao Deng for assistance with data collection in the field and Daniel Noble for advice on statistical methods. We thank two anonymous reviewers for their constructive comments that improved this manuscript.
This project was supported by grants from the National Natural Science Foundation of China to YQ (grant numbers 31201723, 31572273, 31872233) and to YW (grant number 31801980).
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
We obtained the appropriate permissions for conducting the study at the Xiaman Conservation Station in the Zoige Wetland Nature Reserve from the Forestry Department of the Sichuan Provincial Government and the Management Office of the Zoige Nature Reserve. Handing of lizards followed approved protocols from the Experimental Animal Ethics Committee of the Chengdu Institute of Biology, Chinese Academy of Sciences (protocol number 2017005). We also adhered to the ABS/ASAB “Guidelines for the treatment of animals in behavioral research and teaching.”
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