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Function predicts the allometry of contest-related traits, but not sexual or male dimorphism in the amazonian tusked harvestman

Abstract

Animal contests involve threatening displays and physical coercion, which are respectively performed by threat devices used in mutual evaluation of size or strength, and weapons used for grasping, stabbing, striking, or dislodging a rival. According to the functional allometry hypothesis, directional selection consistently favors hyper-allometry in threat devices, whereas the allometry of weapons depends on the way they are used in contests. Here, we tested this hypothesis using the Amazonian tusked harvestman Phareicranaus manauara (Arachnida: Opiliones), a male-dimorphic species, as a study system. Behavioral observations allowed us to recognize four contest-related traits and three control traits, not used in contests. Two weapons used to grasp or prod the opponents from afar and one threat device were hyper-allometric, whereas one tactile signaling device (used to tap the opponent) and all control traits were either iso- or hypo-allometric. These findings support the hypothesis that function predicts the allometry of contest-related traits. However, function does not explain allometric differences in homologous traits between males and females (whose traits also were used as controls). We suggest that if a trait used in contests by males is used by both sexes in another context, natural selection and cross-sexual genetic correlations may constrain its developmental trajectory, preventing the evolution of sexual dimorphism in allometric slopes. Therefore, using female traits as controls for homologous contest-related male traits may not be appropriate. Finally, we show that function does not explain differences between male morphs in the allometric slopes of male-dimorphic traits. Thus, an important next step in allometric studies is to understand what factors affect the slopes of male-dimorphic traits.

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Acknowledgements

We thank Elisa Garcia, Douglas Bastos, and José da Silva Lopes for helping with fieldwork, John Uribe for helping with the lab work, shooting the videos for the supplementary material and taking the pictures presented in Figs. 1, 2 and 3 and also those used to measure the individuals, the curator of the of the Entomological Collection of the Instituto Nacional de Pesquisas da Amazônia (INPA) for allowing us to measure individuals deposited in the collection, the staff of Ducke Reserve for logistical support, William G. Eberhard for discussions on the function of some contest-related traits, and Matthew Symonds and three anonymous reviewers for valuable comments on the manuscript.

Funding

AVP was supported by a post-doctoral grant from São Paulo Research Foundation (FAPESP 2016/22679-3), SGH was supported by a student grant from São Paulo Research Foundation (FAPESP 2017/05283-1), and GM is supported by research grants from São Paulo Research Foundation (FAPESP 2021/00915-5) and National Counsel of Technological and Scientific Development (CNPq 306550/2014–8).

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Correspondence to Glauco Machado.

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The analyses reported here can be reproduced using the data and scripts that are available on the Mendeley repository (doi: https://doi.org/10.17632/9nh9nj5y7w.1).

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GM conceptualized the study; SGH and GM collected data in the field; AVP and SGH conducted the behavioral observations in the laboratory; AVP, SGH, BAB, and GM analyzed the videos; SGH assisted in the measurement of the individuals and curated the data; AVP, SGH, BAB analyzed the data; AVP, SGH, BAB, and GM wrote and edited the manuscript; SGH and GM performed visualization; GM acquired funding.

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Palaoro, A.V., García-Hernández, S., Buzatto, B.A. et al. Function predicts the allometry of contest-related traits, but not sexual or male dimorphism in the amazonian tusked harvestman. Evol Ecol 36, 605–630 (2022). https://doi.org/10.1007/s10682-022-10152-1

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Keywords

  • Alternative reproductive tactics
  • Intrasexual dimorphism
  • Male-male contests
  • Threat device
  • Weapon