Data from: Sperm competition games: a general model for pre-copulatory male-male competition

Linear (mm) measures of the forearm, and weight (mg) of the body and testes of quacking frogs collected from 10 natural populations in Western Australia.

Reproductive males face a trade-off between expenditure on pre-copulatory male-male competition – increasing the number of females that they secure as mates – and sperm competition – increasing their fertilisation success with those females. Previous sperm allocation models have focused on scramble competition in which males compete by searching for mates and the number of matings rises linearly with pre-copulatory expenditure. However, recent studies have emphasised contest competition involving pre-copulatory expenditure on armaments, where winning contests may be highly dependent on marginal increases in relative armament level. Here we develop a general model of sperm allocation that allows us to examine the effect of all forms of pre-copulatory competition on sperm allocation patterns. The model predicts that sperm allocation decreases if either the ‘mate-competition loading’, a, or the number of males competing for each mating, M, increases. Other predictions remain unchanged from previous models: (i) expenditure per ejaculate should increase and then decrease, and (ii) total post-copulatory expenditure should increase, as the level of sperm competition increases. A negative correlation between a and M is biologically plausible, and may buffer deviations from the previous models. There is some support for our predictions from comparative analyses across dung beetle species and frog populations.

本数据集包含从西澳大利亚10个自然种群中采集的鸣蛙（quacking frogs）的前臂线性尺寸（单位：毫米），以及其体质量（单位：毫克）与睾丸质量（单位：毫克）。繁殖期雄性面临繁殖前雄性间竞争投入与精子竞争间的权衡：前者用于提升获得交配机会的雌性数量，后者则旨在提高与这些雌性交配时的受精成功率。过往的精子分配模型多聚焦于掠夺式竞争（scramble competition），该模式下雄性通过搜寻配偶展开竞争，且交配次数随繁殖前竞争投入呈线性增长。但近期研究则强调了涉及繁殖前军备投入的争斗式竞争（contest competition）——在此类竞争中，雄性能否获胜高度依赖于相对军备水平的边际提升。本研究构建了一个通用的精子分配模型，可用于分析所有类型的繁殖前竞争对精子分配模式的影响。该模型预测，当配偶竞争负荷（mate-competition loading）参数a，或是参与单次交配的雄性数量M增加时，精子分配量会随之下降。其余预测结果与过往模型保持一致：其一，单次射精投入会先升后降；其二，随着精子竞争强度提升，总交配后投入也会相应增加。参数a与M之间的负相关关系在生物学上具有合理性，或可缓解与过往模型的偏差。针对蜣螂物种及蛙类种群的比较分析结果，为本文的预测提供了一定支撑。