Saimiri are neotropical primates with seasonal reproduction, males develop a seasonal fattening condition that has been suggested as a pre-copulatory sexual selection strategy. Furthermore, females mate with multiple males in the same season. This could also favor the evolution of a postcopulatory sexual strategy by sperm competition. In this study, we tested the hypothesis that there is a relationship between the fatted condition and advantageous seminal characteristics in Saimiri collinsi and evaluated its implications for sperm competition. Adult males (N = 10), aged 5–15 years, housed in mixed or only-male groups, were analyzed from June, 2015 to July, 2016. Measurements of weight, axilla, and arm skinfold, and testicular volume were taken monthly, and semen was collected by electroejaculation. A fattening index was developed to quantify and identify fatted males, biometrics, and seminal parameters were compared between the non-fatted and fatted conditions. The fatted males present a larger testicular volume. This is related to the increase in spermatogenic activity necessary to sustain a high ejaculation frequency during the mating season. An increase in seminal volume and in frequency of semen coagulation were detected in fatted males, advantages related to sperm protection in the female reproductive tract. Age and social context were not significant sources of variation for both morphological and seminal traits. A decrease in response from the fatted males in obtaining semen and an increase in the frequency of azoospermic ejaculates were observed. These unexpected results may be due to intense reproductive activities in a short period. The fattening phenomenon has many implications in the sexual selection of squirrel monkeys, and they are still not entirely unveiled. Our results corroborate the idea that, in S. collinsi, the fatted male condition is related to sexual selection, and we found evidence suggesting it may be also expressed by a post-copulatory component, sperm competition.

中文翻译：

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雄性增肥与松鼠猴（Saimiri collinsi）精液质量的提高有关：对精子竞争的影响

Saimiri是具有季节性繁殖的新热带灵长类动物，雄性会发展出季节性育肥状况，这被认为是交配前的性选择策略。此外，雌性在同一季节与多个雄性交配。这也可能有利于通过精子竞争进化出交配后的性策略。在这项研究中，我们检验了Saimiri collinsi的肥胖状况与有利的精液特征之间存在关系的假设，并评估了其对精子竞争的影响。成年男性（N = 10)，年龄在 5-15 岁，混居或单独的男性组，从 2015 年 6 月到 2016 年 7 月进行了分析。每月测量体重、腋窝和手臂皮褶以及睾丸体积，精液是通过电射精收集。开发了一个增肥指数来量化和识别肥胖的雄性，生物特征和精液参数在非肥胖和肥胖条件之间进行比较。肥胖的男性睾丸体积更大。这与在交配季节维持高射精频率所必需的生精活性增加有关。在肥胖男性中检测到精液量增加和精液凝固频率增加，这与女性生殖道中的精子保护有关。年龄和社会背景不是形态和开创性特征变异的重要来源。观察到肥胖男性在获得精液方面的反应降低，无精子射精频率增加。这些意想不到的结果可能是由于短期内强烈的繁殖活动造成的。肥胖现象对松鼠猴的性选择有很多影响，但它们仍未完全揭开。我们的结果证实了这样的想法，在 而且它们还没有完全揭开。我们的结果证实了这样的想法，在 而且它们还没有完全揭开。我们的结果证实了这样的想法，在S. collinsi，肥胖的男性状况与性选择有关，我们发现有证据表明它也可能通过交配后的成分，即精子竞争来表达。