Parasites are globally widespread pathogenic organisms, which impose important selective forces upon their hosts. Thus, in accordance with the Adjustment to parasite pressure hypothesis, it is expected that defences among hosts vary relative to the selective pressure imposed by parasites. According to the latitudinal gradient in diversity, species richness and abundance of parasites peak near the equator. The uropygial gland is an important defensive exocrine gland against pathogens in birds. Size of the uropygial gland has been proposed to vary among species of birds because of divergent selection by pathogens on their hosts. Therefore, we should expect that bird species from the tropics should have relatively larger uropygial glands for their body size than species from higher latitudes. However, this hypothesis has not yet been explored. Here, we analyse the size of the uropygial gland of 1719 individual birds belonging to 36 bird species from three Neotropical (Peru) and three temperate areas (Spain). Relative uropygial gland volume was 12.52% larger in bird species from the tropics than from temperate areas. This finding is consistent with the relative size of this defensive organ being driven by selective pressures imposed by parasites. We also explored the potential role of this gland as a means of avoiding haemosporidian infection, showing that species with large uropygial glands for their body size tend to have lower mean prevalence of haemosporidian infection, regardless of their geographical origin. This result provides additional support for the assumption that secretions from the uropygial gland reduce the likelihood of becoming infected with haemosporidians.

中文翻译：

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探索对寄生虫压力假说的调整：古猿和新热带鸟类的尿道腺体积和血孢子虫感染的差异

寄生虫是全球性的致病性生物，对它们的宿主施加重要的选择力。因此，根据对寄生虫压力的调整假设，寄主之间的防御相对于寄生虫施加的选择性压力是不同的。根据多样性的纬度梯度，物种丰富度和寄生虫的丰度在赤道附近达到峰值。泌尿腺是重要的防御性外分泌腺，可抵抗鸟类中的病原体。由于鸟类病原体对宿主的病原体选择不同，因此建议在各个鸟类中尿py腺的大小会有所不同。因此，我们应该期望来自热带地区的鸟类物种的体格大小应比来自高纬度地区的鸟类更大。但是，尚未对此假设进行探讨。在这里，我们分析了来自三个新热带地区（秘鲁）和三个温带地区（西班牙）的属于36种鸟类的1719只个体的尿道腺的大小。热带地区鸟类的相对尿道腺体积比温带地区大12.55％。这一发现与寄生虫施加的选择性压力驱动的防御器官的相对大小一致。我们还探讨了该腺体作为避免血友病感染的一种手段的潜在作用，表明具有大尿壶腺体型的物种，无论其地理起源如何，其血友病感染的平均患病率均较低。该结果为以下假设提供了额外的支持：尿道腺的分泌物减少了感染血孢子虫的可能性。这一发现与寄生虫施加的选择性压力驱动的防御器官的相对大小一致。我们还探讨了该腺体作为避免血友病感染的一种手段的潜在作用，表明具有大尿壶腺体型的物种，无论其地理起源如何，其血友病感染的平均患病率均较低。该结果为以下假设提供了额外的支持：尿道腺的分泌物减少了感染血孢子虫的可能性。这一发现与寄生虫施加的选择性压力驱动的防御器官的相对大小一致。我们还探讨了该腺体作为避免血友病感染的手段的潜在作用，表明具有大尿壶腺体型的物种，无论其地理起源如何，其血友病感染的平均患病率均较低。该结果为以下假设提供了额外的支持：尿道腺的分泌物减少了感染血孢子虫的可能性。不论其地理来源。该结果为以下假设提供了额外的支持：尿道腺的分泌物减少了感染血孢子虫的可能性。不论其地理来源。该结果为以下假设提供了额外的支持：尿道腺的分泌物减少了感染血孢子虫的可能性。