# Proteomic analysis reveals dynamic changes in cloacal fluid composition during the reproductive season in a sexually promiscuous passerine

## TL;DR

This study shows how the protein composition of cloacal fluid in barn swallows changes during the reproductive cycle, potentially influencing sperm function and cryptic female choice.

## Contribution

The study identifies dynamic proteomic changes in cloacal fluid during the avian reproductive cycle, offering new insights into cryptic female choice mechanisms.

## Key findings

- Protein composition in the female reproductive tract differs between receptive and nonreceptive phases.
- Three proteins showed increased abundance during the receptive phase and may influence sperm functionality.
- Gene ontology terms enriched in the proteome shifted with the reproductive cycle phases.

## Abstract

Cryptic female choice (CFC) is a component of postcopulatory sexual selection that allows females to influence the fertilization success of sperm from different males. While its precise mechanisms remain unclear, they may involve the influence of the protein composition of the female reproductive fluids on sperm functionality. This study maps the protein composition of the cloacal fluid across different phases of female reproductive cycle in a sexually promiscuous passerine, the barn swallow. Similar to mammals, the protein composition in the female reproductive tract differed between receptive (when females copulate) and nonreceptive phases. With the change in the protein background, the enriched gene ontology terms also shifted. Within the receptive phase, distinctions were observed between proteomes sampled just before and during egg laying. However, three proteins exhibited increased abundance during the entire receptive phase compared to nonreceptive phases. These proteins are candidates in cryptic female choice, as all of them can influence the functionality of sperm or sperm-egg interaction. Our study demonstrates dynamic changes in the cloacal environment throughout the avian breeding cycle, emphasizing the importance of considering these fluctuations in studies of cryptic female choice.