Sperm Size Decreases With Increasing Pheomelanin Pigmentation but Not With the Amount of Glutathione in the Barn Swallow

Pigment-based coloration is prevalent in animals, but its expression greatly varies across species, populations, and even among individuals in the same populations. Some animals are highly pigmented and thus have conspicuous coloration, whereas others are modestly pigmented and thus have drab coloration. A possible explanation for the variety in pigmentation is a resource-based tradeoff, in which resources invested in pigmentation are unavailable for other functional traits, and thus animals that need to invest in the latter have limited resources to invest in pigmentation. Resource-based tradeoff is plausible in theory, but direct tests are scarce, partially because of many components of pigment-based coloration (i.e., multiple pigments, integument microstructure, and stains) that affect coloration, preventing the use of coloration as an index of pigmentation. Here, using the barn swallow,