Researchers studying flycatcher birds have found that sperm morphology—and thus reproductive success—are linked with the age and attractiveness of males. The size of white patches on the birds’ foreheads, a measure of their sex appeal, as well as the shape of their sperm determined how many hatchlings among a mixed brood were a male’s own. For instance, male flycatchers with smaller forehead patches had longer sperm, probably to ensure the swimmers beat the competition, while males with larger patches were successful fathers even with shorter sperm. These findings were published August 9 in Behavioral Ecology.
M. Ålund et al., “Optimal sperm length for high siring success depends on forehead patch size in collared flycatchers,” Behav Ecol, doi:10.1093/beheco/ary115, 2018.
Clarification (August 21): We removed the word “several” from “several sperm morphology” in the first sentence for clarity.