Animal signals have undergone spectacular diversification across the tree of life. They function in communication within and between species, playing a prominent role in the evolution of mate choice and reproductive isolation, and in mediating intra- and interspecific competition. They are therefore key to understanding speciation and species interactions in ecological communities. We conduct field experiments and broad-scale comparative analyses focusing on acoustic and visual signals in birds, as well as lab experiments using stalk-eyed flies, to disentangle the roles of ecological selection, sexual selection and social competition as drivers of signal evolution and signal divergence among species. We are particularly interested in the ornaments and armaments of females, as these shed light on poorly understood mechanisms such as male choice, sexual conflict and female-female competition, all of which have important implications for sexual selection theory and for understanding the evolution of sex differences in animals.