Fig. 2

Experimental manipulations during tests. a Focals were exposed to staged intrusions by either a bigger or smaller opponent, under a white-noise treatment or a no-noise control (via speaker in experimental lid, in place in all contests); nest presence and size were recorded before contests and aggressive motivation probed via the startle recovery approach using interruptive marble drops through access tubes in the lid. b Recordings of underwater sound conditions with a hydrophone indicated greater changes in sound pressure level and c increases in wavelength frequency during noise treatment, compared to the control condition. Frequencies ranged up to expected hearing maxima for B. spelndens (dashed line: auditory structures and sensitivity; Ladich and Yan, 1998 [47]; Ladich and Popper, 2001 [48]). d Underwater sound profiles under noise exhibited sound pressure changes at higher frequencies, which reflected anthropogenic noise near light terrestrial and shipping traffic (full line: adapted from Dahl et al., 2007 [49])