Summary: Benedict et al. (2012)

Animals often signal at low frequency or with harsh sounds to indicate aggression. For some species, larger body size is correlated with lower frequency sound production and can potentially indicate fighting ability. Low frequency and harsh sounds may also indicate motivation to be aggressive and to attack. Benedict et al. (2012) investigated whether canyon wrens Catherpes mexicanus alter vocal behaviour (facultative adjustment) in the low frequency range, and whether they utter more harsh calls, in response to territorial intrusions (playback simulations). They found that territory holders altered their song type usage, lowered their frequency and increased song production rate in response to simulated intrusions. Territory holders were also more likely to attach harsh notes to the ends of songs. Benedict et al. (2012) indicate that these results support the motivation-structural hypothesis.