Vocal cooperation between the sexes in Little Spotted Kiwi Apteryx owenii ANDREW DIGBY, 1 * BEN D. BELL 1 & PAUL D. TEAL 2 1 Centre for Biodiversity and Restoration Ecology, School of Biological Sciences, Victoria University of Wellington, PO Box 600, Wellington, 6140, New Zealand 2 School of Engineering and Computer Science, Victoria University of Wellington, PO Box 600, Wellington, 6140, New Zealand Sexual call dimorphism in birds is usually associated with sexual size dimorphism. Departures from this relationship can be used to infer call function, but research into inter-sexual call differences, as with song function in general, has been restricted by a bias towards male passerines. The nocturnal and flightless New Zealand kiwi (Apterygidae) are acoustically similar but taxonomically and ecologically very different from other birds, so provide a contrast in exploring avian call function and evolution. However, kiwi acoustic ecology is poorly understood, with the calls of only one of the five kiwi species spectrally described, and acoustic differences between the sexes virtually unknown. We conducted the first bioacoustic study of Little Spotted Kiwi Apteryx owenii, and assessed sexual call dimorphism in this species. There were significant inter-sexual differences in call temporal and frequency characteristics that were not related to size dimorphism. Contribution to duets and variation in temporal structure with call context also differed between the sexes. We suggest that these differences indicate divergent call function, with male calls more suited for territory defence, and female calls for pair contact. There was a striking lack of overlap in the frequency spectrum distributions of male and female calls, which was also unrelated to size and was further emphasized by the presence of formants in female calls. We propose that this provides evidence for inter-sexual acoustic cooperation in call frequency, of a type which to our knowledge has not previously been described in birds. This may result from selection for enhanced joint resource defence in kiwi. Keywords: Apterygidae, duet, sexual dimorphism, vocal communication. An inverse relationship between vocalization fre- quency and body size is found in many taxa (Ryan & Brenowitz 1985), including diverse bird groups (e.g. Bertelli & Tubaro 2002, Martin et al. 2011). This is because larger individuals usually have lar- ger vocal structures, which produce sounds with a lower fundamental frequency (Wallschl€ager 1980, Ryan & Brenowitz 1985, Seddon & Sorenson 2005). In sexually dimorphic bird species, the lar- ger sex usually has lower frequency calls (Maurer et al. 2008, Geberzahn et al. 2009). However, there are exceptions. In some sexually dimorphic species, the larger sex has higher-pitched calls (Herting & Belthoff 2001), whereas in others there are no inter-sexual call differences (Janicke et al. 2007). In contrast, some sexually monomorphic species have heterogeneous calls (Bourgeois et al. 2007, Volodin et al. 2009). Furthermore, the slope of the size–frequency regression varies among avian clades (oscines, suboscines and non-passe- rines; Ryan & Brenowitz 1985). Call frequency can also be influenced by other factors, such as habitat type (Bertelli & Tubaro 2002, Nicholls & Goldizen 2006), ambient noise (Slabbekoorn & Peet 2003, Slabbekoorn & den Boer-Visser 2006, Luther & Derryberry 2012), bill morphology (Grant & Grant 2010) or even the context of calls (Geberzahn et al. 2009). Departures from the expected frequency–size relationship make sexual call dimorphism an important mechanism for determining call *Corresponding author. Email: andrew.digby@vuw.ac.nz © 2013 British Ornithologists’ Union Ibis (2013), 155, 229–245