The fruits of courtship: Male great bowerbirds provide directed dispersal of the Burdekin Plum, a good predictor of mating success. (#16)
Virtually all known cases of mutualism involve reciprocal food and energy flow, and almost nothing is known about mutualisms where the fitness benefit to one partner is not food1. My study investigated a possible mutualism where the benefit to one partner is not food but an advantage in sexual display, and the benefit to the other partner is directed dispersal of seeds.
Male great bowerbirds (Ptilonorhynchus nuchalis) build their bowers underneath dense shrubs in semi-arid savannah2. Such plants are known as nurse shrubs and may facilitate the establishment of other plant species3,4,6. The male bowerbirds also collect fruit as ornaments to display on bower courts and directly present to prospective females5. Neither the male or female bowerbird consumes the display fruit, instead the fruit is eventually tossed off the bower and left to rot7.
I used motion sensing cameras to record mating behaviour and conducted intense surveys of seedlings at active bowers (currently occupied), abandoned bowers (previously occupied) and random sites. Of eight species of fruit collected only one, the Burdekin Plum (Pleiogynium timorense), was significantly associated with mating success when directly presented to the female. Colour of the fruits was not associated at all.
Conversely, bowerbirds chose sites with denser canopies to build bowers and as such brought Burdekin Plum fruits to a non-random ideal microhabitat for their germination. Burdekin Plum seedlings occurred in significantly greater proportions under abandoned bowers and in greater density than expected from seed arrival alone suggesting a differential survival of Burdekin plum seedlings at abandoned bowers.
This is only the second instance worldwide of mating success, rather than a food reward being quantified as the fitness benefit received by the seed disperser in a seed dispersal interaction. Furthermore, because the bowerbird-Burdekin Plum system involves sexual selection, and influences the reproductive success and distributions of both species, the potential to favour coevolution, and even speciation, may be much stronger than has been appreciated previously.
- Bronstein J. L. (1994) Our current understanding of mutualism. Quarterly Review of Biology 69, 31-51.
- Frith C. B. & Frith D. W. (2004) The Bowerbirds: Ptilonorynchidae. Oxford University Press, New York.
- Barnes P. W. & Archer S. (1996) Influence of an overstorey tree (Prosopis glandulosa) on associated shrubs in a savanna parkland: implications for patch dynamics. Oecologia 105, 493-500.
- Callaway R. M. (1992) Effect of shrubs on recruitment of Quercus douglasii and Quercus lobata in California. Ecology 73, 2118-28.
- Endler J. A., Gaburro J. & Kelley L. A. (2014) Visual effects in great bowerbird sexual displays and their implications for signal design. Proc. R. Soc. B 281.
- Tewksbury J. J. & Lloyd J. D. (2001) Positive interactions under nurse-plants: spatial scale, stress gradients and benefactor size. Oecologia 127, 425-34.
- Borgia G., Pruett-Jones S. G. & Pruett-Jones M. A. (1985) The evolution of bower-building and the assessment of male quality. Zeitschrift für Tierpsychologie 67, 225-36.