Characteristics of green rimu fruit that might trigger breeding in kakapo : a thesis presented in partial fulfilment of the requirements for the degree of Masters in Zoology at Massey University, Turitea Campus, Palmerston North, New Zealand
Species living in unpredictable habitats cannot prepare for a regular reproductive year, and must reproduce when conditions are favourable. The timing of reproduction of the seed predators involves anticipating an abundant food supply by tracking cues or triggers which initiate reproduction. These cues are poorly understood. The most supported theory is that a cue or cues is the size or abundance of the developing fruit crop, and possibly involves nutritional compounds in the crop.
The rimu (Dacrydium cupressinum) is a masting podocarp tree strongly linked to reproduction in kakapo (Strigops habroptilus). On Codfish Island kakapo only breed in rimu mast years and nesting is timed so chicks hatch when the rimu fruit crop (the sole food source for chicks) is due to ripen. This thesis investigates the theory that some component in unripe rimu fruit is involved in timing of nesting in kakapo. I characterised seasonal changes (over one year) in nutritional components (gross nutrients), polyphenolic compounds and other potential target polyphenolic substances with possible hormonal activity, including isoflavones and antioxidants, during rimu fruit development.A number of the compounds found in rimu fruit are known to affect reproduction in other species and could be potential triggers for reproduction in kakapo. In non-mast years insufficient fruit is available to trigger reproduction and there is no breeding, supporting the idea that the abundance of the fruit crop is also involved in the timing of reproduction. The trigger for breeding in these birds could be ingestion of a threshold level of a trigger substance or substances, or, as in the antbird, it could be entirely visual. The most likely contenders for the trigger in green rimu fruit include increasing levels of crude protein, crude fat, individual fatty acids, dietary fibre (changing the digestibility of other nutrients), calcium, phosphorus, calcium, total polyphenolic content and antioxidant levels, some flavenoids or a combination of these.
It is possible that the physiological changes necessary for reproduction are triggered by the abundance of the unripe fruit crop but that nesting itself is triggered by changes in the green, undeveloped fruit associated with ripening.