The adjustment of reproductive threshold to prey abundance in a capital breeder

1. Many organisms rely upon stored energy reserves to support reproduction and do not initiate breeding until their reserves exceed some 'reproductive threshold'. However, the determinants of such thresholds are poorly understood; for example, we do not know if they are fixed (invariant) or respond dynamically to fluctuations in resource availability. 

2. An 8-year field study on water pythons (Liasis fuscus Peters 1873) in tropical Australia shows that individual female pythons adjust their reproductive thresholds in response to annual variation in prey abundance. 

3. In every year of the study, female pythons that reproduced were in better condition (mass relative to body length) than non-reproductive females. However, in years with low abundance of rats, female pythons reproduced despite being in relatively poor condition. Indeed, the mean condition of reproductive pythons in one particularly 'bad' year was as low as the mean condition of non-reproductive females in a 'good' year. 

4. Clutch sizes were slightly reduced in a 'bad' year, but the main effect of the lowered threshold was greater emaciation of the females after laying. 

5. Recapture records of marked snakes show that the annual variation in thresholds is due to flexibility of individual females, not to differential representation of cohorts with different reproductive thresholds. 

6. The dynamic adjustment of threshold levels fits well with predictions from life-history models and is likely to be a widespread phenomenon.