IF an organ serves several functions, two or more of these may conflict at times in their contribution to the immediate fitness of the organism. Among lower invertebrates the tail is commonly used as a defence against predators but it has other functions such as fat storage, locomotion, respiration and courtship. Experiments have shown that tail loss can depress reproductive output in lizards1,2, but this has not been reported for natural populations. Data reported here suggest that loss of the tail frequently depresses reproductive output in natural populations of the plethodontid salamander Batrachoseps attenuatus, which is widespread in California. The dual use of the tail for energy storage and for protection may increase the adaptive flexibility of the salamander for coping with an unpredictable environment.