The plasticity of metamorphic traits in the Chinese brown frog tadpoles fails to obey Richards' hypothesis

According to Richards' hypothesis, algae or cells in the intestinal tract has been considered the cellular inhibitory factor, when they fall off and hide in the faeces, excreted together with the faeces. If the animals feed on these faeces with algae or cells, and bring them into their systems, then the cellular inhibitory factor would play the main responsibility for growth inhibition. Here, we surveyed the effects of different combinations of faeces and food level on growth rates, survivorship, larval age and mass, and SVL at metamorphosis of the Chinese brown frog Rana chensinensis. Our results showed that food level can influence the length of the larval period of Chinese brown frog tadpoles, suggesting that delayed metamorphosis is caused by low food supply, indicative of a function of effective energy. Our data also clearly indicated that tadpoles in the presence of faeces were on average larger in body mass than those in the absence of faeces, which failed to obey Richards' hypothesis. Moreover, our results found evidence that faeces have a positive effect on the growth rate of tadpoles. Thus, there is no evidence for Richards' hypothesis, suggesting that this novel mechanism is selected for where coprophagy is likely to prove profitable, irrespective of the abundance of alternative food. Keywords: Rana chensinensis, Richards' hypothesis, faecal material, mass at metamorphosis, growth rate