I'm studying allometric relationship between traits of insects. The pattern of the relationship from scatter plot is not clear. So, I would like to find the best fit model using regression analysis by comparing the data to various statistical models. I used lmodel2 and smtr packages for R, but they seemed work only with linear regression. What should I do if I would like to find equations from more complex models (such as polynomial and logistic) using reduced major axis method in R ?  

Thank you in advance 

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