et al. 2004; Salim et al. 2007). Star formation rates are only used for those galaxies classified as ‘star-forming’ by the definition of Kauffmann et al. (2003). Specific SFRs are determined by combining the SFRs described above with the stellar masses from Mendel et al. (2014).

-   The halo masses come from the group catalogue of Yang et al. (2007, 2009).

-   Local environmental densities are computed as $\Sigma_n = \frac{n}{\pi d_n^2}$, where $d_n$ is the projected distance in Mpc to the $n^{th}$ nearest neighbour within $\pm$1000 . Normalized densities, $\delta_n$, are computed relative to the median $\Sigma_n$ within a redshift slice $\pm$ 0.01. In this study we adopt $n=5$.

-   Stellar mass density is defined as log($\mu_{*i})= \log(M_*) -\log(2 \pi R_{50i}^2$) in which M$_*$ is the stellar mass and R$_{50i}$ is the radius (in kpc) enclosing 50 per cent of the total Petrosian $i$-band flux.

A simple example of HI mass estimation from non-linear methods {#sec-example}
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Before launching into a complex many-parameter ANN application for  estimation, we show a simple example of comparing the linear estimation of Zhang et al (2009) to a non-linear method. Zhang et al (2009) used a sample of $\sim$ 800 galaxies with HI 