What makes the first 3 alkanes decrease in melting point while after that they rise?

What makes the first 3 alkanes decrease in melting point (m.p.) while after that they rise?


$$
\begin{array}{lr}
\hline
\text{Alkane} & \text{m.p.}/\pu{°C} \\
\hline
\text{Methane} & -182 \\
\text{Ethane} & -183 \\
\text{Propane} & -188 \\
\text{Butane} & -138 \\
\text{Pentane} & -130 \\
\text{Hexane} & -95 \\
\text{Heptane} & -91 \\
\text{Octane} & -57 \\
\hline
\end{array}
$$


