Resizing images whist keeping their ratio on a Mac

Hi, I was hoping someone could help as I have always used Indesign on Microsoft and recently have started using a Mac. 

 

I am basically looking for the Mac version of Ctrl and Shift when resizing images in Indesign. I have tried all different combinations and cannot get the same outcome on the Mac. (the images do not keep their ratio) 

 

Thanks!