This is an excellent point. I bet they do have a daily proof of life.
In general, I think Alan's way of surviving is much more sustainable than Sam's. Like someone previously mentioned, Sam hasn't even tried to fish or set up something like Alan and Lucas had. Did Alan give up on his water bottle fish traps and the large basket he made? Wasn't a single mention of it last episode and I figured those would keep bringing in food.
I believe in one of the extra episodes of "behind the scenes" stuff Alan commented on how those bottles provided a small but consistent source of food from the point he created them on. These be Alan's version of mice.
Doug