R-3.2.4 Mac/Linux different in < on characters

Linux:

x<-c("0","1");y<-c("a","-1"); x<y

[1] TRUE TRUE

Mac:

x<-c("0","1");y<-c("a","-1"); x<y
[1]  TRUE FALSE


On both systems, LC_COLLATE/LC_CTYPE/LANG are set to en_US.UTF-8

In Java,FWIW, I get the Mac answer if I use String.compareTo and the 
Linux answer if I use Collator.compareTo, but the result is consistent 
on Mac and Linux.