The default format is 16-bit little-endian unsigned ints.
The manual page of hexdump explains the default format as well:
-x Two-byte hexadecimal display. Display the input offset in hexadecimal, followed by eight space-separated, four-column, zero-filled, two-byte quantities of input data, in hexadecimal, per line. (...) If no format strings are specified, the default display is equivalent to specifying the -x option.