Arp records in solaris

["Edward J. Aivazian" <stealth () arminco com>]

I'd like to start the question with some quotings from authorized sources.
The man page of arp command under solaris says the following: 


>    -s        Create an ARP entry for the host  called  hostname
>              with  the Ethernet address ether_address. The Eth-
>              ernet address is given as  six  hexadecimal  bytes
>              separated  by  colons. The entry will be permanent
>              unless the word temp is given in the  command.

When I make a record with -s option, in a very short period of time (sometimes less than half-an-hour) this record is 
deleted. 
One more article is available here, but it's in russian - http://www.void.ru/content/856


Can anyone give a reasonable explanation to this?