Tempest — Common
Muscle Sliver rulings:
10/4/2004 : Yes, it does give the +1/+1 bonus to itself.
7/1/2013 : Abilities that Slivers grant, as well as power/toughness boosts, are cumulative. However, for some abilities, like flying, having more than one instance of the ability doesnt provide any additional benefit.
7/1/2013 : If the creature type of a Sliver changes so its no longer a Sliver, it will no longer be affected by its own ability. Its ability will continue to affect other Sliver creatures.