ABC’s “Cavemen” could become extinct if it suffers more ratings declines like the one it endured this week, when it lost 26% of its solid premiere audience.

The comedy, which got a cool reception from critics but drew enough curiosity-seekers to deliver a pretty good opening-week rating, ran third in its timeslot Tuesday night. Companion laffer “Carpoolers” fared a bit better, building slightly on “Cavemen” but down 20% vs. its opening week.

According to “live plus same-day” DVR playback estimates from Nielsen, “Cavemen” averaged a third-place 2.6 rating/8 share in adults 18-49 and 6.96 million viewers overall, down from its first-place demo finish of the previous week (3.5/10, 9.19m). It placed second in adults 18-34 (2.2/8) and led its slot in key male demos.

The 26% decline among 18-49ers is rather steep, but it’s not the biggest falloff for a new show this fall: NBC’s “Bionic Woman” tumbled by 28% from its first to second week.

At 8:30, “Carpoolers” (2.8/8 in 18-49, 7.27m) declined from its premiere (3.5/9, 9.06m) but did manage to slightly improve on its lead-in. Show placed second in 18-34 (2.4/7).

Elsewhere Tuesday, a different network captured each hour of primetime with a drama, led by Fox juggernaut “House” (7.4/18 in 18-49, 18.03m), which clobbered its closest competitor, ABC’s “Dancing With the Stars” results show (4.0/10, 16.00m). Both “House” and its lead-in, “Bones” (3.2/9, 8.48m), were up a couple of ticks week to week.

CBS prevailed in the opening hour with “NCIS” (3.4/10 in 18-49, 16.71m), and NBC’s “Law & Order: SVU” cruised at 10 o’clock (4.9/14, 12.30m), with the latter logging its best delivery since February.

“SVU” is opening up some ground on its competish at 10, as both ABC’s “Boston Legal” (2.7/8, 10.58m) and CBS rookie “Cane” (2.3/7, 9.11m) were down.

Another new drama, CW’s 9 o’clock entry “Reaper” (1.6/5, 3.21m), dropped a bit week to week even though it had a stronger “Beauty and the Geek” lead-in (1.6/5, 3.21m). “Geek” rose 20% week to week in 18-34 (1.8/6 vs. 1.5/5).

Perhaps the best opportunity for “Reaper” to pick up some new viewers will come next week, when Fox sits “House” for postseason baseball. Unlike in previous seasons, though, when “House” may have rested for three or four weeks, this month will see the show preempted for baseball only that one time.