Discrete call types referring to predation risk enhance the efficiency of the meerkat sentinel system

1. What they did

In some group-living animals, one member keeps watch while the others feed. This guard, or sentinel, often gives soft calls to tell the group they are on duty. In other species studied so far, sentinels use a single call type, and the rate of calling tells the group how risky things are. Meerkat sentinels are different: they produce six call types. The authors asked what these different calls mean.

They worked with nine habituated meerkat groups (8 to 24 animals each) at the Kuruman River Reserve in the Kalahari, South Africa, between May and November 2014. All meerkats were dye marked so each could be recognised, and one or two per group wore radio collars so the group could be found. They grouped four of the six call types into two categories:

• Short note calls: the common “single note” and “double note” calls.
• Long calls: the rare “di-drrr” and “wheek” calls.

They ran two playback experiments:

• Does danger change what the sentinel says? They played meerkat alarm calls (for aerial and land predators, at high and low urgency) from a speaker at least 7 m from a sentinel, and recorded the sentinel’s calls for two minutes before and five minutes after. There were 32 playbacks in eight groups.
• How do foraging meerkats respond to each call type? They played five-minute blocks of short note calls, long calls, close calls (the ordinary contact calls meerkats give while foraging, used as a control) and background noise to one foraging adult at a time, from a speaker 1 to 2 m away, while no real sentinel was on guard. There were 32 playbacks to four adults per group. An observer recorded the test animal’s foraging and vigilance on a handheld computer.

2. What they found

What sentinels said after an alarm call

• In the 30 seconds after an alarm call, sentinels gave fewer short note calls and more long calls than in the two minutes before.
• Over the full five minutes after the playback, there was no clear change. The authors note that meerkats usually relax within about 60 seconds of hearing an alarm call.

How foraging meerkats responded

• Foraging meerkats responded in the same way to close calls and to background noise, so not just any soft call changes their behaviour.
• With short note calls, they spent more time foraging than with close calls or long calls.
• With long calls, they foraged less and spent more time scanning, both on all fours (quadrupedal scanning) and standing upright (bipedal scanning), compared with short note calls and close calls.
• Short note calls did not change scanning compared with close calls.

3. What they concluded

• Meerkat sentinel calls carry information about small changes in the current risk of predation, and the rest of the group adjusts their behaviour to it.
• Short note calls work as an “all-clear” signal, like the Watchman’s song of other species. Long calls are a step before an alarm call: the group becomes more watchful but does not need to flee, and knows a sentinel is still on guard.
• The authors suggest calling the two categories “calming calls” and “warning calls”. A separate warning call has not been described in any other sentinel system.
• In species that only lower their call rate when risk rises, silence could be mistaken for no sentinel at all, so the group must respond either way. The extra warning call lets meerkats keep foraging when there is no urgent threat, which matters in a harsh environment where food is hard to get.
• Further research is needed on the two call types not tested (“triple note” and “multiple note” calls), and on whether the order of sentinel calls carries information.

6. Other technologies, skills or innovations

• Behavioural focals recorded with a program written in CyberTracker (version 3.376) on a Palm TX Tungsten handheld computer.
• Sound recorded with a Sennheiser directional microphone and Marantz recorder, held 0.2 to 0.5 m from the calling meerkat, using a telescopic pole when the sentinel was up a tree.
• Playbacks from a small rechargeable speaker held 0.3 to 0.4 m high, a height natural sentinels often choose.
• Long-term dye marking and radio collars from the Kalahari Meerkat Project allowed every animal to be identified and every group to be found.