Selfish sentinels in cooperative mammals

Science

Volumn 284, Issue 5420, 1999, Pages 1640-1644

  Clutton Brock, T H ^a   O'Riain, M J ^b   Brotherton, P N M ^a   Gaynor, D ^b   Kansky, R ^b   Griffin, A S ^c   Manser, M ^a  

^a UNIVERSITY OF CAMBRIDGE   (United Kingdom)

^b UNIVERSITY OF PRETORIA   (South Africa)

^c UNIVERSITY OF EDINBURGH   (United Kingdom)

Author keywords

[No Author keywords available]

Indexed keywords

ALTRUISM; ANIMAL BEHAVIOR; ARTICLE; CROWDING; FIGHTING; FOREST; HUNGER; MAMMAL; NONHUMAN; PREDATION; PRIORITY JOURNAL; WEANING;

ANIMALS; BEHAVIOR, ANIMAL; COOPERATIVE BEHAVIOR; FEEDING BEHAVIOR; FEMALE; HERPESTIDAE; MALE; NUTRITIONAL STATUS;

EID: 0033522806     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.284.5420.1640     Document Type: Article

References (35)

1. Examples include lookout or sentry duty in fighting units, watch keeping by sailors, and supervision rotas in some industries.
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12. Our study used a total of 18 different groups of meerkats, ranging in size from 2 to 20 adults, living within 50 km of Nossob in the Kalahari Gemsbok National Park (25°17′S, 20°32′E). The study area consisted of a mixture of dry riverbeds with river terraces on either side and sparsely vegetated sand dunes, and potential predators were abundant [see T. H. Clutton-Brock et al., Afr. J. Ecol. 37, 69 (1999)].
13. S. P. Doolan and D. W. Macdonald, J. Zool. 239, 697 (1996); Behavior 134, 837 (1997).
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15. T. H. Clutton-Brock et al., J. Anim. Ecol., in press.
16. A. S. Griffin, thesis, University of Edinburgh (1999).
17. The principal foods of meerkats are beetles and their larvae, scorpions, solifugids, millipedes, and small lizards (8, 9).
18. The main predators of meerkats are raptors (including martial eagles, tawny eagles, and bateleurs) and medium-sized carnivores (including black-backed jackals and African wild cats). Predators are abundant in the park, and annual rates of meerkat mortality are around 0.68 per year for adults (9).
19. 2,10 = 7.16, n = 6 groups, P = 0.01).
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22. M. Manser, thesis, University of Cambridge (1998).
23. We measured the amount of time that different individuals spent on raised guard (GT) as well as the amount of time they spent guarding at burrow entrances (GB) and the amount of time they were vigilant while foraging (GA) in five different groups in the park. All animals in our study groups could be recognized individually and were habituated to observers walking in the middle of the group. Except when predators approached or during the heat of the day, when they rested below ground, the animals spent most of their time foraging in the open, and it was usually possible to keep all individuals in view. Individuals were sexed during capture or by close observation and were classified as pups from 0 to 3 months of age, as juveniles from 3 to 12 months, and as adults at 12 months and over. The time spent guarding was estimated by scanning all group members at 10-min intervals and recording their activity on handheld computers during observation periods and was expressed as a percentage of group foraging time (periods when at least 75% of group members were foraging). In practice, activity was closely synchronized among group members, and (with the exception of sentinels) usually either all or no group members were foraging.
24. A guard was recorded as looking toward the group when the group fell within 180° of its direction of gaze. The time spent looking toward versus away was sampled by recording the direction of gaze of guards at 10-min intervals. The proportion of time each individual spent looking toward versus away was calculated using an average of 46 scans per individual.
25. Distances were sampled by recording the distance to the nearest bolt-hole whenever individuals went on raised guard and also by recording the same distance for a randomly identified foraging adult. An average of six records were collected for each of 14 individuals (range, 3 to 11 records per individual).
26. Our second study area was dose to Vanzylsrus, approximately 150 km southeast of Nossob, where we monitored the behavior of individuals in eight different groups. Here all predators except goshawks were less abundant, and annual rates of mortality were around 0.34 for adults (9).
27. For each group, we calculated the percentage of foraging time spent on raised guard by each individual on at least 5 days. Where more than one subordinate of each sex was present in a group, means were calculated across individuals of the same sex to compare with values for dominant males and dominant females in the same group.
28. J. H. Zar, Biostatistical Analysis (Prentice-Hall, London, 1996).
29. Unrelated immigrants were identified by a knowledge of pedigrees combined with analysis of genetic microsatellite markers (70). Though we were only able to compare related and unrelated individuals in seven groups, the results of this comparison coincide with more extensive analyses of individual contributions to babysitting and other cooperative activities.
30. T. H. Clutton-Brock et al., Proc. R. Soc. London Ser. B 265, 185 (1998).
31. To investigate the alternation of guarding bouts, we recorded the start and end time and the type of every guarding bout over a 5-day period for each of seven groups. To investigate the effect of the presence of a raised guard on the frequency of raised guarding by other group members, we scored whether each raised guard was initiated during the presence or absence of another guard. We then compared these observed frequencies with expected frequencies, calculated from the proportion of the groups' foraging time during which at least one guard was present and controlling for the fact that fewer individuals were available to initiate guards when one animal was already on guard.
32. We investigated patterns in the order in which individuals performed raised guards within each group by recording the number of intervening guards between consecutive guards by the same individual. We then compared the observed distribution of guard intervals with expected distributions calculated by randomizing each sequence 1000 times.
33. As well as recording the distribution of guarding times, we were able to curtail individual guarding bouts by gently shaking the branch on which the animal was standing until it descended and started to forage again. Animals rarely attempted to return to raised guard immediately but, if they did so within a minute of ceasing to guard, we repeated the process until they desisted. In addition, to investigate the effects of nutrition on guarding, we fed some individuals with hard-boiled egg, comparing their behavior with that of unfed controls of similar age and sex.
34. Uninterrupted guarding bouts: For eight individuals, we calculated the mean latency from the end of a period of uninterrupted raised guarding to its return to guard, for an average of 14 bouts per individual. Only periods of raised guard lasting at least 30 s were included, and samples were restricted to guarding bouts in the first half of the morning feeding period. Interrupted guarding bouts: Each individual in this sample was interrupted after guarding for 2 min on an average of eight occasions, and the mean latency to the individual's next return to guarding duty was calculated for these bouts. Interrupted and egg: We subsequently interrupted each individual once and fed it with 25 g of hard-boiled egg and again measured the latency to its next return to raised guard. Eight individuals were sampled one to three times each.
35. We are grateful to the National Parks Board of the Republic of South Africa for permission to work in the Kalahari Gemsbok Park. For help and support we thank E. le Riche and D. Engelbrecht, wardens of the park; P. Novellie; A. Hall-Martin; D. Ras; J. Herrholdt; G. de Kock; S. de Waal; and Mr and Mrs H. Kotze for permission to work at Vanzylsrus. The study would not have been possible without the support of members of the Mammal Research Institute of the University of Pretoria, including J. Skinner, P. Richardson, A. MacKenzie, M. Haupt, and G. van Dyk. For access to genetic analyses, we are grateful to J. Pemberton and T. Marshall (Institute of Cell, Animal and Population Biology, University of Edinburgh). Thirty-five assistants contributed to data collection: G. Avey, J. Barnard, C. Britten, J. Chadwick, P. Chadwick, S. Clarke, A. Crichton, S. Davies, P. Dixon, P. Elsmere, J. Garner, J. Kewido, J. Kinns, C. MacCallum, A. MacColl, C. Macleod, T. Maddox, A. Marais, G. Marais, K. McKay, S. Mercenaro, H. Nicholls, I. Olyn, M. Peterson, L. Postgate, G. le Roc'h, L. Sharpe, M. Shaw, S. Slater-Jones, R. Smith, R. Tait, B. Themen, A. Toole, A. Turner, and R. Yarnell. For advice, access to data, assistance, or comments, we are grateful to J. Skinner, S. Creel, S. Doolan, T. Jackson, H. Kokko, L. Kruuk, D. Macdonald, M. Manser, G. Mcllrath, J. Nel, and R. Woodroffe. This research is funded by grants from the Natural Environment Research Council and the Biology and Biotechnology Research Council.