Courchamp investigated this awkward trade-off by modelling how the costs of a babysitter change with decreasing pack size. This showed that packs of more than five adults should be able to feed all the pups and still spare a babysitter. But with smaller packs, either the hunting or the babysitting suffers, or the animals have to compensate by increasing the number of hunting excursions - which itself carries a cost to the pack.

　　Field observations in Zimbabwe supported the model. Packs of five animals or fewer left pups unguarded more frequently than larger packs did. There was also evidence that when they did leave a babysitter, they were forced to hunt more often.

　　A pack which drops below a critical size becomes caught in a vicious circle, says Courchamp, who is now at Paris Sud University. Poor reproduction and low survival further reduces pack size, culminating in failure of the whole pack And deaths caused by human activity, says Courchamp, may be what reduces pack numbers to below the sustainable threshold. Mammal ecologist Chris Carbone at Londons Institute of Zoology agrees. Maintaining the integrity of wild dog packs will be vital in preserving the species, he says.

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