In many vertebrates, parents protect their young by detecting predators and aggressive conspecifics before they attack. But parental investment in protection is a limited resource, whose allocation to offspring should reflect optimization strategies. Thus, we tested if maternal investment in vigilance varied with the risk faced by young black howler monkeys (Alouatta pigra). Twelve females in six groups were compared, including those with and without young and those with immatures of differing ages. The greatest increase in vigilance was seen when immatures were conspicuous, and to a lesser extent, among mothers of dependent young (neonates and infants). These findings support the idea that parents adjust their behavior to both chronic and episodic risk faced by young. We explore the possible role of predation and infanticide in shaping the vigilance of mothers.