Daily rhythms are controlled by a circadian clock, entrained to the overriding cue of light intensity (a ‘zeitgeber’ in the terms of Lorenz & Kickert, 1981), and in evolutionary terms, responding to a zeitgeber facilitates efficient use of the environment (Kronfeld-Schor et al., 2001). Here, it triggers appropriately timed, physiological and behavioural responses (Heldmaier et al., 1989; Refinetti, Nelson & Menaker, 1992; Aronson et al., 1993), and facilitates interspecific coexistence (Schoener, 1974; Richards, 2002). Though temporal partitioning in communities has

never been a strong focus of ecology (Kronfeld-Schor Wnt inhibitor & Dayan, 2003) and biologists are aware that there is a degree of rigidity in the response to light, there are few field data to reveal the plasticity of this endogenous rhythmicity.

In particular, little is known of what triggers are likely to mask the zeitgeber, although there are examples where one species causes another to adopt an opposite activity pattern [e.g. mink Neovison vison : otter Lutra lutra and fox Vulpes vulpes : rat Rattus norvegicus interactions (Fenn & Macdonald, 1995; Harrington et al., 2009)]. Furthermore, in the context of landscapes increasingly dominated by people, behavioural plasticity may reduce the threats to a species but will incur a cost [e.g. hyaenas Crocuta crocuta (Boydston et al., 2003)]. With the African wild dog or painted hunting dog (Courchamp, Opaganib in vivo Rasmussen & Macdonald, 2002) Lycaon

pictus (hereinafter referred to as Lycaon) representing a monotypic genus and listed as endangered by the International Union for Conservation of Nature/Species Survival Commission (Woodroffe, Ginsberg & Macdonald, 1997), the aim of this article is therefore to explore (1) the relationship between activity patterns of Lycaon and sympatric competition under ‘natural’ see more conditions of coexistence; (2) plasticity in response to high anthropogenic activity; (3) potential costs of sub-optimization and masking behaviours. We present data from two parapatric Lycaon populations in Zimbabwe, and their competitors. As circadian entrainment is essentially light driven, we make our measurements relative to solar and lunar light cues. Lycaon are eusocial (Sherman et al., 1994; Rasmussen et al., 2008) kin-selected, obligate cooperative breeding canids (Courchamp et al., 2002), living in packs of up to 20 adults. Usually, only the alpha pair breeds, with the remaining adults being reproductively suppressed. It is among the most endangered large carnivores in Africa, with most of the remaining packs being in populations too small to be viable (Woodroffe et al.