One of the most widely accepted explanations for monogamy is the need for biparental care.
However, the occurrence of monogamy combined with biparental care is extremely rare in
oral incubating (mouthbrooding) cichlid fishes. Few studies have been performed on cichlid
species that exhibit this behaviour, and therefore the ecological factors that favour
monogamy in these cases remain obscure. Here we present new information on the natural
history and reproductive biology of Eretmodus cyanostictus (Boulenger 1898), a
monogamous biparental mouthbrooder from Lake Tanganyika. The populations studied consisted
of territorial pairs and a male-biased non-territorial population of smaller 'floater'
individuals. We present the first detailed description of spawning in this species, show
that breeding does not appear to be synchronised within the population, and provide
evidence that parental care is costly. We discuss the implications of this information
for our understanding of monogamy.