Reproductive investment of giants and dwarfs: specialized tactics in a cichlid fish with alternative male morphs

1. Adopting alternative reproductive tactics may require divergent solutions to reproductive competition among individuals of a population. Often investment in reproduction differs sub-stantially between individuals pursuing bourgeois and parasitic tactics, which may result in dif-ferent trade-offs and limitations. 2. Here we identify divergent behavioural, morphological and physiological traits of bourgeois and parasitic male morphs in Lamprologus callipterus, a Lake Tanganyika cichlid with an extreme size dimorphism among males. We focus on limiting factors and compare these between large, nest-building males and dwarf males parasitizing their reproductive effort. 3. Only nest males invest in courtship, and they exhibit much more aggression than dwarf males. In contrast, dwarf males spend 20 % of their time feeding, whereas nest males hardly ever feed. 4. Nest males accumulate reserves before breeding and use these up before taking a reproductive break, thereby performing a ‘capital breeder ’ strategy. In contrast, dwarf males use assimilated energy immediately for reproduction, thus acting as ‘income breeders’. This is a requirement of their spawning tactic, which only works out with a small and slim body. 5. A field experiment showed that nest males lose weight by their restricted feeding opportunitie

Sperm-limited males save ejaculates for future matings when competing with superior rivals

Adjusting ejaculates to sperm competition can lead to sperm limitation. Particularly in polygynous species, males may face a trade-off between investing sperm in current or future mating opportunities. The optimal sperm allocation decision should depend on the relative intensity of sperm competition experienced in a mating sequence. Here we asked how males respond to this trade-off in polygynous fish with alternative male mating tactics, intense sperm competition and sperm limitation. Large bourgeois males of the shell-brooding cichlid Lamprologus callipterus build nests consisting of empty snail shells, in which females spawn and raise offspring. During spawning, nest males release ejaculates into the shell opening. Genetically distinct, parasitic dwarf males enter shells during spawning to fertilize the eggs from inside the shell. These dwarf males were previously shown to be superior sperm competitors to nest males. Here we showed that when spawning with several females simultaneously, nest males reduced the spawning duration for each clutch and the number of ejaculations per female tended to decrease, reflecting sperm limitation. Experimental exposure of nest males to sperm competition with dwarf males reduced the number and duration of ejaculations by roughly half. Hence, when exposed to competition with a superior rival, nest males did not increase their sperm expenditure as predicted by sperm competition risk models, but in fact saved sperm for future mating opportunities as predicted by sperm competition intensity theory. This seems to be adaptive because of the considerable sperm demands in this species, which is partly due to their high degree of polygyny