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

Abstract

Ejaculate adjustments 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 ask how males respond to this tradeoff 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 show that when spawning with several females simultaneously, nest males reduce the spawning duration for each clutch and the number of ejaculations per female tends to decrease, reflecting sperm limitation. Experimental exposure of nest males to sperm competition with dwarf males caused a reduction of the number and duration of ejaculations roughly by half. Hence, when exposed to competition with a superior rival, nest males do not increase their sperm expenditure as predicted by sperm competition risk models, but in fact save 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.

The evolution of strategic male mating effort in an information transfer framework

“…Thus, by ‘marking’ females as mated, the male mating first will benefit, as the male eventually mating second will as a consequence downgrade the value of these matings (cf. Schütz et al ., ). When there is first male sperm precedence ( r < 1), on the other hand, the second male's knowledge increases his optimal sperm investment to compensate for this disadvantage (Parker et al ., ), and this is to the first male′s disadvantage.…”

Alternative male morphs solve sperm performance/longevity trade-off in opposite directions

“…2 to 4 ). In contrast, nest males save investment in ejaculate production when competing for fertilizations with dwarf males ( 59 ).…”

“…Separate ejaculates are released for the fertilization of each egg, which involves a tight coordination process between female and male during spawning ( 45 ). More than 20 females may simultaneously breed in a nest ( 65 ), and nest owners may spawn with up to four females at a time ( 59 ). After spawning, females care for the eggs and hatched larvae inside the shell for a period of 10 to 14 days ( 40 ).…”

Effects of past mating behavior versus past ejaculation on male mate choice and male attractiveness

“…Similarly, investment into ejaculates may impose costs that affect male mate choice ( Lemaître et al 2020 ). Males that ejaculate frequently experience faster sperm depletion ( Montrose et al 2004 ; Schütz et al 2017 ; Abe 2019 ) that can lead them to selectively allocate ejaculates to different females (e.g., Hunter et al 2000 ).…”