Paternal Investment or Mating Effort

A nuptial gift can benefit a male in two ways. The gift can function as paternal investment, where transferred nutrients or defensive compounds increase the number or quality of resultant offspring, or it can function as mating effort, which increases the male's fertilization success with respect to other males that mate with the same female (Eberhard, 1996). The hypotheses are not mutually exclusive, and there is debate on the distinction between them, centering mainly on the degree to which a donating male has genetic representation in the offspring that benefit from the gift. The latter is dependent on female sperm-use patterns, the length of her non-receptive period following mating, and the time delay until the female lays the eggs that profit from the male's nutritional contribution (reviewed by Vahed, 1998).

The incorporation of male-derived urates into oothecae of B. germanica suggests paternal investment, supported by three lines of evidence (Mullins et al., 1992). First, urate levels in eggs steadily decrease during development. This strongly suggests that the uric acid is metabolized during embryogenesis (Mullins and Keil, 1980), presumably via bacteroids transmitted transovarially by the female (e.g., Sacchi et al., 1998b, 2000). Second, 14C radioactivity not attributable to 14C urate is present in tissue extracts of oothecae (Mullins and Keil, 1980; Cochran and Mullins, 1982; Mullins et al.,1992).As pointed out by Mullins and Keil (1980), however, the 14C radiolabel reflects pathways involving carbon atoms and not neces sarily the path of nitrogen contained in urates. In subsequent work, however, Mullins et al. (1992) demonstrated that, third, 15N from uric acid fed to females did find its way into the nitrogen pool of oothecae, and was incorporated into four different amino acids. The question nonetheless remains as to whether the uric acid derived from a particular male ends up in the offspring that he sires (Vahed, 1998). Female B. germanica expel the empty spermatophore with the adhering urates about 24 hr after mating, then consume them between 4 and 18 days later, depending on her nutritional status (Mullins and Keil, 1980). Females typically transfer 90% of the food reserves accumulated during the pre-oviposition period into the next ootheca (Kunkel, 1966). It seems reasonable to assume, then, that the majority of the uric acid transferred during a given copulation is incorporated into the eggs of the next reproductive bout, particularly in un-sated females. Young females rarely mate more than once prior to their first ootheca (Cochran, 1979b), so during the first oviposition period a male can be reasonably certain that his nuptial gift will benefit his own offspring. Females may, however, mate between ovipositions. Paternity of subsequent oothecae is variable, but there is a tendency for first-male precedence (Fig. 6.6). The nuptial gifts of male consorts following the first male, then, may benefit some nymphs fathered by other males.

Gwynne (1984) argued that uric acid donation should not be classified as paternal investment, because, as a waste product, uric acid is likely to be low in cost. Vahed (1998) countered that it is likely to be a true parental investment precisely because of the low cost. If a gift is cheap, just a small resultant benefit to offspring will maintain selection for the investment. Neither author appreciated the fact that males deplete their uricose glands with each copulation, and actively forage for uric acid by seeking it out in bird and reptile droppings. The degree to which this foraging activity entails a cost in predation risk and energetic expense is an additional consideration.

Although a demonstration that male-derived nutrients are incorporated into eggs supports the paternal investment hypothesis, it does not necessarily rule out the mating effort hypothesis (Vahed, 1998). Because female cockroaches feed on male-provided urates after sper-matophore transfer, the nuptial gift cannot influence overt mate choice. The possibility remains that after copulation, females may bias sperm use based on the size or quality of the urate gift. In many species, females preferentially use the sperm of males that provide the largest nuptial gifts (reviewed by Sakaluk, 2000). With four separate chambers for sperm storage (discussed below), female B. germanica certainly have potential for exercising choice. The existence of substantial variation in sperm precedence suggests that she may be doing so.

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