Parental Costs

Indirect reproductive costs of oviparity in cockroaches include the time, energy, and predation risks involved in concealing the ootheca in the environment and the metabolic expense of producing a protective oothecal case. The case consists primarily of quinone-tanned protein (Brunet and Kent, 1955) (Table 4.5), much of which can be recovered after hatch if the parent or neonates eat the embryonic membranes, unviable eggs, and the oothecal case after hatch (Roth and Willis, 1954b; Willis et al., 1958). In several species of cockroaches, oothecal predation by adults and the ingestion of oothecal cases after hatching by nymphs increases when other protein sources are lacking (WJB, unpubl. obs.).

Live bearing permits females to dispense with producing a thick, protective oothecal case, and allows them to channel the protein that would have been required for its manufacture into present or future offspring or into their own maintenance. Nonetheless, the burden of "wearing" the next generation may be metabolically expensive and impair mobility, with consequences for predator evasion and foraging efficiency. In B. germanica, however, Lee (1994) found no correlation between the physical load on the female and oxygen consumption, and in N. cinerea the mass-specific metabolic heat flux of pregnant females at rest was actually reduced in relation to non-pregnant females. This suggests that the energetic demands of gestation in these species do not translate into increased metabolic rates (Schultze-Motel and Greven, 1998). Still, most female cockroaches feed little, if at all, during gestation, even when offered food ad libitum in the laboratory (e.g., Blattella—Cochran, 1983b; Hamilton and Schal, 1988; Rhyparobia—Engelmann and Rau, 1965; Trichoblatta— Reuben, 1988). The most commonly offered explanation for fasting at this time is that the cumbersome bodies of pregnant females may increase their vulnerability to predation. This seems reasonable, given that, first, the mass of the reproductive product is 30% or more of female body weight in both B. germanica (Mullins et al., 1992; Lee, 1994) and N. cinerea (Schultze-Motel and Greven, 1998), and second, pregnant N. cinerea are demonstrably slower than virgin females of the same age (Meller and Greven, 1996a). Agility also may be affected. Ross (1929), however, opined that pregnant B. germanica "do not show any signs of being impeded by their burden" despite the clumsy ootheca dragging from their nether regions. Loss of agility may not be an issue in cockroaches that rely on crypsis or thanatosis to escape predators, but the larger body of gravid females requires a larger crevice in species that seek protective shelter (Koehler et al., 1994; Wille, 1920). It is unknown whether the physical burden of an egg clutch hinders flying in those species that depend on it for evasion. Blattella karnyi females can take to the air while carrying an impressive ootheca of up to 40 eggs (Roth, 1985).

In viviparous D. punctata, gravid females normally double their body weight during gestation but nonetheless forage; the nutrient secretion of the brood sac is derived from the maternal diet rather than stored nutrients, particularly in early pregnancy (Stay and Coop, 1974; WJB, unpubl. data). This species has hard, dome-shaped tegmina (common name = "beetle cockroach") and impressive defensive secretions (Eisner, 1958; Roth and Stay, 1958) that may permit some bravery when under attack by ants (Fig. 1.11A). Vertebrate predators, however, are threats, and lizards, toads, and birds have been observed eating them in the field (Roth and Stay, 1958; WJB, pers. obs.). It is possible that D. punctata females rely on readily accessible, predictable sources of high-quality food for supporting the explosive growth of their embryos. Their diet, however, appears little different from that of many other cockroaches.

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