BIPARENTAL INVESTMENT AND REPRODUCTIVE SUCCESS IN A SUBSOCIAL DESERT BEETLE - THE ROLE OF MATERNAL EFFORT

Parastizopus armaticeps is a nocturnal subsocial detritivorous desert tenebrionid that produces very few offspring per brood. The two enviro nmental factors that constrain reproduction, rapid sand desiccation ra te and food scarcity, are countered by biparental effort. Males dig an d extend breeding burrows, maintaining their moisture level; females f orage on the surface at night for high-quality detritus, the larval fo od. This was shown to be a scarce and unpredictable resource for which there is high competition. When food was supplemented in a field expe riment, offspring number and survivorship doubled and burrow failure d ue to desiccation dropped from approximately half, the typical failure rate for unsupplemented burrows, to zero. Food supplementation did no t, however, increase larval foodstore size and there was no difference in the size of the offspring produced. Supplemented females reallocat ed their time, foraging less and digging more with the male. This chan ge in maternal behaviour patterns resulted in deeper burrows which rem ained moist longer, thus extending the larval production period. Femal e foraging efficiency, particularly food retrieval speed, determined h ow much time females could allocate to digging, consequently increasin g the reproductive success of the pair. Burrow depth and sand moisture level at the burrow base were the major correlates of reproductive su ccess, but the scarcity and unpredictability of high-quality food on t he surface and the competition for this resource influenced the number of offspring indirectly through their effect on female behaviour.