- Contrary to the theoretical prediction that the optimal investment per offspring should be independent of maternal size, maternal size and investment per offspring are often positively correlated. However, some previous studies have reported a weak correlation or no correlation. Thus, it remains unclear why a positive correlation between maternal size and the investment per offspring persists in some organisms contrary to the theoretical prediction.
- In species in which the parents provide food for their offspring, the main components of parental investment are egg size and provisioning mass. Dung beetle (Onthophagus atripennis Waterhouse) parents produce dung brood balls, each of which is the whole provisioning mass for one offspring to grow to adulthood.
- Parental body size, egg size, and brood ball size were measured in O. atripennis. No correlation was found between maternal size and egg size, but a positive correlation was found between maternal size and brood ball size. Thus, the effect of maternal size differed between egg size and brood ball size.
- The positive correlation between maternal size and brood ball size may be a by-product of the brood ball construction process, because the egg maturation period determines brood ball size. Thus, the brood ball construction process may prevent the beetles from investing their resources optimally. It may often however be an advantage for the beetles because it allows the female to adjust the brood ball size according to variable dung quality.