Dung beetle (Onthophagus atripennis) parents provision their future offspring by collecting dung. They transport it to a series of underground tunnels that they have excavated for that precise purpose. Once at the blind end of a tunnel, the dung will be shaped into a neat little “brood-ball.”
Mom will deposit an egg into a specific chamber on the top of the ball. She seals it with more dung. These balls play an important role in the development of the beetle larvae: this is all the provisioning that the parents will ever provide.
One would assume that the quality of the dung balls is critical to the successful development and emergence of dung beetle babies.
Many factors, including resource quality and predation pressure, play an influential role in the plasticity of parenting. When resources are of high quality and abundance, we would expect to see parents taking advantage of the bounty and having larger broods or larger offspring.
Dung beetle parents are capable of assessing the quality of dung and adjusting their investment accordingly. When they encounter a high-quality pile, such as that produced by fruit-eating monkeys, beetle parents will use it to produce a large number of small brood masses. This ultimately translates to a large number of high-quality emerging offspring.
Conversely, when the dung is of low quality, such as that produced by ruminant cows, parents will produce a lower number of offspring, each with a larger brood ball. Unfortunately, even with larger fecal-balls to eat, beetle larvae reared on low-quality dung are inevitably smaller at emergence than those provisioned with high-quality food.
Morphology and Social Context Alter Paternal Investment
In addition to the quality of the food resource, the level of paternal investment by male dung beetles varies with social context and morphology.
Males in the genus Onthophagus have one of two distinct morphotypes. Those larger than a critical body size develop a pair of disproportionately long horns, and those smaller than said critical size only develop rudimentary horns.
There are costs and benefits to both morphologies for males. Those with the large horns tend to take on a dominant role in reproduction, whereas those with small ones gain reproductive success by sneaking.
Paternal investment also varies between the morphotypes. When no other males are present, males with large horns provide a large degree of parental care in the form of help with tunnel excavation and dung transport, while those without horns do not.
However, when other males are present (i.e., when there is competition for mates), males will cease any paternal behaviour in favour of mate-guarding.
For dung beetles, competition for breeding opportunities is particularly intense. Dung patches rapidly lose quality (within twenty-four hours) and so they must be immediately exploited if they are to be useful. If a lone pair happens to find a rich resource, a greater level of investment into offspring can be realized. When competitiors are present it becomes more important for males to secure their female rather than to help her with offspring provisioning. In this way, the social context has an effect on the level of parental investment.