Young beetles engage in sibling rivalry when competing for parental attention just like children

Sibling rivalry is not just commonplace in human families, but in beetles too. 

A new study suggests young beetles of the common sexton beetle (Nicrophorus vespilloides) compete with each other when it comes to receiving care from their parents. 

A team of researchers, led by Darren Rebar, from Emporia State University in Kansas in the US, studied 22 generations of the burying beetle to find out more about family interactions within the species. 

These insects are seen as ‘the undertakers of the animal world’ as they bury dead and decaying animals, such as mice and small birds, and, even more gruesomely, feed on their corpses.  

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Larvae of the burying beetle Nicrophorus vespilloides developing in the absence of parental care

Parental care-giving can vary among sexton beetle populations, with some parents continuing to tend to their young until they reach the larvae stage, while others leave shortly after laying eggs. 

In two of the experiments, the authors separated the parents from their brood to examine their behaviour. 

They found young beetle siblings which were cared for by their parents were more competitive towards each other, while those which received no care appeared to be more co-operative. 

Generally, when parents provide care, a behavioural response in multiple species is to exhibit greater levels of competition for the resource that adults provide – in the form of movements towards the parent, for example, or aggression to their sibling rivals. 

This can sometimes result in ‘siblicide’ – the killing of an infant sibling – which can actually benefit the surviving offspring and the parents.

After 22 generations, the authors created mixed broods of newly hatched larvae and measured larval weight after five days as a proxy for fitness. 

Larvae from the full-care populations showed evidence of competition, but when parents stopped providing care for their offspring, the rival siblings started co-operating with each other.

The findings parallel human siblings who might start to help each other in the family home when the parents are otherwise absent.  

This cooperation can be seen to compensate for the lack of parental care and to aid each other’s survival. 

‘Rapid evolutionary switching between sibling rivalry and sibling co-operation is possible because siblings induce greater levels of rivalry (or co-operation) in each other,’ the authors wrote in Proceedings of the National Academy of Sciences.

Larvae of the burying beetle Nicrophorus vespilloides (pictured) as it appears while being cared for by an attentive parent

‘This generates positive evolutionary feedback, rapidly locking larvae into evolving greater levels of competition, or co-operation, in the presence, or absence, of parental care.’ 

The researchers then created mixed broods so that they contained equal numbers of co-operative and competitive larvae. 

They found that once most of the offspring in a brood started to express co-operative or competitive behaviour, it induced greater levels of co-operation or competition, respectively, within the rest of the group. 

The scientists said both parental strategies, be it caring for their offspring or leaving them to fend for themselves, can ‘increase an individual’s genetic fitness’. 

‘In natural populations of burying beetles, it is likely that the supply of care fluctuates greatly from one generation to the next, maintaining a mixture of co-operative and competitive larvae and so preventing any evolutionary runaway to purely competitive or purely co-operative broods,’ the study reads.