by Jo Ohm
When I think of aggression, several things come to mind: basketball games, imperialist countries and violent television. But aggression encompasses much more than our human associations with the term. Aggression affects relationships and situational outcomes across the animal and plant kingdoms, with ant-plant interactions serving as a prime example of where aggression can go wrong.
In ant-plant mutualisms involving tree cholla cacti, Opuntiae imbricata, and ant visitors, the cactus is known to associate with two competing species of ants which vary in their levels of aggression: Crematogaster opuntiae and Liometopum apiculatum. Though the benefits, such as decreased herbivory, which these ants confer on their host plants is well known, scientists debate whether there is a cost to having too aggressive ant guards. I think we can all relate to this debate. Aggression is a positive force in many defensive strategies. In sports games, an aggressive team scores more points; in families the size of Cheaper by the Dozen, the aggressive sibling is first in line for the shower, dinner and teeth-brushing; in plant-ant interactions, the aggressive ant gets to eat the most herbivores. But can there too much of a good thing? Where do we see costs of aggression?
A recent study published in 2011 by Dutch researchers at the University of Utrecht found that preschoolers with aggressive behavior had costly affects on their families and society. Aggression in toddlers, researchers found, is both expensive to society and to the children’s families as this type of behavior merits intervention, driving up the school district’s costs for educational, health and social services. In addition, the parents of these children often suffer from the stress of having an aggressive child, which results in either sleep deprivation, strain at work or in the home and dealing with other consequences of aggressive behavior, such as property damage. In this case, aggression can come at an exorbitant price.
For ant-tended plants, ants are like children. They too can be costly when aggressive. In 2006, Josh Ness, a researcher at Skidmore College, presented findings on the effects that ant species with varying levels of aggressive behavior have on cacti pollination by cactus bees. Ness found that not only do pollination rates vary with ant-guard identity, but pollination rates are correlated inversely with aggression level. This means that more aggressive ants lead to less frequent pollinator visitations and later, less seeds and smaller fruits. Aggressive ant bodyguards are putting plants between a rock and a hard place: which is more important, protection from herbivory or reproduction? That’s the choice between having progeny and being eaten alive.
The cost of aggression seems to be a risk taken to gain some other greater fitness trade-off. Costs and benefits are weighed and balanced for these types of relationships to persist. Hopefully, by understanding the cost of aggression, I think we will be able to better understand how relationships between unrelated species form and how the ratio of costs to benefits affects fitnesses of each participating member. The implications for this research could range across almost all organisms, or anything involved in interspecific interactions (which is you, me, all organisms everywhere). Perhaps one member can be manipulated to defer costs or maximize benefits. Perhaps more third party interactions could show additional costs or benefits of aggression in mutualistic interactions. Perhaps this will affect our future parenting skills to foster less aggressive kids.
Ness, J. 2006. A mutualism’s indirect costs: the most aggressive plant bodyguards also deter pollinators. Oikos (113): 506-514.
Raaijmakers, Maartje A., Jocelyne A. Posthumus, Ben A. van Hout, Herman van Engeland, and Walter Matthys. 2011. “Cross-Sectional Study into the Costs and Impact on Family Functioning of 4-Year-Old Children with Aggressive Behavior.” Prevention Science. DOI: 10.1007/s11121-011-0204-y.