The mighty insect colonies of ants, termites and bees have been described as superorganisms. Through the concerted action of many bodies working towards a common goal, they can achieve great feats of architecture, agriculture and warfare that individual insects cannot.
That’s more than just an evocative metaphor. Chen Hou from Arizona State University has found that the same mathematical principles govern the lives of insect colonies and individual animals. You could predict how quickly an individual insect grows or burn food, how much effort it puts into reproduction and how long it lives by plugging its body weight into a simple formula. That same formula works for insect colonies too, if you treat their members as a collective whole.
Life is fundamentally about the use of energy, about effectively harvesting it from food and channelling it into existence and offspring. As animals get bigger, their changing use of energy ripples across all aspects of their lives. Because of economies of scale, larger and more complex animals need less energy for each individual cell. They grow and reproduce more slowly and they live longer.
The astounding thing is that this variety can be captured by a deceptively simple equation. An animal’s metabolic rate is proportional to its mass to the power of three-quarters (0.75). So a cat that is 100 times heavier than a mouse would have a metabolic rate that was around 32 times greater, and a human that is 10 times heavier than a cat would have a metabolic rate around 6 times greater. This beautiful three-quarters “power law” links all animals from mice to elephants.
Hou showed that it applies to insect colonies. He gathered data on over 168 species of social insects and noted the total mass of all their members. They ranged from species of fire ants whose colonies weigh little more than 2 milligrams, to African termite colonies that tip the scales at around 4kg.
If you’ve ever complained about having bad sex, you really have no idea. Human women may have to complain about poor stamina or incompetent technique but the female seed beetle (or bean weevil; Callosobruchus maculatus) has to contend with her partner’s nightmarish penis – an organ covered in hard, sharp spikes. Just see if you can look at the picture on the right without wincing.
It’s no surprise then that females sustain heavy injuries during sex. But why have male beetles evolved such hellish genitals? What benefits do they gain by physically harming their partners?
It’s possible that the injuries directly benefit the males, either because they stop the females from mating again or spend more efforts in raising their fertilised eggs to avoid the strain of future liaisons.
The alternative is that the spikes could give the males an edge in “sperm competitions“, where they compete with rivals not through direct combat, but through fertilising as many eggs as possible. In this theory, the spines are important for winning these competitions, and the wounds they inflict are simply a nasty side-effect.
Cosima Hotzy and Goran Arnqvist from Uppsala University think that the latter theory is right. They have found that the penile spines are vital to a male’s success – those with the longest spikes fertilise the most eggs and father the most young. Size, it seems, really does matter.