Originally published by ReefWatch SA on 5 August 2015.

Photo by Carl Charter

All animals have to reproduce. It’s how the next generation is created and how an animal’s genetics are passed on. But how do two male cuttlefish fight for a mate? Most fish don’t have to compete, as they release clouds of sperm and eggs into the water and fertilisation occurs. Most large land mammals have epic battles that end in the crashing of two large bodies together, and some whales have marathon races that last for hours, so that the female can determine the fittest and most suitable mate.

Giant Australian cuttlefish don’t have the ease of releasing clouds of eggs and sperm into the water and nor do they have the capacity to gauge their opponents’ capabilities like a mammal or bird, or so we thought. A recent study from Macquarie University has found that male cuttlefish do, in fact, ‘size up’ possible rivals before engaging in combat and react accordingly. This ability to size up rivals means that only 20% of all male interactions actually end in physical combat.

By applying game theory, behaviouralists in Sydney were able to measure and witness the ways in which the male cuttlefish’s actions changed dramatically according to the length of its rival. Whilst this has been recorded when small, unpaired males are faced with much, much larger, paired males, it wasn’t known how far this behavioural adaptation carried through their activities. In other words, if a submissive, unpaired male was faced with another small, unpaired male that was obviously shorter than it (measured by the mantle length), then the first male would put on an aggressive display. This consists of flashing colours along their sides, the waving and stretching of tentacles and swimming towards the interloper. But, if that same male was approached by a male that was longer than it, it immediately stops the light display and removes itself from the situation.

But why should we care? It’s long been known that many species of mammal and bird when engaging in male-male conflict will employ ‘resource-holding potential’ theory. In other words, if the other male is bigger, nastier and scarier, the smaller one will admit defeat and try its luck elsewhere. If two males are of similar size and strength, then they will commit to combat and the winner is left with his choice of females while the loser, yet again must admit defeat and leave.

The fact that ‘resource-holding potential’ is evident in Australian cuttlefish tells us a number of things about these fascinating creatures. For starters, it is just another piece of behavioural evidence that shows how intelligent these colourful and intriguing creatures are. This ability to judge another and compare a rival to oneself is also a very good indication of analytical capabilities. And lastly, the ability to judge a rival and assess your chances of winning in combat shows that this behavioural capability may have evolved because of selection pressures across many different groups of animals.

So there you go, even male cuttlefish measure one another up before starting a fight.

Academic Paper Link: http://www.sciencedirect.com/science/article/pii/S0003347215002237