Mourning cuttlefish (Sepia plangon), generally solitary animals, gather together by the thousands once a year to mate. These groups' sex ratios are decidedly male-biased, and so males battle fiercely for the change to snag a mate – and once they have one, they guard her closely, preventing any competitors from stealing her away. But smaller, younger cuttlefish that are less capable of winning a mate in the traditional cuttlefish fashion have an ingenious way of making sure their genes get passed on. Much in the same manner as Homer's Odysseus slipped the Greeks into Troy in a wooden horse, smaller and weaker male mourning cuttlefish are able to approach and in many cases successfully inseminate an already-spoken-for female by posing as a female themselves. The male will split his skin coloration down the middle; the flank facing towards the female he's wooing bears the smooth, striped pattern characteristic of a male, while the flank facing away from his would-be mate – towards the male he's cheating – will flash yellow and mottled, the mark of a female cuttlefish.
Sending false messages to other individuals – or lying – is something we may be accustomed to among our fellow humans. But animals sending each other false signals isn't as straightforward as it seems. Evolution should, in theory, steer species towards honest displays and communication. If they aren't honest, natural selection will over time lead the recipient to ignore the false signals (think "the boy who cried wolf"). Game theory offers an explanation for how cheaters can succeed: if a signal is generally reliable (say, if you're a mourning cuttlefish, color pattern as an indication of sex), then a small number of cheaters can trust that they'll be trusted. Nearly 40% of males in the presence of one female and one rival tried to steal away the female. Brown, Garwood, and Williamson suggest that males shied away from attempting a cheat when there were other, larger males around that might catch them in the act. Among many animals, social punishment is generally a big deterrent for would-be fakers, and cuttlefish, it would seem, are no exception to this rule.
While this study offers an exciting snapshot at a fascinating behavior, there's still more to learn about these cuttlefishes' trickery. For example, we don't know for sure which male's sperm goes on to fertilize the female's eggs. Cuttlefish are thought to be capable of storing multiple males' sperm and later choosing which she will use (known as "cryptic mate choice"). It's possible that the female chooses the sperm from the larger, stronger male that fought for her over sperm from the smaller, weaker cheater. The study did not extensively explore other means by which cuttlefish might tell the sexes apart; it is possible that there are pheromonal cues, for instance, that the study does not control for. In addition, the study is limited in scope by its geographic confines; the behavior's context and frequency could differ in other cuttlefish populations.
This is an exciting discovery for many reasons. It gives us greater insight into how false signaling and deception play out between animals, a phenomenon not yet fully understood. It also serves as one of very few examples of deceptive tactics in invertebrates, and further illustrates to us how cognitively advanced cephalopods really are. We still have a lot to learn about cephalopods: how complex social interaction shapes their evolutionary path, what's happening at the neurological level for these animals when they solve problems (cephalopod brains are almost entirely alien to us in their structure), and to what degree females really do choose "brains over brawn" as they appear to in this study. Hopefully future research will further our understanding in these areas.
Originally written as an assignment for the University of Melbourne's open online Animal Behavior class.