Over the past several years, I like to think I’ve been a sort-of successful spy on the secret lives of herps. As a behavioral ecologist with a focus on reproductive behavior, invading animal privacy comes as part of the job. Thankfully for my conscience, my subjects don’t seem to mind… or really even notice. One of the plus sides of working on reproductive behavior is that— at least when sex is in the offing—animal subjects seem to care a lot less that big, lumbering, non-predatory researchers are nearby. In this post, I’d like to share with you the delightfully graphic details of one of my spying expeditions on eastern fence lizard (Sceloporus undulatus) sexual competition.
Males of all species tend to fight. A lot. Not exactly a ground-breaking revelation. (Females fight too, but generalizing over all species… let’s face it: males on average tend to fight more.) What is immediately apparent across the animal kingdom is that fighting with males of your same species can be very, very costly. Think about some examples of “classic” male weaponry – antlers, teeth, tusks, horns – they’ve evolved to wound, maim, injure, and even kill.
Fence lizard males—despite their modest size, lack of obvious weaponry, and generally sweet nature (a spy’s personal opinion)—also can be absurdly aggressive when it comes to sex. Quick natural history lesson: Male fence lizards defend home ranges that abut or overlap those of other males; females have smaller home ranges that are nestled inside one or more male home ranges. When males fight with each other, it’s often over access to females who, from the males’ perspective, wander too much. Females watch males fighting and may make their own mate choice decisions based on what they see. So, you might imagine that it would behoove males to give each fight their all, not only to win the right to the lady, but also to impress her. However, that’s not what we’ve observed. Instead, we see a wide variation in fighting strategies: some male lizards are downright timid, backing down pretty quickly, while others go right for the throat, so to speak.
Above: A bout of territorial competition between two Sceloporus (either graciosus or occidentalis) involving chases and whole-body shudders.
As part of our research, we staged encounters between males to figure out why so much variation exists in how and when males escalate fights. We placed males in a laboratory arena, separated from each other (and a tempting, lovely, lady lizard) by clear glass to prevent any real bloodshed between competing males. Instead of observing who the winners and losers of real, pitched lizard battles were, we used an excellent indicator of contest escalation—display behavior. Display behaviors are body postures and movements that convey messages to competitors, for instance, lizard “pushups” and whole-body shuddering that are signals of territory assertion. Using video recordings, we carefully documented every display behavior each male sent to his competitor—essentially transcribing a non-verbal dialogue between two increasingly angry males. We were able to “decode” this dialogue, assisted by information collected and previously published by other researchers around the world. And what we found after decoding these secret messages was pretty surprising.
Because fighting can be so costly (not only is fighting exhausting, but there’s a high risk of getting a serious bite), we initially guessed that differences in the tendency to escalate fights may exist because males that can afford to lose more (“big and strong” males) were more willing to take the chance of having a costly encounter than scrawnier males. What we observed, however, was almost the exact opposite. Of the two males in each trial, we found that the wimpier of the two consistently responded to his competitor’s display behavior with more aggressive behaviors. The really cool part about this is that the tendency to respond with aggression isn’t hardwired— it’s not that small males were always more aggressive, but instead that the males that were smaller than their competitors were more aggressive. A nuanced, but important, distinction. Put a medium sized male with a bigger competitor, and he’d act like a tough guy; put the same lizard with a tiny competitor, and he wouldn’t bother to give the challenger the time of day. So, it seems like fence lizard males are somehow assessing their own chances of success in a fight against every individual competitor that they face, and adjusting their strategies accordingly. Being willing to escalate a fight may be crazy risky, but also could literally be the only way to defeat a physically formidable opponent. Littler guys need to be willing to take the risk in order to reproduce. Bigger males are more chill: there may be no need to waste time and energy doing displays when they don’t need to, since they’ve really got the size advantage if the confrontation were to come to blows (or bites!).