Intrigued by the flagging tape on the trees in the University Gardens on the University of Florida campus in Gainesville?
Curious about the brown anoles with beads on their tails? Wondering why there are people wandering the grounds every day with binoculars and long fishing poles?
It’s all part of my Ph.D. thesis research on the social lives of lizards! This project is funded by Harvard University, the Animal Behavior Society, and a Young Explorers Grant from National Geographic.
Briefly, the aim of this project is to measure how individual brown anole lizards vary in their movement patterns—are some lizards wanderers and others homebodies? Understanding variation in movement will in turn let us understand how different lizards encounter their mates—do the wanderers encounter more mates on their travels, for instance, or do the homebodies with the largest homes attract the most mates? Knowing how reproduction is structured in brown anoles is crucial to figuring out how these lizards, with their sticky feet and their showy throat-fans, have evolved. Understanding brown anole reproduction is also crucial to uncovering how this widespread invasive species has been able to invade and persist so well in new environments. Keep reading for more details!
Who are the Brown Anoles?
Brown anoles belong to the genus Anolis, a group of over 400 species of lizards found in North America, the Caribbean, Central America, and South America. Anoles are active during the day, eat insects, and are especially good at living in the trees. Biologists have been studying anoles for decades! You can read all about the biology of Anolis lizards at the Anole Annals blog and in Lizards in an Evolutionary Tree, a book about all things anole written by Dr. Jonathan Losos.
Brown anoles (Anolis sagrei) are native to Cuba and the Bahamas, but have spread across the south eastern United States in the last 150 years. (How did they make the trip? They might have hitched a ride on a boat, or perhaps their eggs were transported in a potted plant). In the warmer months, you can see them all across cities and towns in Florida. Seriously, they’re everywhere!
What’s the Question?
Biologists have thought for many years that most anoles, and brown anoles in particular, are territorial and polygynous. What does this mean?
The conventional wisdom on anoles’ social lives goes like this: imagine that the blue line is the boundary of the territory of the male anole (the male is in the centre, with the red-and-yellow fan or “dewlap” under its throat). If this lizard were perfectly territorial, he would do two things
- He would stay in this one location throughout the breeding season (March to August)
- He would defend the area within his territory against other males.
Females are thought to be similarly territorial, maintaining smaller territories that they defend against other females (shown in red).
If this male is perfectly territorial, we expect him to mate with all the females whose territories are contained within his, and to prevent these females from mating with anyone else. If brown anoles are in fact perfectly territorial, then we expect males to mate with multiple females, and we expect females to mate with just the one male in whose territory they live. This mating system is known as polygyny.
BUT, are brown anoles actually polygynous? We have one very convincing piece of evidence that suggests they aren’t. By using DNA sequencing to identify the fathers of baby lizards, biologists have shown that female anoles often mate with more than one male. This multiple mating by females means that brown anoles cannot be perfectly territorial—males may change their locations, females may change their locations, and males may not exclude other males from their territories completely. Any or all of these possibilities can result in the breakdown of territorial polygyny, but which is it in the case of the brown anole? If lizards are moving out of their territories, then who’s moving and how much? Which other lizards do they interact with when they move? If multiple male and female lizards’ territories overlap with one another, then who mates with whom? If they aren’t territorial and they aren’t polygynous, then what is the brown anole’s mating system? These are some of the questions I’m trying to answer.
What are we actually doing?
To understand where individual lizards are moving and who they’re interacting with, we’re catching as many of the lizards in the University Gardens as we can. To catch a lizard, we slip a small slipknot over the lizard’s head and tug gently–the slipknot tightens enough to capture the lizard but not enough to hurt it. The slipknot is attached to the end of a retractable fishing pole, which lets us reel the lizard in and extricate it from the slipknot as soon as possible. We then mark each lizard permanently at the base of its tail with a unique combination of coloured beads (it’s like giving the lizard a piercing!). These tags let us identify individuals from far away, with binoculars. Then, every time we see a lizard, we figure out who it is, note the time at which we see it, and write down where in the garden it is.
We’re also making a map of the trees in the University Gardens, measuring lots of distances between the trees and then plotting their locations. Combined with our observations, this map allows us to quantify each lizards’ movement in space and time. We’ve marked more than a hundred lizards so far, and will observe them as often as we can until the end of the breeding season in August.
The preliminary data we collected last year suggests that while some brown anoles stick resolutely to a single tree, others move a little bit, and some move a LOT more than we’d expect if they were territorial. And individuals don’t behave the same way all the time–some lizards stick to one tree for weeks before suddenly shifting somewhere else. We’re observing these lizards for five months, through most of their breeding season, so that we can measure variation across individuals and across time.
Why Are We Doing This?
The way in which an organism reproduces has HUGE consequences for how populations of this organism evolve. For a particular trait of an organism to evolve by natural selection, it must vary among individuals and this variation must be inherited by an individual’s offspring. But most importantly, this trait must affect how good the individual is at surviving and reproducing. Who an individual lizard mates with, and how many mates it has, will undoubtedly affect how successful it is at producing babies.
Consider, for example, the brown anole’s dewlap, the colourful flap of skin under the male’s throat. Arguably it’s most striking trait, this fan-like structure is displayed, along with little push-ups, by males to females and to other males. It seems to play a role in communication between lizards about reproductive decisions.
Biologists (including none other than Charles Darwin!) have long thought that the dewlap has evolved by sexual selection (a subset of natural selection), which means we expect male brown anoles with bigger, brighter dewlaps to have more babies than males with smaller, duller dewlaps. If brown anoles were territorial and polygynous, then we’d imagine that the dewlap is used by males to settle disputes over territories–males with larger dewlaps would defeat males with smaller dewlaps and would lord over larger territories that include more females, and would thus father more offspring.
But if brown anoles are not territorial and not polygynous, then what is the dewlap used for, and has it evolved by sexual selection? We won’t be able to answer these questions until we understand the brown anoles’ mating system. Now consider the dramatic variation in dewlaps across anoles. Surely we want to know how traits as striking and varied as this have evolved? My research is one step along the way to answering that question.
If you aren’t yet convinced that we need to figure out how brown anoles reproduce, consider that these lizards are a widespread invasive species, not just in the mainland U.S. but also in Hawaii, Taiwan, and parts of the Caribbean. Knowing how populations of a species reproduce is crucial to understanding how that species invades and persists in new locations and environments, and therefore to understanding how we might be able to limit their spread.
I’ll be blogging about this project here, as it unfolds. Feel free to leave a comment or question below, or come chat with me about this research if you see me in University Gardens!