Monogamy, or the formation of stable pair bonds between males and females for reproductive purposes, is thought to be relatively rare across animals. While social pair formation is observed (commonly in birds and occasionally in reptiles), genetic assessments of parentage have revealed that mating fidelity is infrequent. Social monogamy is therefore not equivalent to genetic monogamy. However, the reasons for the persistence of social monogamy despite promiscuous mating remain unclear.
Sleepy lizards are the best known example of pair-bonding in lizards (photo by J. Todd Kemper)
A new paper by Alexis Harrison revisits one of the only examples of social pair-bonding known from anoles–a population of Anolis limifrons in the La Selva Biological Station in Costa Rica. While most anoles are polygynous, with the territory of one male overlapping the territories of several females, Talbot (1979) noticed that 70% of adult A. limifrons in La Selva were found in pairs of a single male and female in close proximity to each other. However, such pair bonding has not been documented in any other population of the species, making La Selva an intriguing outlier.
A pair of Anolis limifrons (photo by Jason Weigner)
Harrison (2013) first asks whether adult A. limifrons in the La Selva population are found in pairs, and the answer is an equivocal yes–about 45% of the adults were found in pairs, a lower proportion than found by Talbot (1979). The remaining individuals were found in a variety of social situations, including solitary animals, polygynous groups, and pairs of males. However, members of male-female pairs were often seen interacting with each other, and males of a pair were seen defending the female against intruding males, suggesting that these pairs are in fact social units and not simply found together by chance. Check out the paper for more fascinating behavioural observations! Of course, it remains to be seen whether socially monogamous A. limifrons are also genetically monogamous.
Second, Harrison (2013) measured the morphology of individuals in pairs and in other social situations. She found evidence for size-assortative pairing in monogamous individuals, but saw no differences in morphology between individuals in pairs and solitary animals. This result suggests that pair formation in A. limifrons is not a consequence of mutual mate choice for a morphological trait–for example, if males and females prefer to pair with large individuals, one would expect paired individuals to be larger than unpaired individuals. Other possible explanations for size-assortative pairing are discussed in the paper.
So why are A. limifrons in La Selva pair-bonded? Any explanation would need to be site-specific (though some preliminary evidence from a population near Limón, Costa Rica, suggests pair-bonding in that habitat also; A. Rojas, pers. comm.). The lack of parental care in anoles makes it unlikely that pair bonds are formed for the purpose of rearing offspring. Two major factors likely play a role. First, habitat structure and the distribution of resources are widely thought to drive social organization and mating systems in animals, and could be an important explanation for variation in social organization across the species. Second, high predation rates might favour the formation of pairs to increase vigilance and reduce the odds of an individual being attacked.
Both resource distributions and rates of predation can be challenging to measure in nature, but measuring them will be essential if we want to understand social organization in anoles. Documenting the patterns of variation in social organization is the first step in this endeavour, and by confirming one of the most intriguing examples of anole social behaviour, Harrison’s (2013) paper makes an important contribution to this field.
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sarah
I have a pair of Anolis Carolinensis living in my yard. I’ve seen them together for at least 6 years, maybe longer. They hang out together. This year, after all the spring rain they finally started laying eggs all over my yard. I’ve had the best summer ever discovering newly hatched and young anoles all over the yard, providing water, watching them grow, and photographing them. I can tell you, they very definitely mate and stay together. Not only that, but I believe he tells he when to, or when it’s safe to, lay an egg. Because every 10-14 days he’d appear on the railing of my back deck/porch, and he’d parade back and forth the length of the rail, which is a good 25 ft, doing his dewlap dance, while the hatchlings appeared at just about the same rate. It’s Oct 20th now, and I just saw the pair sunning themselves today, together. Also saw a very young one, so she was still laying eggs recently. She doesn’t look as gravid as she did back in Aug, so she might be done now. But they’ve been very busy.
DAVID DRAKE
I have 3 carolina anoles in my tank, 2 females and 1 male. I caught one of the females and the male in the same 6ft area. The other female I caught by my gym. They have all been in the same tank for 2 months now. The male anole only breeds with the female I caught at the same time as him. He completely ignores the other female even though I can confirm that she laid 2 eggs and 1 hatched before I introduced the other 2. She is clearly old enough to breed and yet he does not court her