Author: Jonathan Losos Page 77 of 130

Anolis ventrimaculatus. Photo by Diego Garcés

Morphological differences between the sexes and among populations have been studied extensively in Caribbean anoles, but—like so many other aspects of biology—not so much in mainland species. Studies in the islands suggest that differences, both sexual and geographic, often represent adaptation to different conditions, either the sexes partitioning niches or populations adapting to different circumstances.

Martha Calderon and colleagues have just published a paper in Revista de Biología Tropical on variation in the Colombian anole A. ventrimaculatus. Examing museum specimens from seven populations, they find consistent size dimorphism (males larger) and substantial dimorphism in body proportions. The extent of these dimorphisms, however, varies among sites.

Unfortunately, at this time little is known about the habitat use and general ecology of this species, much less about differences among populations, so evaluation of the potential adaptive significance of this variation awaits further fieldwork.

The paper’s abstract:

Variation in body characteristics related to lizard locomotion has been poorly studied at the intraspecific level in Anolis species. Local adaptation due to habitat heterogeneity has been reported in some island species. However, studies of mainland species are particularly scarce and suggest different patterns: high variability among highland lizards and poorly differentiated populations in one Amazonian species. We characterized interpopulation variation of body size and shape in the highland Andean Anolis ventrimaculatus, an endemic species from Western Colombia. A total of 15 morphometric variables were measured in specimens from the reptile collection of the Instituto de Ciencias Naturales, Universidad Nacional, Colombia. The study included individuals from seven different highland localities. We found size and shape sexual dimorphism, both of which varied among localities. Patterns of variation in body proportions among populations were different in both males and females, suggesting that either sexual or natural selective factors are different in each locality and between sexes. Since this species exhibits a fragmented distribution in highlands, genetic divergence may also be a causal factor of the observed variation. Ecological, behavioral, additional morphological as well as phylogenetic data, may help to understand the evolutionary processes behind the geographic patterns found in this species. Rev. Biol. Trop. 61 (1): 255-262. Epub 2013 March 01.

Not effective at a great distance

Think about when you want to communicate with someone, but first you have to get their attention. Let’s start with verbal communication. If Fred is across the room, you probably holler out “Hey Fred” a lot louder than if he’s sitting next to you. Now, suppose you’re the non-verbal sort. If Fred’s a long distance away, you’re going to have to wave your hand wildly, maybe even jump up and down, to get his attention. But if he’s nearby, a little wave, even a discreet hand gesture, will suffice. Why we don’t whisper or make slight movements to attract the attention of those far away is pretty obvious–the target won’t hear or see you. But why not yell loudly or gesture emphatically even when the target is nearby (ok, we all know some annoying people who do this, but mostly people don’t)?

Signal modulation is an area of great interest in the field of animal communication, and Steinberg and Leal have just published a fascinating study on the Puerto Rican A. gundlachi in Animal Behaviour (pdf). The key to understanding signal modulation is to investigate how signal detectability changes as a function of distance. Building on prior work by Fleishman on A. sagrei, Steinberg and Leal conducted lab studies in which they move a black disk against white paper to determine how much movement is needed to attract the lizard’s attention. Fortunately, this can be easily done with lizards because they have something called the  visual grasp reflex–when something gets their attention, they shift their eye to gaze right at it. Easy to determine in the lab. So, by moving the disk up-and-down different amounts and varying the distance of the lizard, the authors were able to determine the degree of amplitude of movement in the visual field most detectable by the lizards (see figure on right). Notably, there is not only a minimum visual angle, but also a maximal one, above which response declines (and then increases again; for reasons discussed in the paper, Steinberg and Leal focus on the maximal peak in the 0.25-0.75 degree range).

Of course, because the movement is expressed as an angle relative to the visual field, then as the distance to the target receiver increases, larger amplitude movements would be necessary to be detected. Moreover, looked at the other way, because there may be a degree of movement too great to maximally stimulate a response, the amplitude would be expected to decrease at shorter distances.

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