Category: New Research Page 29 of 67

A. polylepis displaying dewlap.

A commonly observed, but little studied, aspect of tropical herpetology is the seasonal shift in some species’ relative abundance in forested habitat and adjacent, nearby streams. The general pattern is that during the dry season, some species of forest frogs, lizards, and snakes seem easier to detect along streams than in the forest and vice versa during the wet season. Despite this intuitively unsurprising seasonal shift in macrohabitat use being noticed in the 1960s by researchers like Jay Savage and Norm Scott, there has been little work done to document it. In an upcoming issue of the Journal of Herpetology is a paper titled: Seasonal Shifts in Relative Density of the Lizard Anolis polylepis (Squamata, Dactyloidae) in Forest and Riparian Habitats.

The difficulty in documenting seasonal macrohabitat shifts is twofold. First, field sampling must encompass both seasons and be continuous. Second, simultaneous sampling needs to occur in both forest and streams across seasons. For many tropical herpetologists, the opportunity and time for such a study do not come about often. In December 1999, I had this opportunity when I spent three years studying the herpetofauna along the south-central Pacific coast of Costa Rica. I was a young, precocious and budding herpetologist and wanted to understand the ecological habits of all the local amphibians and reptiles. So, out of curiosity I set up transects in a 25-hectare forest patch and a stream that ran through the forest at the Tropical Forestry Initiative (TFI) research station. For 29-months, with the help of field assistants (Deborah Merritt and Yemaya Maurer St. Clair) we sampled the transects regularly, documenting and observing species diversity and habitat use in the forest and stream. While I was organizing the data, an interesting pattern emerged in regard to Anolis polylepis. Of all of the species in the local lizard fauna, A. polylepis showed the strongest seasonal shift in relative density between the two habitats!

Anolis polylepis is the most common anole along the Pacific coast of Costa Rica, reaching densities of up to 300 individuals per hectare (Andrews 1971; Scott 1976). The species can be found in a wide variety of forested habitats ranging from old growth forest to gardens with ample shade trees. In my experience, the only necessary habitat requirement for A. polylepis is shade from a closed canopy. The high density and generalist habits of A. polylepis make it a wonderful study species.

Like many forest anoles, A. polylepis is active in the understory during the day. However, obtaining accurate population counts can be difficult because individuals are wary and can be difficult to detect. For example, A. polylepis will jump to the ground or circle around a tree when observed. This avoidance behavior can be problematic when attempting to obtain reliable counts by increasing the likelihood of missing a lizard. To counter this difficulty, I surveyed for A. polylepis at night, which facilitated easier detection. Anolis polylepis, like many species of anoles, sleeps visibly on leaf tops, twigs, branches and vines from 0.5 to 4 meters above the ground. Thus, it is easier to obtain better counts of relative density for some anole species when lizards are sleeping and inactive. Nocturnal surveys can be very informative for addressing certain questions related to anole biology.

In total, 41 nocturnal surveys were conducted between January 2001 and February 2002, covering one wet and one dry season. We found significant seasonal differences in A. polylepis relative densities between the wet and dry season. During the dry season, A. polylepis density was 0.052 lizards per meter in the stream and 0.010 lizards per meter in the forest. This pattern reversed in the wet season when stream relative density was 0.002 lizards per meter and forest relative density was 0.036 lizards per meter. This seasonal change in relative abundance suggests that wet-dry seasonality influences macrohabitat use in A. polylepis in Costa Rica.

One major limitation of our study was that we did not use mark-recapture. Use of such an approach would give insight into the individual movements associated with our observed patterns. For example, we could test whether lizards are moving large distances to the stream during the dry season, or whether deep forest lizards are moving to moist microhabitats within the forest such as tree buttresses, to name two possibilities.

As with many pilot field projects, ours documents a novel pattern, but raises additional questions. Future work on this issue should extend to other species and regions and use mark-recapture or radio telemetry to elucidate the details of seasonal migrations. An understanding of seasonal movements in environments with distinct wet and dry seasons has implications for how anoles and other herps can tolerate the harsh dry season.

References:

Andrews, R.M. 1971. Food resource utilization in some tropical lizards. Unpubl. PhD diss. University of Kansas, Lawrence.

Scott, N.J. 1976. The abundance and diversity of the herpetofauna of tropical forest litter. Biotropica 8:41-58.

A. polylepis on tree trunk.

Sleeping A. polylepis. Courtesy of Cesar Barrio Amoros.

Martin Whiting of Macquarie University began his talk at the Animal Behaviour Society 2014 meeting by lamenting how little we know about the social lives of lizards, especially when compared with mammals, certain insects and fish, and most of all, those pesky other reptiles, birds. But the more we examine lizard social behaviour and cognition, the more apparent it becomes that these animals are capable of substantially more complexity than we previously thought possible. Whiting presented some recent research on the Eastern Water Skink, Eulamprus quoyii, that bolsters this view.

Eastern Water Skink, from the Whiting Lab Page

Though not often social, many lizards, including Eastern Water Skinks, live at densities high enough to allow individuals to be within sight of each other. This is a sufficient prerequisite for social learning, defined as learning a task by observing others and modifying one’s own behaviour accordingly. Whiting asked whether Eastern Water Skinks were capable of social learning by training “demonstrater” individuals to perform certain tasks, letting “observer” individuals watch these demonstraters, and then measuring whether this exposure to the demonstraters enhanced the observers’ success at the task at hand.

The answers to Whiting’s questions were not simple. First, age matters—young individuals were twice as likely to demonstrate social learning than old individuals. Second, the task matters—lizards learnt to associate a colour with a food reward by watching others, but the prerequisite task of actually flipping over the coloured cap to access a mealworm was not spurred by observing other individuals do the same.

In the future, Whiting and his students hope to conduct similar experiments with a variety of lizard species that differ in their degree of sociality. These experiments will definitively address the role of learning in shaping the social lives of lizards, and I can’t wait to see they find!

Following up on yesterday’s post, more research results from the Warner Lab on egg incubation were presented at JMIH. Corey Cates, a masters student from the Warner Lab, presented his data on developmental plasticity in Anolis sagrei. He used an experimental approach to test whether lizards incubated under dry conditions would survive better in a dry habitat than lizards incubated under moist conditions and vice versa. The idea for the study came from the observation that habitat and substrate differs among small islands in Florida. Some islands are scarcely vegetated and have dry substrate consisting of broken shells. Other islands are more densely vegetated and have dark soil that contains organic matter.

Corey collected 128 breeding pairs from four islands and incubated the eggs using the two different substrates. He also tested two different moisture conditions (wet and dry). He found that lizards incubated under wet conditions hatch on average 4-5 days later and hatchlings were significantly heavier than those incubated under dry conditions. In addition, lizards hatch significantly later when incubated in the soil substrate, which retains moisture longer than the broken shells. Corey further tested whether lizards raised under dry conditions have higher desiccation tolerance than lizards from wet conditions. He measured body mass before and after keeping the lizards in a desiccation chamber. Lizards that had developed under wet conditions lost 5% more mass than lizards developed under dry conditions.

Hatchlings incubated under wet conditions lost significantly more mass than hatchlings incubated under dry conditions.

This suggests, that plastic responses to different developmental conditions have an effect on physiological traits that might increase survival in a specific habitat. To test this, Corey then released the hatchlings on four experimental islands and measured hatchling survival using a recapture method.

Significantly more hatchlings survived in the open, arid habitat when eggs were incubated under dry conditions.

He found that significantly more hatchlings survived in open, arid habitats when eggs were incubated under dry conditions. No effect of incubation condition on hatchling survival was found in the shaded, moist habitat.