Topic 4.5: Reptiles

Content

Topic Introduction

Humid tropical forests provide very favourable conditions for amphibians, being warm and humid. In contrast, drier forests provide conditions that are very favourable to reptiles, although reptiles can also do extremely well in warm, humid conditions. A key is the ability to regulate body temperature: this is easier in some environments than in others. All modern reptiles exhibit some degree of cold-bloodedness – and show a mix of poikilothermy (major temperature fluctuations), ectothermy (reliance on external heat sources) and bradymetabolism (major differences between active and resting metabolism).

As with many other groups of animals and plants, species richness of reptiles increases towards the Equator. There are many possible reasons for this, and some of these are examined in the accompanying course ‘Ecological processes in tropical forests’.

In this module, we have followed standard taxonomies in treating turtles, crocodilians, snakes, amphisbaenians, lizards and the tuatara together. This does not reflect their evolutionary relationships, and these groups do not represent a monophyletic grouping (i.e. they are not all descendants of a common ancestor). For example, crocodilians are more closely related to birds than they are to lizards. (In modern taxonomy, a group known as the diapsids include all reptiles and the birds).

The earliest known reptiles (proto-reptiles) date back to the Carboniferous period, 310–320 Ma. They evolved on land, and many lineages are now extinct as they appear to have been particularly susceptible to the mass extinction events that have occurred in the past. The event that marked the boundary between the Cretaceous and Paleogene appears to have been particularly important, resulting in the loss of many groups, including the pterosaurs, plesiosaurs, ornithischians and sauropods. Other groups, such as the troodontids, dromaesaurids, tyrannosaurids, abelisaurids, Crcodyliformes and squamates experienced major reductions in the number of species.

All reptiles, including the snakes, descended from four-limbed ancestors. Most reptiles are oviparous (i.e. they lay eggs), but a few squamates are viviparous, with the fetus developing inside the mother in a placenta. Unlike the majority of amphibians, there is no aquatic larval stage. Sexual reproduction is the most common form of reproduction amongst reptiles (given that they lay eggs), but asexual reproduction in the form of parthenogenesis is probably more widespread than reported to date. In some species, there is strong evidence of temperature-dependent sex determination, with the proportion of male and female hatchlings being determined by the incubation temperature of the eggs.

The metabolism of reptiles enables them to survive under very different conditions to, for example, mammals. Their very low rates of metabolism while resting allow them to conserve energy, in turn requiring a lower food intake. For example, some crocodiles can survive 6 months without eating. This ability to survive in areas where there is very low net calorie availability is one of the reasons for the success of reptiles in dry environments such as deserts and dry forests.

By the end of this topic, you should be able to:

Describe the unique features of reptiles.
Discuss why reptiles are an important element of tropical forests.
Identify the main groups of reptiles and, for example, know the difference between a legless lizard and a snake.
Explain why reptiles are more abundant in some areas than others.
Contrast the reptile faunas of humid and seasonally dry tropical forests.

Topic Readings

Required Readings

Ghazoul, J. and Sheil, D. 2010. Tropical Rain Forest Ecology, Diversity, and Conservation. Oxford, Oxford University Press, 516 pp. Sections 4.3.2.

Optional Readings

For amphibians and reptiles, I strongly recommend: Pough, F.H., Andrews, R.M., Crump, M.L., Savitzky, A.H., Wells, D.D. and Brandley, M.C. 2018. Herpetology. 4^th New York, USA: Oxford University Press and Sinauer Associates.
Shine, R. 1998. Australian Snakes. A Natural History. Sydney, Australia: New Holland Publishers. 224 pp.

Graded Assignments (For certification)

Six Sentence Answer #7

If you are not familiar with the Six Sentence Answer (6SA) format, please refer to the course pages on graded assessments to see details of the 6SA exercise. You should not attempt to do this assessment until you have read and fully understood the requirements.

The Case

So far, we presented the situation that you are a member of a major international forest consultancy firm that specializes in doing surveys of forests around the world. You have access to field teams and the latest surveying equipment, including LiDAR. Your company has taken on a contract with the UN-REDD (United Nations Collaborative Programme on Reducing Emissions from Deforestation and Forest Degradation in Developing Countries) to survey the forests on a very large island in South-East Asia, and you not only have the full cooperation of the government but they are also asking your company for advice on a range of other issues.

You have already produced a map of the island’s forests, and assessed the growing stock. Your field teams have done on-the-ground surveys to develop the allometric equations that you needed to produce estimates of the carbon stored in different types of forest on the island. You have found major differences, as you might expect, between the moist tropical forests on the west of the island and the dry forests in the rain shadow of the volcano on the east of the island. You have identified that the trees in the tropical moist forest are much smaller than those on the island of Borneo, located only a few hundred kilometres to the west and you have found a plausible explanation for this.

The government has indicated that it is interested in developing ecotourism on the island and are building an interpretative walk through a section of rain forest on the west of the island. They are still at the initial stages of planning, and have asked your company to help them. You have already indicated how you would prioritize the botanic features along the walk and how the plants got there. They have now asked for an interpretative sign that indicates the mammals present on the island. Using the 6SA format, please indicate why you would choose particular mammals to include on the sign.

Remember to follow the structure of the 6SA outlined in the section on graded assessments.

Topic Summary

The reptiles are a species-rich class of animals in comparison to the mammals. They are not as diverse as the birds, but birds and reptiles, particularly crocodylians, are related. They have adapted to many different environments, with some being primarily aquatic and others completely arboreal. All reptiles have lungs, so must breathe air.

As reptiles are cold-blooded, they are largely reliant on heat from external sources, primarily the sun. They are capable of generating heat through muscle movement but, with a few exceptions, this is insufficient as the sole source of heat. As a result, most reptiles spend time basking in the sun, particularly in the early morning. This also enables them to maintain temperatures during periods of low metabolism, an adaptation that allows reptiles to go without food for considerable lengths of time.

Within tropical forests, reptiles are an important part of the food chain. They include herbivores, omnivores and predators, and many reptiles prey on smaller reptiles, as well as other organisms. Many reptiles prey on mammals, although they may be limited by size. Crocodylians deal with this problem by tearing their prey apart. Many snakes can dislocate their jaws, allowing them to swallow large organisms, including humans.

Topic Self-review (For self learning)

To review what you have learned in the topic, you are encouraged to use the following reflection questions as a study guide to do a self-review for the topic:

What distinguishes the reptiles from other taxonomic groups?
What ecological roles do reptiles play in tropical forests?
What are the main groups of reptiles, and how would you recognize them.
Why are reptiles are more abundant in some areas than others
To what extent do the reptile faunas of humid and seasonally dry tropical forests differ? Are any groups unique to either habitat?
How many different types of prey (e.g. fish, insects) are you aware that the different groups of reptiles prey on?
Are reptiles primarily herbivorous or predatory?

FODE 008-01

Topic 4.5: Reptiles

Content

Topic Introduction

Topic Readings

Required Readings

Optional Readings

Further Readings

Fundamental Concepts

PPT Presentations

Self-check Quiz

Pre-readings and Discussion (For certification)

Graded Assignments (For certification)

Six Sentence Answer #7

Topic Summary

Topic Self-review (For self learning)