Developmental Changes in Diet
Sometimes, the diet of a species changes as it ages. This is known as an ontogenetic diet shift, a marked change in diet as the species develops over time. This is especially evident in the genus Ctenosaura, a group of 15 species of lizard known as spinytail iguanas. I have become somewhat familiar with them because of the research of two of my fellow graduate students in my university herpetology lab.
The species pictured above, the black spiny-tailed iguana, Ctenosaura similis, is originally native to Central America going slightly north and south into Mexico and Colombia respectively. However, a non-native population has been introduced right into my own backyard. Some decades ago, a group of these iguanas was brought to Keewaydin Island, just south of Naples, Florida. This particular island is undeveloped and serves as important habitat for gopher tortoises and nesting sea turtles and shorebirds. One of my colleagues set up a genetic analysis to determine the original location of the population and the likely number of separate introductions. The other conducted a diet study to see what kind of impact their feeding habits might have on the native species of the island.
As an organism develops, its nutritional needs can change. The energy taken in from food can be distributed to one of four areas: maintenance (day to day necessities), growth (getting bigger), reproduction (producing the next generation), or storage (saving up for a rainy day). The ratio of energy allotment will change over time and the diet reflects that. Juvenile individuals will usually invest a larger portion in growth and nothing into reproduction. As they mature, that ratio will begin to flip.
Species with faster rates of growth require large amounts of protein in their diet in order to fuel this. While this is true for individuals across a wide variety of species, the growth rate of Ctenosaura is particularly high, especially compared to other herbivorous reptiles. This means that the shift is even more pronounced. Adult ctenosaurs are primarily generalist herbivores, eating leaves, fruits, stems, and (like the picture above) flowers. However, juvenile diet is not plant-based at all, despite the fact that they do already possess the physiology to digest plant matter at that age. Instead, the juveniles are insectivores. Insects provide an excellent source of easily digested protein for a developing individual (this is why they are also a large portion of the diet for many baby birds).
As the iguanas reach maturity and adult size, they no longer need to dedicate so much energy to growth and their protein requirements drop. This is when the diet shift occurs and they transition to a herbivorous diet and foraging strategy. They are opportunistic and will still occasionally catch insect prey, but their needs have shifted and it no longer dominates their diets. However, regardless of where along the diet continuum an individual ctenosaur is (things in nature are rarely just one thing or another, but tend to fall on a gradual line between extremes), they are still diet generalists. While the stomach content analysis performed by my colleague did indicate certain preferences, the overall composition of each individual diet varied widely in terms of insect groups or plant structures.
And for those curious about the result of the genetic analysis, all captured iguanas showed the same markers for the particular coding section under study. While it’s only a small portion of the genome, the analysis suggests that the black spiny-tailed iguanas on Keewaydin all come from a native population on a small island off the coast of Honduras called Utila. This indicates the likelihood of a single introduction event of a small number of individuals.
The point of this week’s story is to keep in mind that diets and foraging behaviors are rarely set in stone. They can change over the lifetime of a single individual, and while many of these changes are developmental as described above, plenty of them have an underlying environmental component. If the ecosystem changes (either seasonally over the course of a year or because of a catastrophic event), those individuals that can best adapt their diet to the change have a greater chance of survival and passing down that genetic material to the next generation. In the end, food is just an energy source for powering the engine of an organism’s body. They go about it in different ways at different points in their life, but the end goal is the same.
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