Image Credit: Goutham89, CC BY-SA 4.0

The evolution of crocodilian nesting ecology and behavior (2020) Murray et al., Ecology and Evolution, https://doi.org/10.1002/ece3.5859

The Crux

One goal of evolutionary ecology is to understand the links between microevolution and macroevolution, meaning evolution in the short term (multiple generations) and how that scales up to the long term (millions of years). In macroevolution, a group of organisms is thought to be successful if it not only exists for a long period of time, but if it also boasts a large number of species. With those criteria in mind, crocodilians (alligators, crocodiles, gharials, and caimans) are one of the most successful lineages to have ever existed on the planet. Though they may not be the most diverse group of organisms with only 25 species, they have been around for about 100 million years. To put that into perspective, dinosaurs went extinct about 65 million years ago, meaning that the crocodilians not only lived with dinosaurs, but they survived the mass extinction that the dinosaurs didn’t.

This longevity as a lineage raises some questions as to what it is about the crocodilians that made them so successful, when their cousins the dinosaurs died out. An interesting aspect of crocodilians is that there is very little variation among these organisms, as they are all generalist carnivores, live aquatic lives, exhibit mating vocalizations, their sex is determined by the temperature of their eggs (see Did You Know?), and they care for their eggs and young. Despite these similarities, there are some notable differences in the reproductive ecology and behavior of the different species, specifically how they build and care for their nests. Because of these differences, the authors of today’s study asked if variation in how crocodilians reproduce may have been the cause of their success.

What They Did

The authors collected data on 24 crocodilian species to categorize 14 different parameters relating to the ecology and behavior surrounding reproduction. I won’t list all of the parameters here, but they include the type of nest built (mound nest vs. a hole nest), maternal nest attendance (the mother remains with the nest), nest defense (the mother actively defends the nest), hatchling attendance (the mother remains with the young), and clutch frequency (how many times a female lays a clutch of eggs in a breeding season). These parameters were then mapped onto the most recent phylogeny of crocodilians to infer ancestral states for each of the parameters, meaning the authors used data from extant species to infer how their ancestors reproduced. After determining the inferred ancestral state, the loss/gain of certain traits was mapped onto the phylogeny.

Did You Know: Temperature-Dependent Sex Determination

As mammals, we are familiar with a fairly even mix of male and female children whenever parents have kids, due to the predicted 50% split of male and female children whenever a parent with two X chromosomes reproduces with a parent with an X and a Y chromosome. However, for organisms like crocodilians and some turtles, the ambient temperature that eggs experience determine which eggs are male and which are female. Which temperature determines which sex varies on an organismal basis: in some turtles cooler temperatures result in males, while crocodilian eggs exposed to intermediate temperatures are male and the females must be exposed to either warmer or cooler temperatures. A concern for species that exhibit this type of sex-determination is that rising temperatures will result in all-female clutches, and I don’t need to tell you why that would be an issue for a species.

What They Found

The common ancestor of modern-day crocodilians was a mound-nester, and hole-nesting has evolved five separate times in more recent species. The ancestor both attended and defended its nest, while all but one and two modern-day species attend and defend their nests, respectively. The ancestor also attended its hatchlings, and only one modern species does not attend the hatchlings. Interestingly, the freshwater crocodile (Crocodylus johnsoni), the only species that does not attend its nest, DOES attend its hatchings. The evolution of multiple clutches per year has only evolved twice among crocodilians, with most species sharing the ancestral trait of a single clutch per year.

Problems?

I am by no means an expert, but I’ve done some work with phylogenetic comparative methods in my own research, so I know a little bit about what you can do in that field, and there are a plethora of methods, tools, and models that can be employed to analyze and interpret phylogenetic relationships and ancestral states. The authors of this study used a method that I’m not only not familiar with, but they also didn’t do a great job at describing what these methods are or why they are appropriate for their study, but it seemed to be fairly limited in that it only constructed the ancestral state and where that trait changed in modern day species. Instead of calculating metrics like diversification rates (i.e. extinction rates) and then making rigorous comparisons among the various reproductive parameters, the authors instead made inferences about how and why traits evolved without any kind of statistical backing. The authors may have been limited by the relatively small size of their tree (i.e. there are only 24 tips on the tree), but I feel like there was a missed opportunity to ask some really cool questions.

So What?

Today’s study is a an example of tying together the evolutionary history of a group of organisms to their modern-day representatives to try and understand how and why some organisms are more successful than others. Crocodilians display remarkable homogeneity in their appearance, behavior, and physiology, but there is an interesting amount of variation in their reproductive ecology/behavior. This variation appears to indicate a degree of high evolvability, meaning that crocodilians as a group are likely to be able to respond to a variety of environmental pressures, which may be why they are so successful as a group. By using data on extant species and relating that to the evolutionary history of organisms, we will be better able to understand how and why some groups succeed while others die out.

Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions. You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.