Evolutionary Ecology

I am an evolutionary ecologist who works on parental investment and life history evolution in fishes, both in the lab and in the field. The purpose of my work is to understand the processes of evolution that lead to the creation and resolution of tradeoffs in organismal morphology and behavior. In other words, I try to understand why organisms are they way they are, and why they do the things they do, with a particular emphasis on reproduction and parental behavior.

Cichlid fishes (family Cichlidae) are a natural group for me to focus on because all cichlids provide some sort of parental care. I currently do much of my work on cichlids, though I also work on other groups of fishes (sunfishes, intertidal fishes).

 

Parental Investment

The central question of parental investment research is why animals allocate resources to their offspring, and if they do so, how do they decide how much to provide? For example, why do some species provide parental care while others do not? Why is it sometimes the male and sometimes the female? How large should eggs be? Why should parents of different size value the same offspring differently? These are all issues of parental investment.

 

Philosophy: Tradeoffs

The cornerstone of all my work, and of every evolutionary ecologist, is the notion of the tradeoff. A tradeoff is the idea that if you get more of one thing, you get less of another. Evolutionary ecologists analyze costs and benefits.

For example, if you spend more time on a certain task (e.g., homework for your English class), you will have less time for other tasks (e.g., homework for your Physics class). The benefit of doing your English homework is a better grade in English, the cost is a poorer grade in Physics.

Life is full of tradeoffs for humans and for animals. There are two parts to understanding tradeoffs: identifying what is being traded-off, and identifying which factors affect the tradeoff. In the homework example the tradeoff is between your grade in your English class versus your grade in your Physics class. Things that might affect the tradeoff might be the difficulty of the assignments, whether you intend to become an English major, etc.

Mouthbrooding in cichlids is often regarded as a tremendous benefit to the offspring. They are protected for much of their early lives. But are there costs to mouthbrooding? Certainly. In most cases, the parent doing the mouthbrooding cannot eat during the period of parental care. Further, the clutch size (the number of babies) is limited to the number that can fit in the parent's mouth.

When you start to see the world as the product of tradeoffs, you will start to realize that rather than viewing organisms on a scale from simple to advanced (as is so often taught), rather we can recognize that different organisms have solved the fundamental tradeoffs of life in different ways. So, for example, rather than seeing a parasitic worm as a tiny body devoid of almost any structure, we see it as an organism which has traded off the ability to live where it wants for the ability to devote almost all of its resources to reproduction.

 

My Approach

Constructing Theoretical Models

I use a combination of theoretical and experimental approaches. Some of my work involves constructing theoretical models of how parental fishes ought to behave given what we know about their fundamental biology. The goal of this work is to make predictions, based on a relatively small amount of knowledge, about how the organisms will behave. If the predictions turn out to be correct then we can be satisfied that the information we used to make those predictions is in fact the biological factors of key importance to understanding the situation.

Why is this important? From a philosophical viewpoint, if we can isolate the "important" factors, then we can claim to "understand" the biology. From a practical point of view, it is simply not possible to measure everything about the physical and biological world. We need ways of predicting what the world is like and how it will respond based on a relatively few measurements. For example, consider weather forcasting. One approach to weather forecasting is to put instruments over every square inch of North America and then collate all the information to say what the weather is like. A far more practical and ultimately useful approach is to deploy a limited number of instruments, then determine the rules by which weather works, and from that not only describe the weather as it is happening, but also predict what will happen tommorrow or even a week from now.

Understanding the biology of organisms is a similar problem. We can either attempt to measure and record absolutely everything about every kind of organism, which is an impossible task, or we can hypothesize that certain key factors are likely to be important, construct models of how these factors will interact, make predictions, measure and record some organisms and see how accurate our predictions were. If our predictions were accurate, fine. If not, we have to reevaluate our models and go through the cycle again.

For a concrete example, consider the issue of mouthbrooding in cichlids. Which cichlids are mouthbrooders (fish in which the eggs and/or fry develop in the parent(s) mouth? We could examine each and every species of cichlid to determine this, but there are over 1500 species to examine and many are hard to locate, let alone observe breeding. There are also other fish families with mouthbrooders. Do we examine each of those as well? Alternatively, we may ask, which factors cause a cichlid to be a mouthbrooder, and if it is a mouthbrooder, what characteristics will it have? By this approach we examine a relatively small number of organisms, make observations, models and ultimately predicitons and if these hold true, we have determined general knowledge that will apply to all mouthbrooders.

Testing the Models

Some researchers focus on constructing models, and others on testing them with real organisms. I do both. To test my models, I work both in the laboratory, i.e. with fish in aquaria, but also in the field (Costa Rica mainly). The laboratory work is important in conducting precise manipulative experiments, i.e. experiments where I can closely control the environment and as many variables as possible to critically test some aspect of parental investment theory. For example, I might be interested in knowing exactly how does the number of young affect a parent's willingness to defend its offspring. In the lab, I can carefully count and provide a parent with precisely 50 babies.

Manipulative experiments are difficult in the field, but the field provides the context in which the behavior evolved. All interesting behavior is the product of natural selection and it is only relevant in the context in which it evolved. Continuing with the example above, I wouldn't ask how much a convict cichlid parent would defend 10,000 babies, because they never have that many in the wild. But I may ask how they would respond to a reduction of 50% of their kids because predation by other fish on a cichlid's brood is common and parents would have evolved an ability to respond to these events appropriately. In other words, there are many questions not worth asking and studying organisms in the field helps you to avoid wasting time asking pointless questions.

 

Four lines of investigation

Parental investment research can be broken down into four key lines of investigation:

1. The Origin of Parental Care: Should one, both or neither parent provide parental care to the offspring?
2. The Amount of Care: If care is to be provided, how much should a parent provide?
3. The Form of Parental Care: Which specific behavior should care involve and why does the form differ between species?
4. Interactions with Life History: How does the provision of parental care interact with other aspects of an organism's life history?

 

Amount of Care

 

The bulk of my recent work concentrates on the issue of the amount of care. Parents in many species take care of their kids. But why? And how much? How do they decide? What affects these decisions? Why don't all parents take care of their kids? Why do some parents eat their kids? For example, if a predator approaches a parent guarding its offspring, how hard should the parent defend? Should it defend to the death? Or should it give up at some point? What factors might affect this decision (e.g., the age and number of young)?

The issue of egg size is another manifestation of this same question. The question of how large an egg to lay be can be rephrased as "how much investment should a female put into each egg at one point in time?" This question is particularly interesting because there are such radical differences in egg size across species.

When I see a parent taking care of its kids, as an evolutionary ecologist I can conclude several things. First, parental care must be advantageous for these parents, i.e., it pays them in terms of the survival of their offspring to allocate some of their resources (time, energy) into their offspring. Second, there will be costs to this parental care. Parents do not have unlimited time or energy so every bit of their own resources that they put into the existing kids are resources they cannot use themselves. In particular, these are resources the parents will not have to invest in future offspring. This gives us the basic components of a "trade-off": each parent must tradeoff investment into the current offspring against investment into future offspring. How do parents make such decisions?

My recent work has centered around the concept of "value". How much should a parent value its offspring? And, what will change that value? Thinking in these terms can lead to some surprising discoveries. For instance, the same physical objects, such as 100 babies, can have very different values to two different parents. And these parents will invest in these offspring differently.

In the particular case of fish, which grow throughout their lives, and in which fecundity increases for both sexes with size, the notion of value leads to the counter-intuitive prediction that larger fish should take less care of a given number of offspring than a smaller parent. Sure enough, this turns out to be true when tested experimentally.

From this, I have been developing and testing a model of investment allocation in biparental species. I am interested in understanding how parents resolve the life-history tradeoffs and game-theoretic problems of investing in their offspring when they must consider not only their own investment into the young, but also that of their partner. In particular, I have been exploring the effects of asymmetries in the willingness of the two parents to invest and testing my predictions with cichlids. Because two parents may value the same offspring differently, there will be conflicts over investment into those offspring.

My experimental tests show that parents do assess their offspring relative to themselves and that they incorporate the investment decision of their partner in their own decision rules.

I have also become very interested in detailed investigation of egg size variation and the interplay between egg size allocation, other components of parental investment, ecology, and developmental morphology. I have been pursuing a number of inter-related experiments to test and explain the enormous egg-size variation among fishes. In species such as cichlids, which provide parental care of eggs and fry, the tradeoff is not just between egg number and egg size, because egg size has ramifications for post-laying parental investment.

I am currently completing experiments exploring the effects of temperature on hatching time, egg size on hatching time, temperature on the costs of fanning, temperature on spawning site choice, egg size on hatchling size and hatchling size on swimming ability. Together these experiments show that cichlid fishes are making extraordinarily precise allocation decisions for egg size in response to numerous complex environmental variables.

For example, one species I am focussing on, Cichlasoma tuba, appears to lay unusually large eggs (and suffer a massive drop in fecundity) so that it can lay these in unexploited fast-water portions of rivers. The large eggs would normally die of insufficient oxygen, but the fast river current makes up for inadequate parental fanning. The cooler temperature in faster water also lengthens development time, but reduces oxygen demand. The eggs hatch into unusually large offspring which can swim in the current, unlike the offspring of species laying smaller eggs. These other species may temporarily exploit the fast water, but are swept away during storms. This kind of study is only possible because of the enormous size and diversity of the family Cichlidae (upwards of 1500 species), and because many species can be bred in captivity.

To obtain a much larger dataset on egg size variation in cichlids four years ago I initiated the Cichlid Egg Project. This project encourages hobbyists from around the world to participate in science by donating samples of eggs from species of cichlids they have bred. I obtain this data through traditional channels such as hobbyist clubs, but I have also harnessed the power of the World Wide Web. The online project (http://cichlidresearch.com/eggproj.html) has been phenomenally successful to date: I now have precise quantitative data on the eggs of over 220 species of cichlids. This data set is answering not only my original questions about egg size allocation across taxa, but has also opened up several new areas of investigation, including characters useful for deciphering the complex phylogeny of cichlids.

 

Further Reading

 

Probably the best book to read to help you understand the way an evolutionary ecologist sees the world is the book, "The Selfish Gene" by Richard Dawkins. Dawkins is a scientist, but has the rare gift of writing understandable and entertaining books. This book is available either new at most any larger bookstore, or in many used bookstores. The original edition was published in 1976. There is a newer 1989 edition as well which contains additional notes. Either edition is well worth the time to read and can change the way you view the world.

Dawkins, Richard. 1989. The Selfish Gene (new edition). Oxford University Press, Oxford.

ISBN 0-19-217773-7 (hardback) 0-19-286092-5 (paperback)

Not all evolutionary ecologists (myself included) agree with everything Dawkins says, but the general arguments and tone of the book are right on the mark.

 

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TERMS

evolutionary ecologist: an evolutionary ecologist is a type of biologist who is concerned with why plants and animals do the things they do (e.g., breed in a certain way, grow in a certain way) with particular reference to how this shapes and is shaped by the whole of the animals life time, i.e., its life history. Evolutionary ecologists often speak of costs and benefits of either a behaviour or a particular structure (e.g., the long neck of a giraffe).

decide, decision: this is a tricky word for some people to swallow, particularly those not familiar with modern evolutionary ecology. When I say "a parent decides", that does not NECESSARILY mean that the animal says to itself, "well, I could do this, or I could do that...". Rather the term means "to take a course of action" and does not specify the exact brain functioning that goes into the decision. There need not be any brain function of the kind humans typically associate with decisions; if upon receiving a certain set of inputs the animal does a certain action, that is called a "decision" offspring: the kids, babies, eggs, fry... resource: time, energy, risk. These three are frequently interconnected to a large extent. Risk is a particularly challenging resource to analyze. Time and energy are straight-forward in that once they are used, they are gone. If you have a chocolate bar and you eat it, it is gone. That doesn't mean you can't get another one, BUT (very important point), if you do get another one, that is not the same as just having the first one and not eating it. Why? Because if you get another one, and had you not eaten the first one, you would now have two chocolate bars. Risk is harder to understand and still the topic of much study and debate. If you run across the road in front of a truck, but make it to the other side unharmed, are you the same as if you had run across the road with no truck approaching???