A single silver, violet and cornflower yellow fish flits through the water and glides through the seaweed in each tank. The brightly colored fish, a male with a sword on his back and lines up his sides, attracts female mates. How he does it is what interests the researchers peering through the glass.

Dr. Molly R. Morris has spent more than 20 years studying swordtail fish (Xiphophorus multilineatus) and their mating strategies. She specializes in sexual selection and its role in the evolutionary process, and she is currently studying how certain social factors influence sexual dominance, activity and attraction. Research apprentice Nicole Kleinas, a sophomore studying Biological Sciences, is working with Morris to test the hypothesis that genetics is not alone in dictating speciation.

“[Fish] are becoming sort of a model system for studying how sexual selection plays a role in increasing biodiversity, and they’re very manageable in the lab,” Morris said. She has pinpointed two social factors she believes are instrumental in the growth rate, and thus sexual performance, of the fish: maternal investment and exposure to adult males. Specifically, how much attention and nourishment did the mother provide? Were there any male role models or potential sources of competition around?

The researchers are interested in determining how maternal investment and exposure to adult males affect aggression, which is tied to courting behavior. Morris has not studied aggression in depth before, and she hopes to learn how males possessing different sex-selected traits react to competition. After the eligible bachelors have grown a healthy amount, Morris and Nicole test their aggression. They position the fish by mirrors and quantify their reactions to their reflections, which they perceive as menacing competitors, within a given period of time. These aggressive responses vary from mundane (swimming back and forth) to violent (furious rushing and biting).

The research gets complicated in the expression of size. There is a window of size into which many of these fish fit—dictated by the genotype, the DNA coding, on the Y chromosome. However, within that genotypically influenced range of size, the fish are plastic; they can grow as they will. The difference is in genotype and the expressed physical phenotype. Morris has learned that seeing a large male as a child compels a fish to grow larger through a variety of means. But if all of them possess the ability to grow faster and larger, why don’t they?

“I think people think of growth rate as something that you should just do as fast as you can, and what this is making us realize is that growth has a cost,” Morris said. “You can grow fast and you can grow well. Those are two different things.” In some cases, depending on maternal attention and the presence of a large male, for example, growing faster or better is not necessarily the best or most obvious choice.

One of Morris’ former graduate assistants bred several populations of fish based on maternal care and exposure to an adult male. Over time, these populations have been continually bred and separated into groups based on growth. The fish are categorized based on which have had greater maternal care, which have been exposed to older males, which have had experienced both and which have received nothing.

Nicole photographs the fish next to measurement devices and compares their growth over a period of time. She also scrutinizes the morphological changes, such as the vertical violet bars. Once they’ve grown, she runs the aggression tests with the mirrors, gauging how physical attributes affect aggression and which fish with which social background are aggressive. Aside from taking these measurements, she performs a number of odd jobs around the lab. “I’m helping with cleaning, feeding,” Nicole said, and this is no small task. “We have a lot of fish. A ton of fish.” She also runs the preference test on the females to determine which social history in males yields more attractive specimens. (See What Women Want below)

But the effects these mothers and older males have on the young fish vary, as do the actions of these older figures. For example, the results are different by gender. “The mothers seem to invest differently in males than in females and how males and females respond to the maternal investment is different,”

Morris said. “Those are really new findings we’re pretty excited about.”

The results so far support Morris’ hypotheses. In males, a larger display of maternal investment leads to a higher growth rate, as does the presence of an older male. However, when female young are on a high quality diet and they see a larger male, their growth rate goes down. But social forces aren’t limited to determining growth in fish. Every animal grows up in an influential environment.

Aside from fish, these experiments tell Morris and other scientists interesting things about sexual selection in many species, including humans, and support theories pressing social interaction as a major factor in growth. More startling by far are surfacing connections being made between growth rate and diabetes. The research suggests diabetes is an evolutionary strategy tied to growth in times of scarce resources.

“There’s a hypothesis out there that diabetes might be an alternative growth strategy,” Morris explained. “One of the reasons that we have the genetic aspect of diabetes is because the genes that produce people more sensitive to diabetes are selected for in that they allow individuals to grow when there’s very little food. . . . They call it ‘the thrifty genotype.’” Such revelations explain diabetes’ advantage, and enable researchers to better understand why the disease is genetically engrained in so many humans.

But this diabetes correlation is a more surprising result of the research. It is these smaller, sideline discoveries that are so useful in connecting research on the swordtail fish with humans.

The fish are useful to study: They are efficient for a lab, beautiful creatures to spend time working with, and “you can do a lot more manipulation with fish than you can with humans,” Morris said.
The males wait in their bachelor pads, circling in flashy purple movements, bred to teach us about how important our social pasts are to our aggression and our sexual attraction.

What Women Want

What Morris, her apprentice, and many other researchers already know is what women want. The female swordtail fish respond positively to the sword, and even more energetically when the male is large. The vertical bars are also an attractive sign of masculinity, and symmetry plays a part in just how tantalizing the markings are.

Nicole runs experiments on females, dancing cut-out photographs of male specimens of varying sizes and morphological structures on either side of a tank and recording how much time the female fish spends near one specimen as opposed to the other.

“These fish have a lot of sexually selected traits,” Morris explained. “We have this sword, an extension off the back; we have this pigment pattern we call body bars. Females like those bars.” The traits Morris describes are a long, trailing portion of the tail fin that juts out from the bottom and a series of vertical marks along both sides of the fish, deep purple in color. The females, by contrast, do not possess this sword feature, and their markings are limited to a single, thick, horizontal dash of purple. In humans there are similar physical traits. Big muscles in men are a surefire point of attraction for females. Machismo is also a factor: The bigger the fish is (or the human male), then the higher the chances that he will be aggressive.