The Phenotypic Plasticity of Death Valley's Pupfish
Desert fish are revealing how the environment alters development to modify body shape and behavior
Aggression and the AVT System
Male pupfish are pugnacious. During the breeding season, males display, chase and nip as they establish and defend reproductive territories. As graduate students at the University of California, Los Angeles, during the 1960s, David L. Soltz and Richard Liu made detailed behavioral studies of Death Valley's pupfishes. Their work revealed that pupfish populations vary in behaviors including aggression and courtship. For instance, populations in large spring habitats such as Big Spring show territorial breeding where males are highly aggressive. Pupfish in Devil's Hole, in contrast, are not aggressive but court females intensely.
As previously mentioned, however, the behavior of some populations varies seasonally. This variation results from shifts in pupfish reproductive activity in response to seasonal changes in water temperature and other environmental factors. Not all populations show similar behavioral changes, though. Pupfish in variable habitats such as the Amargosa River alter their aggression with seasonal variation in pupfish population density and water temperature, whereas fish in comparatively stable habitats such as Big Spring show territorial breeding year-round.
Given that behavior can vary so widely among populations, has the behavior of Death Valley's pupfishes evolved in response to their ecologically dissimilar habitats? And, if so, what proximate mechanisms might underlie that behavioral evolution? One possible mechanism came from studies on the bluehead wrasse (Thalassoma bifasciatum) by John Godwin and Katharine Semsar of North Carolina State University. The bluehead wrasse is a coral reef fish that changes sex from female to male in response to social cues. Besides changing sex, the wrasse also shifts between territorial and nonterritorial behaviors according to social conditions.
Godwin and Semsar demonstrated that these shifts in territoriality are mediated by arginine vasotocin (AVT). AVT is a peptide hormone produced in the preoptic area of the hypothalamus. AVT is secreted into circulation to regulate hydromineral balance, but also acts as a neurotransmitter within the brain to modulate behavior. In the bluehead wrasse, the AVT system responds rapidly to social cues and underlies shifts in territorial behaviors. Could the AVT system play a similar role in regulating differences in territoriality and aggression among Death Valley's pupfishes?
To begin to answer this question, Nevitt and I explored whether pupfish populations in the wild differ in their neural AVT phenotype. We focused these studies on pupfish from two Death Valley habitats, the Amargosa River and Big Spring. These two populations have been isolated from each other for less than 4,000 years and provide an opportunity to examine the process of behavioral evolution as it is taking place. Using an antibody to AVT, we labeled and quantified AVT-synthesizing neurons in the brains of pupfish from these two Death Valley habitats. Males and females from the Amargosa River showed larger AVT neurons in two specific regions of the preoptic area of the hypothalamus—the parvocellular and magnocellular nuclei—relative to same-sex individuals from Big Spring.
Neurons in the parvocellular and magnocellular regions have been linked to the regulation of stress responses, hydromineral balance and social behavior in fish, so the differences in neural AVT phenotype that we found might be associated with variation in any of those functions. As a next step, we examined how AVT influences pupfish behavior by administering AVT to male Amargosa River pupfish under both controlled laboratory conditions and in their natural habitat. Under both testing conditions, exogenous AVT caused male pupfish to become less aggressive, demonstrating that AVT indeed regulates pupfish behavior.
All the same, it remained unclear whether the AVT system had evolved between the populations. We found the answer to this question by breeding pupfish from the Amargosa River and Big Spring habitats in captivity. Offspring were reared under different conditions of salinity (0.4 parts per thousand or 3 parts per thousand) and temperature (stable or daily fluctuating) to represent the range of environmental variation experienced in the populations' two habitats. This experimental design permitted us to examine how salinity and temperature influenced the AVT system and behavior of pupfish, while also allowing comparison of how plasticity in the AVT system might differ between the Amargosa River and Big Spring populations.
Results from this experiment revealed that Big Spring pupfish are more aggressive than fish from the Amargosa River, providing evidence that these two pupfish populations have evolved differences in behavior. Big Spring pupfish show a highly territorial breeding system with little seasonal variation, and pupfish in this habitat have evolved higher aggression in response to these social challenges. The Big Spring and Amargosa River populations also evolved differences in how their AVT systems respond to salinity and temperature conditions. These evolutionary changes, however, vary among different AVT-producing neuron groups in the brain. Parvocellular AVT neurons, which appear to mediate physiological responses to stressors, showed plastic changes to temperature in Big Spring pupfish but not Amargosa River pupfish. The populations also differed in how the parvocellular neurons responded to salinity. Magnocellular AVT neurons, in contrast, show plastic responses to salinity and temperature, but this plasticity is similar in both populations.
So far, these studies have established that phenotypic plasticity can contribute to population-level variation in the neural pathways underlying aggression and that plasticity in brain pathways can evolve in as little as 4,000 years. Nevertheless, we are only beginning to unravel the role that phenotypic plasticity and the AVT system play in behavioral diversification of Death Valley's pupfishes.