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Research Topics for Jeff Arendt  
Evolutionary Ecology of Growth Rate
My primary interest is in the evolutionary ecology of growth rate and body size. It has become increasingly clear that growth rates (increase in size per unit time) are optimized for local conditions rather than always being maximized so that individuals always grow as fast as physiologically possible. That is, the average growth rate that evolves within a population is a balance of the costs and benefits of rapid growth. Part of my research involves determining what conditions favor slow growth (when there is minimal advantage to getting large fast and the costs of rapid growth dominate) and what conditions favor rapid growth (when the advantages outweigh the costs). For example, my doctoral research looked at how the competition between pumpkinseed sunfish (Lepomis gibbosus) and bluegill sunfish (L. macrochirus) influenced growth rate. These species compete when they are small, but at around 70 mm in length each shows a major diet shift. Bluegill shift to open waters where they feed on plankton and pumpkinseed are now large enough to start crushing the shells of snails, a major component of their diet. The result is a size-refuge from competition favoring rapid growth to reach this refuge as quickly as possible. However, in lakes where bluegill have never been present there is no advantage to rapid growth. Measurements of growth conducted in the wild show that pumpkinseed with bluegill grow very slowly, but this is a direct effect of having a competitor present (i.e., there is simply less food for everyone). Under common garden conditions, pumpkinseed from lakes with bluegill have a faster growth rate than do pumpkinseed from non-bluegill lakes (for theory and an empirical test see Arendt and Wilson 1997). In fact, in a reciprocal transplant experiment the non-bluegill fish were starving when raised with bluegill (Arendt and Wilson 1999)
Costs of Rapid Growth I: Skeletal Development
It seems clear that rapid growth is good for pumpkinseed when bluegill are present, so why haven't the non-bluegill pumpkinseed evolved to grow just as fast? One reason is that rapid growth results in a delay in skeletal development. Juvenile fish from a fast-growth bluegill population show a delay in the onset of mineralization in their bones by several days (Arendt and Wilson 200) and weaker scales (Arendt et al. 2001). Weaker skeletal elements may compromise swimming speed (a tradeoff between growth rate and swimming speed has been demonstrated in several fish species), reduce the strength of defensive elements (scales and fin spines), and, especially important for pumpkinseed, delay the switch to feeding on snails as the pharyngeal bones used to crush shells takes longer to fully mineralize. It is possible that this cost is never paid when bluegill are present because competition limits the realized growth rate, but when there are no bluegill pumpkinseed from these populations may simply grow so fast that skeletal development cannot keep up (this is also why large breeds of dogs often suffer from hip-displasia).
Costs of Rapid Growth II: Muscle Development
My interest in exploring developmental costs of rapid growth, such as skeletal development, has lead me to my secondary interest, understanding the building blocks that determine body size and growth rate. I am currently looking at how cell recruitment and cell growth contribute to overall growth and body size. I am working with spadefoot toad tadpoles because this genus includes species that can develop from hatching to metamorphosis with just 8 days and other species that take a more leisurely 2 months. I have also found a tradeoff between growth rate and swimming speed (Arendt 2003) similar to that seen in fish. I am now looking at how recruitment and growth of muscle cells mediates this tradeoff. I am especially interested in whether similarly sized tadpoles but with different cellular composition (those with many small muscle fibers vs. those with few but large muscle fibers) perform.
Developmental Plasticity in Body Size: food and temperature

As a corollary to studying these trade-offs, I am also interested in how cell growth and recruitment contribute to plasticity in body size. Tadpoles fed more food grow faster than those on restricted diets and tadpoles raised at warmer temperatures grow faster than those raised at cooler temperatures. However, food effects and temperature effects on growth are mediated through different cellular mechanisms. There is also a complex interaction between food and temperature. Early on, cell recruitment is limited at low temperatures so all growth is due to cell growth. However, at high temperatures both cell recruitment and cell growth contribute to overall body growth.

Future work includes studying how general the pattern of developmental plasticity is in other anuran species. In addition, I am interested in the functional consequences of these developmental patterns for swimming ability in these tadpoles. Finally, I am hoping to be able to tie these factors back into variation in local predation pressure and how different populations and different species have evolved to deal with predation pressure depending upon the local ecology.