Form, function & fitness—

Antagonist

J. Brad Johnson1,2, D. Brent Burt1, Thomas J. DeWitt2

1Dept. Biology, Stephen F. Austin State University2Dept. Wildlife & Fisheries Sciences, Texas A&M University

Selection and performance gradients for tadpoles exposed to predatory naiads

(Evolution 62:1243-1251)

AcknowldegementsThanks RB Langerhans, A Pease, B Shipley, D Pfennig and two referees for manuscript review, and to C Kaufman for empirical assistance. Funded by NSF (DEB0344488 to TJD) and Dept. Biology, SFA (JBJ)

Protagonist

2. ConceptsIdentifying selection on a trait is difficult because it may be correlated with other traits. Plus, the correlated traits also may be targets of selection. Lande & Arnold (1983) proposed we use multiple regression to disentangle the complexity of correlated traits under selection. In the same year, Arnold also pointed out that primary traits are not typically direct targets of selection.

3. MethodsWe collected tadpoles from the wild, measured their morphology (per figure), startled them three times while filming with high-speed video equipment, and then exposed tadpoles in groups of 10 to predatory Anax larvae in wading pools. Morphology was summarized as the three major principle components of shape (i.e. relative warps). Survival (fitness) was scored dichotomously and a path analysis was conducted using the expanded Arnold paradigm as depicted above. Surviving tadpoles were also examined for tail damage.

Players:

Now frogs eat usINow frogs eat usI

Lie, wait,Lie, wait,strike!strike!

DragonfliesDragonfliesare mean!are mean!

5. ConclusionsTadpole survival with dragonfly nymphs was increased by deep tailfins and large size. Based on work by uber- awesome ecologists (Van Buskirk et al. 2003), we know deep tails draw predator strikes to nonvital tailfin tissue and hence away from the trunk. Perhaps surprisingly, fast swimming did not seem to confer defense against nymphs.

ReferencesArnold SJ (1983) Morphology, performance and fitness. Am. Zool. 23:347–361.Dayton GH, Saenz D, Baum KA, Langerhans RB, DeWitt TJ (2005) Body shape, burst speed and predator susceptibility of larval anurans. Oikos 111:582-591Garland T Jr, Losos JB (1994) Ecological morphology of locomotor performance in squamate reptiles. In PC Wainwright PC and SM Reilly, eds. Ecological Morphology. U. Chicago Press.Johnson JB, Bert DB, DeWitt TJ (2008) Form, function, fitness—pathways to survival. Evolution 62:1243-1251.Lande R, Arnold SJ (1983) The measurement of selection on correlated characters. Evolution 37:1210–1226.Van Buskirk J, Anderwald P, Lüpold S, Reinhardt L, Schuler H (2003) The lure effect, tadpole tail shape and the target of dragonfly strikes. J.Herpetol. 37:420–424.

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4. Results—Our major result is summarized by the path diagram:

Rather, primary traits such as morphology merely enable aspects of performance. And performance determines survival and reproduction. To admit this complexity, Arnold suggested path analysis as illustrated in the left panel of the figure below. Expanding that model (right panel) can even help uncover aspects of performance due to unmeasured traits (Garland & Losos 1994; Johnson et al. 2008).

Significant standardized

coefficients are in black.

1. Natural historyTwo hypotheses suggest how tadpole morphology mediates survival with predators:1) morphology confers increased swimming speed:

(e.g Dayton et al. 2005)

2) morphology induces predators to misdirect strikes (Van Buskirk et al. 2003)