Abstract

For gape-limited predators, investment in larger heads should occur when larger prey items are more profitable or are the only prey option available. This may result in evolutionary or within-lifetime (plastic) increases in gape size in animals exposed to larger prey. This phenomenon has been well documented in larval but not adult caudates (salamanders and newts). We report here evidence of greater gape size in populations of a newt that co-occur with energetically profitable, large prey. We collected morphological data from populations of Red-Spotted Newts (Notophthalmus viridescens viridescens) that occupied nearby ponds that varied in the presence of eggs and tadpoles of Wood Frogs (Lithobates sylvaticus). Newts were larger in ponds occupied by Wood Frogs, suggesting a potential energetic advantage to this prey. Newts syntopic with Wood Frogs had relatively wider heads, which we interpret as a potential adaptation for consuming seasonally abundant Wood Frog eggs and tadpoles. Male newts had wider heads than did females, suggesting sexual selection also may act on head or gape width in newts. Our findings suggest that adult newts exhibit either phenotypic plasticity in gape width or microgeographic local adaptation in response to variation in food resources. Additional work examining this in other caudates may reveal it to be a phenomenon widespread in adults as well as larvae.