Antipredator defenses are expected to decrease towards higher latitudes because predation rates are predicted to decrease with latitude. However, latitudinal variation in predator avoidance and defense mechanisms has seldom been studied. We studied tadpole antipredator defenses in common frog Rana temporaria populations collected along a 1500-km latitudinal gradient across Sweden, along which previous studies have found increasing growth and development rates. In laboratory common garden experiments, we measured behavioral and morphological defenses as well as swimming performance in tadpoles raised in the presence and absence of a predator (Aeshna dragonfly larva). We also estimated tadpole survival in the presence of free-ranging predators and compared predator densities between R. temporaria breeding ponds at low and high latitudes.
Results/Conclusions Tadpole activity and foraging were generally positively correlated with latitude. While all populations responded to predator presence by decreasing activity and foraging, high-latitude populations maintained higher activity level in the presence of predator. All populations exhibited defensive morphology in body and tail shape but there were indications that defensive morphology was stronger in the high latitude populations. In the southern populations, predator presence tended to have a negative effect on metamorphic size, whereas in the northern populations predators had little or a positive effect on size. Latitude of origin had a strong effect on survival in the presence of a free-ranging predator, with high-latitude tadpoles experiencing higher mortality than those from low latitudes. Swimming performance was not correlated with latitude suggesting that increased activity level was the main factor for increased predation rates in high-latitude populations. In the wild, predator densities were significantly lower in the high latitude than in the mid-latitude breeding ponds. Our results support the notion that latitudinal climatic variation can have a major impact on antipredator defenses. Although the higher activity level in the northern populations seems to confer a significant survival disadvantage under predation risk, it is probably needed to maintain the high growth and development rates, which enable completion of development under harsh high-latitude conditions. Furthermore, the occurrence of R. temporaria at high latitudes may be facilitated by the lower predator densities in the north.
