Researchers are becoming interested in testing whether investment in growth and/or development trades off against investment in parasite defence. We tested this idea by examining relations between development of Wood Frogs (Rana sylvatica) and susceptibility to lung nematodes (Rhabdias ranae). Male and female frogs reared in outdoor mesocosms were the same length and mass at metamorphosis. However, males metamorphosed sooner than females. Lung nematodes were no more likely to penetrate male versus female metamorphs following controlled exposures, but males had higher intensities of adult female worms and the largest worms per host were, on average, of larger size in male metamorphs. Males that took longer to metamorphose carried higher numbers of worms in their lungs than males that metamorphosed early. In comparison, females that developed faster harboured more worms in their lungs than females that took longer to reach metamorphosis. Our results suggest that variation in susceptibility to lung nematodes is influenced by host sex and possibly also by sex-specific relations with developmental rate. Further, male hosts might prove to be a more important source of infective stages of worms than female hosts.
To rear frogs to metamorphosis, 10 tanks (Rubbermaid®; 132 cm × 78 cm × 63 cm) were established as outdoor mesocosms in a fenced-off compound. Each tank was filled with 300 litres of water and covered with 40% shadecloth to prevent oviposition by invertebrates, to prevent predation of frog tadpoles by wildlife, and also to prevent frogs from escaping. Water levels fluctuated naturally with precipitation and evaporation. On 2 occasions, water was removed to prevent overflow. Each tank was seeded with 100 g of leaf litter collected from a mixed deciduous forest stand and 8.75 g of Purina Rabbit ChowⒸ 2 weeks prior to the introduction of the tadpoles. By this time, there also was sufficient growth of algae to provide food. Daphnia spp. were collected using D-frame dipnets from a local pond, and introduced to a ‘ stock’ tank with 300 litres of water. Once Daphnia had established in the stock tank, 750 ml of water containing Daphnia were added to each rearing tank to control excessive algal growth.
Portions of 3 R. sylvatica egg masses were collected from Casselman, Ontario (45°19'N, 75°05'W) on 11 April 2005. Each egg mass was housed indoors in a separate aquarium until hatching, with pond water from their natal habitats and a bubbler. Tadpoles were fed a granular mixture of tadpole food (Ward’s Nat. Sci. Tadpole Food #88V 6534) and Hagen fish flakes (Nutrafin Max Complete Flake Food #668672) until they had developed to Gosner stage 25 (Gosner, 1960). Five stage-25 tadpoles reared from eggs from each of the 3 egg masses were added together for a total of 15 individuals in each of the 10 tanks.
Rana sylvatica metamorphosed from 12 June to 26 June 2005, coinciding with metamorphosis of wild conspecifics in the Ottawa region (O. Dare, personal observations). The first day a metamorph was removed was recorded as day 0 to metamorphosis (DM = 0). Metamorphs were removed from tanks when they developed forelimbs (Gosner stage 42). At this stage, snout-to-vent lengths (SVL) were measured to the nearest 0. 1cm, and each metamorph was also measured to the nearest 0.1 g.