A Skeletochronological Assessment of Age–Parasitism Relationships in Wood Frogs (Lithobates sylvaticus)
Amphibian parasite communities are often suggested to be dependent on host size. However, age-related differences in parasite exposure and susceptibility could lead to differences in parasitism unaccounted for by host size. To address these hypotheses, we determined the ages of Wood Frogs (Lithobates sylvaticus) using skeletochronology and assessed the relationships of host age and size with helminth infections. Neither age nor size was significantly related to helminth species richness. Total infection intensity increased with female age and generally increased with age and size in males. However, these overall measures may mask more complex dynamics with individual helminth species. Although the intensity of Alaria spp. increased in males initially, both the intensity and prevalence of Alaria spp. ultimately decreased with male age. The prevalence of both Fibricola sp. and Echinoparyphium rubrum increased with female age. The abundance of Fibricola sp. also increased with female age, whereas Fibricola sp. prevalence and intensity increased with male size. Rhabdias bakeri abundance increased with female size, but decreased with female age. These results suggest age and size are both related to some aspects of Wood Frog infection dynamics. We hypothesize that the absence of trematode metacercariae in young female frogs may be a result of delayed maturity because female Wood Frogs generally avoid water until they are reproductively mature. In contrast, male parasite-specific relationships may be the result of host immune response, host mortality, or parasite mortality. Because males and females were sampled at different times of the season, differences in parasitism based on host sex alone should be interpreted with caution.Abstract
Typical 20-μm hematoxylin-stained transverse sections from phalanges of Wood Frogs from North Dakota. MC, marrow cavity; ML, metamorphosis line; arrows indicate LAGs unless labeled ML. (A) Male Wood Frog without ML; 4 yr old. (B) Male Wood Frog with the ML intact; 2 yr old.
Age–frequency distribution in the selected sample of female (n = 23) and male (n = 56), respectively.
Probability of an infection (left axis and plotted lines) with (A) Fibricola sp. or (B) Echinoparyphium rubrum in relation to female Wood Frog age. Open circles represent the frequency distribution of individuals in each age class that are either infected (presence = 1, or upper tier of circles) or not (presence = 0, lower tier of circles). Mean infection intensities (black squares) and SEs (vertical lines) are plotted within each age class (right axis).
Positive relationships between (A) Rhabdias bakeri intensity and size (SVL) in female Wood Frogs and (B) TII and age in female Wood Frogs. Negative binomial regressions were used to test for significance; data represented here are log-transformed and fit with a linear regression for visualization purposes.
Probability of infection (left axis and plotted line) with Alaria spp. in relation to male Wood Frog age. Open circles represent the frequency distribution of individuals in each age class that are either infected (upper tier of circles) or not (lower tier of circles). Mean infection intensities (black squares) and SEs (vertical lines) are plotted within each age class (right axis).
Relationship between body length and age in Wood Frogs. Solid lines show the predicted values estimated from the von Bertalanffy growth model. (A) Female von Bertalanffy growth model: L(t) = 55.73[1−e−0.587(t +0.918)]. (B) Male von Bertalanffy growth model: L(t) = 51.26[1−e−0.931(t +0.643)].
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