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.2012 Jan;81(1):58-69.
doi: 10.1111/j.1365-2656.2011.01892.x. Epub 2011 Aug 10.

Synergistic effects of seasonal rainfall, parasites and demography on fluctuations in springbok body condition

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Synergistic effects of seasonal rainfall, parasites and demography on fluctuations in springbok body condition

Wendy C Turner et al. J Anim Ecol.2012 Jan.

Abstract

1. Seasonality of rainfall can exert a strong influence on animal condition and on host-parasite interactions. The body condition of ruminants fluctuates seasonally in response to changes in energy requirements, foraging patterns and resource availability, and seasonal variation in parasite infections may further alter ruminant body condition. 2. This study disentangles the effects of rainfall and gastrointestinal parasite infections on springbok (Antidorcas marsupialis) body condition and determines how these factors vary among demographic groups. 3. Using data from four years and three study areas, we investigated (i) the influence of rainfall variation, demographic factors and parasite interactions on parasite prevalence or infection intensity, (ii) whether parasitism or rainfall is a more important predictor of springbok body condition and (iii) how parasitism and condition vary among study areas along a rainfall gradient. 4. We found that increased parasite intensity is associated with reduced body condition only for adult females. For all other demographic groups, body condition was significantly related to prior rainfall and not to parasitism. Rainfall lagged by two months had a positive effect on body condition. 5. Adult females showed evidence of a 'periparturient rise' in parasite intensity and had higher parasite intensity and lower body condition than adult males after parturition and during early lactation. After juveniles were weaned, adult females had lower parasite intensity than adult males. Sex differences in parasitism and condition may be due to differences between adult females and males in the seasonal timing of reproductive effort and its effects on host immunity, as well as documented sex differences in vulnerability to predation. 6. Our results highlight that parasites and the environment can synergistically affect host populations, but that these interactions might be masked by their interwoven relationships, their differential impacts on demographic groups, and the different time-scales at which they operate.

© 2011 The Authors. Journal of Animal Ecology © 2011 British Ecological Society.

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Figures

Figure 1
Figure 1
Etosha National Park in northern Namibia. The three study areas are indicated with dashed lines. Okuakuejo was the main study area with intermediate rainfall values; Otjovasandu was the drier area and Namutoni the wetter area. The inset figure shows annual rainfall in the years each area was sampled. The satellite image from Terra MODIS shows the large Etosha salt pan (available fromhttp://visibleearth.nasa.gov/view_rec.php?id=4009).
Figure 2
Figure 2
Eimeria oocyst morphotypes from springbok. These undescribed oocysts were considered generically for this study as A)Eimeria A, a tear-shaped oocyst and B)Eimeria B, an elliptical oocyst with micropyle cap (additional research is underway to determine if each morphotype represents a single species). The bar represents 10μm.
Figure 3
Figure 3
Monthly variation in rainfall and parasite prevalence in springbok.
Figure 4
Figure 4
Springbok body condition by age and sex. Condition score is the average (1-5 scale) condition score recorded; means are presented ± standard errors.
Figure 5
Figure 5
Springbok body condition and strongyle infection intensity. A. Strongyle intensity and body condition by demographic groups from the main study area. B. Strongyle intensity and body condition for adult males and adult females in the drier and wetter study areas. Means are presented ± standard errors; condition score is the average (1-5 scale) condition score recorded; strongyle intensity is presented as log(x+1) wherex is the parasite count including zero values; significant differences are indicated with **.
Figure 6
Figure 6
Sex differences in (a) strongyle parasitism and (b) condition in adult springbok. Data are presented monthly in relation to the average timing of reproductive events (Gasaway, Gasaway & Berry 1996; Skinner & Chimimba 2005; W.D. Versfeld pers. obs.). Means are presented ± standard errors; condition score is the mean (1-5 scale) condition scores recorded; strongyle intensity is presented as log(x+1) wherex is the parasite count including zero values.
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References

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