Energetic costs of parasitism in the Cape ground squirrel Xerus inauris

Abstract

Parasites have been suggested to influence many aspects of host behaviour. Some of these effects may be mediated via their impact on host energy budgets. This impact may include effects on both energy intake and absorption as well as components of expenditure, including resting metabolic rate (RMR) and activity (e.g. grooming). Despite their potential importance, the energy costs of parasitism have seldom been directly quantified in a field setting. Here we pharmacologically treated female Cape ground squirrels (Xerus inauris) with anti-parasite drugs and measured the change in body composition, the daily energy expenditure (DEE) using doubly labelled water, the RMR by respirometry and the proportions of time spent looking for food, feeding, moving and grooming. Post-treatment animals gained an average 19g of fat or approximately 25kJd-1. DEE averaged 382kJd-1 prior to and 375kJd-1 post treatment (p>0.05). RMR averaged 174kJd-1 prior to and 217kJd-1 post treatment (p<0.009). Post-treatment animals spent less time looking for food and grooming, but more time on feeding. A primary impact of infection by parasites could be suppression of feeding behaviour and, hence, total available energy resources. The significant elevation of RMR after treatment was unexpected. One explanation might be that parasites produce metabolic by-products that suppress RMR. Overall, these findings suggest that impacts of parasites on host energy budgets are complex and are not easily explained by simple effects such as stimulation of a costly immune response. There is currently no broadly generalizable framework available for predicting the energetic consequences of parasitic infection.

Figure 1

(a) Relative number of ectoparasites per animal and (b) relative number of endoparasites per animal before treatment for parasites (pre) and after treatment (post). Filled bars denote control animals; open bars denote treated animals. Error bars denote standard errors. Asterisk indicates a significant difference between groups.

Figure 2

Mass-corrected daily energy expenditure (DEE) (kJ d⁻¹) and resting metabolic rate (RMR) (kJ d⁻¹) of X. inauris females prior to dosing for parasitism (pre, filled bars) and after dosing (post, open bars). Values were calculated using the mean residual for each of the groups of animals added to the grand mean across all animal groups. Error bars denote the standard errors of the residual mean estimates. Asterisk indicates a significant difference between groups.

Figure 3

Block diagram illustrating energy allocation to DEE and RMR and fat deposition in X. inauris females before and after treatment for parasitism.

Figure 4

Proportion of time spent feeding, foraging, grooming and moving by X. inauris females prior to anti-parasite treatment (pre: filled bars) and post treatment (post: open bars). Error bars denote standard errors. Asterisk indicates a significant difference between groups.