Effects of food abundance and predictability on body condition and health parameters: experimental tests with the Hooded Crow

Abstract

It has been shown that small passerines can counteract variability of food resources by actively regulating their body reserves through an increase of mass. However, the effects of food predictability on body mass regulation and other body parameters, such as immune functions, in larger species have been little studied. To analyse the response of the Hooded Crow Corvus corone to food abundance and predictability, we performed three experiments with controlled food provisioning under laboratory conditions. Body mass, TOBEC (total body electrical conductivity) lean mass index, blood parameters and immune organ masses were measured at the beginning and end of a 15‐day period. In the first experiment, the food release was predictable (same amount each day) but the quantity of food delivered to five groups of birds varied (37, 75, 100, 150 or 300 g/day). Low food levels induced a greater decrease in mass accompanied by an increase in erythrocyte sedimentation rate. In the second experiment, the same average quantity of food (100 g) was supplied according to either predictable or unpredictable (random) schedules. In this case, the crows lost more mass, and their erythrocyte sedimentation rate increased when food was unpredictable. In the third experiment, the same average quantity of food (150 g) was supplied according to either a predictable schedule or two schedules with different levels of variability. The group with a low level of variability did not differ from the control, while the group with a highly variable feeding schedule lost more mass. In this group, the higher mass loss was associated with greater variation of the erythrocyte sedimentation rate and a reduced Phytohaemagglutinin index of immunocompetence and haematocrit value. The results of experiments 2 and 3 contrast with findings in other passerines, which increase their mass when food availability is unpredictable. It appears that a body mass decrease in the Hooded Crow can be induced by a reduction of either food abundance (exp. 1) or food predictability (exp. 2, 3), and it is accompanied by a worsening of health state.

The effects of testosterone on immune function in quail selected for divergent plasma corticosterone response

“…Considering that T implantation had a negative effect on body mass, and body mass in previous studies has been positively related to immunocompetence (Acquarone et al, 2002;Cucco et al, 2002;Fargallo et al, 2002;Lifjeld et al, 2002), it seems somewhat surprising that T did not have a negative effect on immune response. The reasons for this are unclear; however, it is possible that individuals trade-off condition (as fat reserves) for immunocompetence (McNamara and Buchanan, 2005); the result is no perceived cost of T to immunity but a corresponding cost to condition, as found in our study.…”

“…Relating either CORT or T negatively to condition could suggest an indirect cost of maintaining high levels of these hormones. It is also possible that chronic effects of either hormone on body condition will eventually have indirect, negative effects on immunocompetence (Acquarone et al, 2002;Fargallo et al, 2002;Lifjeld et al, 2002).…”

Unpredictable food availability induces metabolic and hormonal changes independent of food intake in a sedentary songbird

“…Although this hypothesis describes an adaptive process, other studies have demonstrated that variable access to food can decrease body mass (Acquarone et al, 2002;Cucco et al, 2002) and growth rate (Boon et al, 1999), and lower plasma testosterone (Bridge et al, 2009). These observations suggest that energy is being mobilized and contradict the adaptive regulation hypothesis.…”

“…In birds and mammals, temporal variability in food availability is associated with fattening (Bednekoff and Krebs, 1995), an increase in daily torpor (Munn et al, 2010), increased or stabilized body mass (Witter et al, 1995; Cuthill et al, 2000) and less behavioral activity (Dall and Witter, 1998). These observations are consistent with the adaptive regulation hypothesis (Witter et al, 1995;Fauchald et al, 2004), which describes how animals coping with unpredictable food availability aim to decrease energetic expenditure and attenuate physiology that depletes energy reserves such as CORT secretion, thus promoting energy conservation, which conserves (or even increases) body mass.Although this hypothesis describes an adaptive process, other studies have demonstrated that variable access to food can decrease body mass (Acquarone et al, 2002;Cucco et al, 2002) and growth rate (Boon et al, 1999), and lower plasma testosterone (Bridge et al, 2009). These observations suggest that energy is being mobilized and contradict the adaptive regulation hypothesis.…”

“…These above studies differed with respect to the methodology used to control food access. They involved manipulations of: (1) the time available to forage (Witter et al, 1995; Boon et al, 1999); (2) the timing of food presentation (Boon et al, 1999); (3) the amount of food provided, but not necessarily consumed (Acquarone et al, 2002;Cucco et al, 2002;Munn et al, 2010); (4) the rate of food consumption, but not the absolute amount consumed (Bridge et al, 2009); or (5) the interruptions to foraging (Dall and Witter, 1998; Bauer et al, 2011). In addition, many of these studies used control groups with access to ad libitum food as the basis for comparisons, but did not determine the actual amount of food consumed.…”