According to the "winter immunoenhancement hypothesis," some species of mammals exhibit an increase in immune function prior to winter, resulting in improved defense from pathogens during harsh winter conditions.  It has also been suggested that the enhancement of immune function is possible, in part, because of a simultaneous reduction in reproductive activity.  As a result, energy and nutrients can be re-allocated to support immune functions.  The occurrence of energy re-allocation would indicate that trade-offs exist between energy use for immune and reproductive functions.  We demonstrated previously that a mild immune challenge was associated with a decrease in testes mass in white-footed mice.  To further investigate the ability of animals to accommodate the energetic costs of immune and reproductive functions, we examined the effects of testosterone and immunochallenges on energy budgets, again using white-footed mice.   Understanding of the trade-offs that may exist between these two systems is important to understanding the potential impacts of the increasing incidence of infectious diseases and parasites in wild animal and human populations worldwide.
 
 

We used a two-by-two design with testosterone level (T) and immunochallenge (I) as the two treatment variables.  Corresponding controls (C) were established.  The result was four groups of adult male white-footed mice.

To determine patterns of energy allocation in each group of mice, we measured resting and daily metabolic rates before and after injections of sheep red blood cells (SRBC) and phytohemagglutinen (PHA).   SRBC challenged the humoral component of the immune system and PHA challenged the cell-mediated component.  Control animals were injected with saline.  The testosterone-treated animals were injected with testosterone propionate daily and the controls were injected with saline.  At the end of the experiment the animals were dissected.

 

Resting metabolic rate (RMR) was significantly greater by the end of the experiment in animals that were immunochallenged (IC) compared with the control group (CC).  Elevated testosterone was associated with suppression of the increase in resting metabolic rate (IT).  Daily metabolic rate (DMR) did not increase significantly during the immunochallenges.

Surprisingly, the reproductive organs (seminal vesicles and testes) were significantly larger in males that were immunochallenged (IC, IT) compared with the control males (CC, CT).  These data indicated two important results. First, the animals were capable of mounting an immune response and increasing energy allocation to reproductive organs.  Second, the increase in resting metabolic rate (graph above) was attributable to the increase in immune function rather than the increase in reproductive organ masses.