Predator threat, copulation effort and immunity in male rats (Rattus norvegicus)

Parental investment theory predicts that organisms should increase their current reproductive investments when they perceive a threat to future reproduction. In such a situation, animals are expected to pay the cost of reproduction by committing a terminal reproductive effort (TRE). We aimed to induce a TRE in male Wistar rats by exposing experimental animals to predator (cat) odour, while the control group was exposed to the odour of rabbit. As response variables, we measured the parameters of male copulatory activity (copulation effort) and resistance to the experimental infection with a neurotropic nematode, Toxocara canis. Contrary to our expectations, control animals received a greater average number of ejaculations in ad libitum copulation trials than predator-exposed animals. Other parameters of copulatory activity (intromission and ejaculation latencies, numbers of mounts and intromissions, ejaculation efficiency, duration of post-ejaculatory interval) did not differ between treatment groups. Exposure to predator odour resulted in growth retardation and immune suppression as revealed by the elevated urinary corticosterone levels and higher parasite counts in brain and muscle tissues as compared with control animals. We failed to detect any immunological cost of increased copulatory activity as no significant correlations between measures of copulatory activity and immune parameters could be found. Individual mount number correlated positively with urine creatinine concentration (an index of muscular activity), suggesting that this aspect of male copulatory behaviour is energetically most demanding. Our experiment thus failed to support the hypotheses that exposure to predator odour increases copulatory activity in male rats and that intense copulatory activity leads to immune suppression. The hypothesis that predator odour exposure leads to immune suppression and reduced infection resistance was supported.