Immunoecology

""The immune system is an energetically expensive self-maintenance complex that, given the risks of parasitism,
cannot be carelessly compromised. Life-history theory posits that trade-offs between fitness components, such as self-maintenance and reproduction, vary between genders and age classes depending on their expected residual lifetime reproductive success, and seasonally as energetic requirements change. Using ruff (Philomachus pugnax), a bird with two genetically distinct male morphs, we demonstrate here a decrease in male immunocompetence during the breeding season, greater variance in immune response among males than females, immunosenescence in both sexes and male morphs, and a seasonal shift in the age range required to detect senescence. Using a phytohaemagglutinin delayed hypersensitivity assay, we assessed cell-mediated immunity (CMI) of males of typical breeding age during the breeding and nonbreeding seasons, and of a larger sample that included females and birds of a greater age range during the non-breeding period. CMI was higher for breeding-aged males in May than in November, but the increase was not related to age or male morph. In November, mean CMI did not differ between the sexes, but the variance was higher for males than for females, and there were no differences in mean or variance between the two male morphs. For both sexes and male morphs, CMI was lower for young birds than for birds of typical breeding ages, and it declined again for older birds. In males, senescence was detected in the non-breeding season only when very old birds were included. These results, generally consistent with expectations from life-history theory, indicate that the immune system can be involved in multifarious trade-offs within a yearly cycle and along an individual’s lifetime, and that specific predictions about means and variances in immune response should be considered in future immunoecological research.""

Alternative reproductive tactics and strategies (ARTS) refer to polymorphic reproductive behaviours in which in addition to the usual two sexes, there are one or more alternative morphs, usually male, that have evolved the ability to circumvent direct intra-sexual competition. Each morph has unique morphological, ecological, developmental, behavioural, life-history, and physiological profiles that shift the balance between reproduction and self-maintenance, one aspect being immunity. Immunoecological work on species with ARTS, which is the topic of this review, is particularly interesting because the alternative morphs make it possible to separate the effects of sex, per se, from other factors that in other species are inextricably linked with sex. We first summarize the evolution, development and maintenance of ARTS. We then review the main immunoecological hypotheses relevant to species with ARTS, dividing them into physiological, life-history, and ecological hypotheses. In context of these hypotheses, we critically review in detail all immunoecological studies we could find on species with ARTS. Several interesting patterns emerge. Generally, greater understanding would occur if hypotheses and predictions were always explicitly articulated. Oddly, there is a paucity of studies on insects, despite the many benefits that arise from working with insects: larger sample sizes, simple immune systems, and countless forms of alternative reproductive tactics and strategies. Of all the hypotheses reviewed, the immunocompetence handicap hypothesis and its derivatives have generated the greatest amount of work, but not necessarily the greatest level of understanding. Integration has been a one-way street, with ecologists delving deeper into physiology, seemingly at the cost of ignoring their organisms' evolutionary history and ecology. One possible useful framework is to divide ecological and evolutionary factors affecting immunity into those that stimulate the immune system, and those that depress it. Finally, the contributions of genomics to ecology are being increasingly recognized, including in species with ARTS, but we must ensure that evolutionary and ecological hypotheses drive the effort. There is no grandeur in the strict reductionist view of life.

""Immunity and resistance to oxidative stress are two  mechanistically related aspects of  self-maintenance that are usually not studied together in connection to ecological or
evolutionary relevant variables. Whereas many studies compare 2 sexes, here we use ruffs (Philomachus pugnaxL., 1758.), a species in which males have 3 alternative reproductive morphs: independents, satellites, and faeders. Previous work suggested that immune function in ruffs depends on energetic constraints or potential of injuries. Based on their behaviour and life history, the 3 male morphs and females can be placed on an ordinal scale with independents at one end and females at the other, and these two explanations predict opposite patterns along this continuum. Innate and cell-mediated immunity decreased along this axis
from independents to females, supporting a risk-of-injury explanation over the energetic constrains hypothesis. No such pattern was evident for oxidative stress or resistance, and no
relationship was detected between immunity and oxidative resistance or stress. Hence, during the breeding season immunity reflected the risk of injury, with faeders located in the immunological continuum between females and the other male morphs. Species with alternative reproductive strategies provide particularly useful systems in which to address the evolution and ecology behind physiological mechanisms.""