Enhanced male coloration after immune challenge increases reproductive potential.

In many animal species, females select a mate on the basis of the expression of secondary sexual traits. A prevalent theory suggests that male ornaments are reliable indicators of immunocompetence, because the cost of immune function prevents cheating. However, sexual signalling is a component of male reproductive effort, and an immune challenge may also alter his perceived future prospects and hence signalling effort. In this study, blue-footed booby males (Sula nebouxii) were inoculated with a diphtheria-tetanus vaccine during courtship to investigate the consequences of mounting an immune response on signalling effort. We found that, after this immune challenge, on average, males increased their signalling effort but lost more body mass compared with control males. Importantly, vaccination affected the partner's reproductive decisions: compared with control females, females paired with vaccinated males laid eggs earlier and increased clutch volume in pairs that laid early. Overall, our results suggest that blue-footed booby males invest more in sexual signals when future breeding opportunities are at risk, eliciting a greater reproductive investment by their partners. Increased signalling effort by infected individuals may contrast the idea of sexual ornaments as signals of infection status.

Vitamins, stress and growth: the availability of antioxidants in early life influences the expression of cryptic genetic variation.

Environmental inputs during early development can shape the expression of phenotypes, which has long-lasting consequences in physiology and life history of an organism. Here, we study whether experimentally manipulated availability of dietary antioxidants, vitamins C and E, influences the expression of genetic variance for antioxidant defence, endocrine signal and body mass in yellow-legged gull chicks using quantitative genetic models based on full siblings. Our experimental study in a natural population reveals that the expression of genetic variance in total antioxidant capacity in plasma increased in chicks supplemented with vitamins C and E despite the negligible effects on the average phenotype. This suggests that individuals differ in their ability to capture and transport dietary antioxidants or to respond to these extra resources, and importantly, this ability has a genetic basis. Corticosterone level in plasma and body mass were negatively correlated at the phenotypic level. Significant genetic variance of corticosterone level appeared only in control chicks nonsupplemented with vitamins, suggesting that the genetic variation of endocrine system, which transmits environmental cues to adaptively control chick development, appeared in stressful conditions (i.e. poor antioxidant availability). Therefore, environmental inputs may shape evolutionary trajectories of antioxidant capacity and endocrine system by affecting the expression of cryptic genetic variation.

Senescent males carry premutagenic lesions in sperm.

As organisms age, DNA of somatic cells deteriorates, but it is believed that germ cells are protected from DNA-damaging agents. In recent years, this vision has been challenged by studies on humans indicating that genomic instability in germ cells increases with age. However, nothing is known about germ line senescence in wild animals. Here, we examine DNA damage in sperm of a wild vertebrate, the blue-footed booby Sula nebouxii. One of the major types of premutagenic DNA damage generated by oxidative stress (a proximal cause of ageing) is loss of single bases resulting in apurinic/apyrimidinic sites (AP sites). We examined AP sites in the sperm of known-age males sampled during courtship on Isla Isabel, Mexico. We show that damage to the DNA of sperm increases with age of male blue-footed boobies. Moreover, we found that sexual attractiveness (foot colour) declines with age and is correlated with germ line damage of senescent males. By choosing attractive males, females might reduce the probability of their progeny bearing damaged DNA. This study reports the first evidence of senescence in the germ line of a wild vertebrate and future studies should investigate whether this burden of senescence is sidestepped by potential sexual partners.

Evolvability of an avian life history trait declines with father's age.

Studies of laboratory organisms have suggested that parental age affects the genetic variance of offspring traits. This effect can engender age-specific variance in genetic contributions to evolutionary change in heritable traits under directional selection, particularly in age-structured populations. Using long-term population data of the blue-footed booby (Sula nebouxii), we tested whether genetic variance of recruiting age varies with parental age. Using robust quantitative genetic models fitted to pedigree, we found a significant genotype-by-paternal age interaction for recruiting age. Genetic potential for adaptive change in recruiting age was greater in progeny of young (age 1-6 years) fathers (males: CV(A)=6.68; females: CV(A)=7.59) than those of middle age (7-9 years) fathers (males: CV(A) = 4.64; females: CV(A)=5.08) and old (10-14 years) fathers (CV(A)=0 for both sexes). Therefore, parental age dependence of heritable variance, in addition to age-related variation in survival and fecundity, should affect the strength of natural selection for evolutionary changes. Our results provide rare evidence for the influence of parental age on the evolutionary potential of a life history trait in a wild population.

Heritability of resistance to oxidative stress in early life.

Oxidative stress has recently been suggested to play an important role in life-history evolution, but little is known about natural variation and heritability of this physiological trait. Here, we explore phenotypic variation in resistance to oxidative stress of cross-fostered yellow-legged gull (Larus cachinnans) chicks. Resistance to oxidative stress was not related to plasma antioxidants at hatching, which are mostly derived from maternal investment into eggs. Common environmental effects on phenotypic variation in resistance to oxidative stress were not significant. Heritability was relatively low and nonsignificant in hatchlings, but interestingly, the chicks of age 8 days showed high and significant heritability (h(2) = 0.59). Our results suggest that resistance to oxidative stress is determined mainly by the genotype as chicks grow. Further work is required to explore the genetic role of oxidative stress in life-history evolution.

The importance of hatching date for dominance in young shags.

In birds, conditions and experience in the nest may influence the social rank of juveniles after they become independent. Three main factors may be involved: (1) mass at fledging, larger birds being better able to compete; (2) brood hierarchy, dominance in the brood continuing after the birds leave the nest; and (3) hatching date, birds hatching earlier having more experience and a greater ability to compete. I studied the effect of these factors on the agonistic behaviour of juvenile European shags, Phalacrocorax aristotelis, in crèches. Of all the factors, only hatching date had a significant influence on the dominance rank of juveniles. Birds that hatched earlier had a higher rank. Additionally, males were somewhat more aggressive than females. The importance of an individual's hatching date for its social rank may have consequences for its future reproductive value. Copyright 2000 The Association for the Study of Animal Behaviour.