Biological ageing, i.e. the loss of physiological functioning leading to increased frailty and mortality risk with advancing age, is influenced both by genotype and environmental factors as well as their interactions. The leading evolutionary hypothesis is that the resource allocation from self-maintenance and repair to growth and reproduction is the cause for this detrimental process. In this thesis I studied redox metabolism and telomere attrition, which have been suggested to be molecular mechanisms of biological ageing across species. Redox status is a dynamic state composed of reactive species (RS) induced oxidative damage and antioxidant (AO) defense. In healthy tissues most of the RS are produced by normal metabolism, however, several stressors can enhance RS production. Telomeres are protective DNA sequences at the end of chromosomes, which shorten with each cell division and advancing age. Telomeres are vulnerable to RS and short telomere length (TL) is linked with increased mortality and age-related disease risk. 

In the first two studies, the main interest was to elucidate the impacts of different stressors on TL and redox status in free-living vertebrates. In the first study, the effects of heavy metal pollution on TL and on redox status were investigated in adult and nestling great tits and pied flycatchers. The results varied remarkably among the species and age groups. Both of these passerine species showed pollution induced redox status alterations, but great tits, especially at an early age were more vulnerable to pollution induced oxidative stress and telomere attrition. In addition to pollution, within-brood competition induced growth stress was also found to cause TL attrition and alterations in AO-defense in great tit nestlings. In the second study, the potential roles of TL and AO defense in pathogen resistance and disease severity were investigated in Tetracapsuloides bryosalmonae-parasite infected brown trout. Parasite load was not directly associated with either AOs or with TL, but some AO-activities were lower in fish with severe disease symptoms than in fish with modest symptoms. It seems that TL may reflect individuals’ ability to tolerate infection, as the fish which were less sensitive to parasite-induced impaired growth had longer TLs and lower levels of certain AOs than the more sensitive fish. 

In the last two studies, experimentally produced selection lines of wild house mice with divergent TLs were used to obtain better understanding of the relationship between redox status and TLs in constant laboratory conditions. In the third study, the age and sex-depended alterations in AOs in relation to TL were examined with a longitudinal approach. Most of the measured AOs increased from the age of 8 weeks to 6 months, and females had, in general, higher AO-levels than males. TL was also found to correlate positively with some of the AOs. In addition, there was a delayed negative effect of sibling competition on females’ AO defense in later life. In the last study, the differences in metabolism and redox status were examined in relation to experimentally selected divergent TLs. Half of the experimental mice were exposed to short-term exercise in order to magnify the possible among TL selection group differences. Males in the long TL group had considerably lowered AO-defense although their cell respiration was relatively high and comparable to control TL males. Short TL group males showed drastic reduction in cell respiration, and yet high AO defense activity. 

Overall, the results indicate that a variety of environmental stressors can accelerate biological ageing via oxidative stress and telomere attrition. Furthermore, unfavorable early life experiences, like growth stress can have longlasting negative consequences for individuals’ future fitness and survival. Taken together the results highlight that changes in redox status and telomeres are highly dependent on the species, sex, the phase of life and the experienced stress. The very same factors determine which parts of the AO defense system are used for self-maintenance.