American Society of Naturalists

“Age-Specific Phenology and Reproductive Success Senescence Vary with Environmental Age in a Wild Bird”

Suzanne Bonamour, Luis-Miguel Chevin, Christophe de Franceschi, Anne Charmantier, and Céline Teplitsky: Read the article

Investigations of aging mechanisms have been doing the rounds for a long time in the research community. However, the more important question to ask is – what factors promote this almost universal phenomenon in living organisms? Aging or senescence involves a reduction in the probability of survival over time and/or a decrease in reproductive performance with increasing age. Variability in senescence between populations, species, genders, and individuals could be attributed to genetic and/or epigenetic factors. Environmental conditions – favorable and unfavorable, can change the physiology of the organism directly, leading to senescence, or indirectly through the organism’s life history trade-offs. For instance, exposure to favorable environments early in life could reduce the decline in reproductive success with increasing age, or breeding could occur at a faster pace leading to a faster senescence. Not much is known about how environmental conditions affect senescence in populations of wild organisms, especially among individuals.

In a recent paper titled “Age-specific phenology and reproductive success senescence vary with environmental age in a wild bird,” researchers from the Norwegian University of Science and Technology (NTNU), Norway and the Centre d’Ecologie Fonctionnelle et Evolutive, France, have used two proxies, namely relative adult mortality based on annual population-level survival of the organisms and relative breeding failure based on annual reproductive success, to measure ‘environmental age’ – that is older for individuals facing harsher climatic conditions when compared to younger individuals that are exposed to favorable climatic conditions. These two proxies therefore precisely quantify the cumulative effects of past environment experienced by an individual organism. They choose the short-lived wild blue tit passerine population in Corsica to analyze the age-specific patterns of egg-laying date and annual reproductive success. They hypothesized that the environmentally young female tits might acquire more resources to display better reproductive performance (after the onset of senescence) leading to a slower rate of senescence – a theory that is in line with the ‘silver spoon hypothesis’. Alternatively, they might allocate more resources to maximize reproductive performance at the expense of a faster senescence.

As expected, egg-laying date and annual reproductive success increased until about 3 years of age, then consistently declined with further aging. Interestingly, wild female blue tits that had experienced favorable past environmental conditions laid eggs earlier and senesced faster. In contrast, female birds that lived in more favorable environments showed higher breeding success, suggestive of a golden life effect, wherein the silver spoon effect is observed through its lifetime, resulting in birds surviving well even when breeding late.

The authors suggest that more information is needed to clearly understand how direct and indirect environmental effects shape senescence across life history traits in wild organisms. The average onset of senescence could be influenced by environmental age; hence, it would be important to also consider the among individual variation in the onset of senescence in the blue tits. Finally, the authors have raised a quite pertinent question – is the variation observed in senescence rate and onset timing between different environmental conditions adaptive? Answering this query would entail long-term studies via time-series data analysis in iteroparous organisms in natural habitats with rapidly fluctuating environments. Overall, this research points to the fact that chronological age and real age of the organism may not correlate with one another, owing to the organism’s real age being affected by several epigenetic factors.

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Shatarupa Sarkar is working towards her Ph.D. in Forest Molecular Entomology under the supervision of Dr. Amit Roy at the Department of Forest Genetics and Physiology, Czech University of Life Sciences, Prague, Europe. Her work focuses on forest protection against wood-feeding insects using molecular tools. In her spare time, Shatarupa enjoys hiking and cooking, and is an avid reader of science fictions and popular science books.