Phenotypic senescence and multilevel variation in different body sizes
in a natural insect population
Abstract
1. Senescence seems to be universal in living organisms and plays a
major role in life-history strategies. Phenotypic senescence, the
decline of body condition and/or performance with age, is a largely
understudied component of senescence in natural insect populations,
although it would be important to understand how and why insects age
under natural conditions. 2. We aimed to a) investigate how body mass
and thorax width change with age in a natural population of the
univoltine Clouded Apollo butterfly (Parnassius mnemosyne,
Lepidoptera: Papilionidae) and b) to assess the distribution of body
mass, thorax width, wing and proboscis length in the population
within/between flight periods. 3. We studied a population 2014–2020
using mark-recapture during the whole flight period each year. Repeated
measurements on body mass and thorax width and single measurements on
wing and proboscis length were performed on marked individuals. We
analysed body mass and thorax width change with age (days since marking)
and initial body mass, thorax width, wing and proboscis length with time
since the flight period’s first day. 4. Both body mass and thorax width
declined significantly with age in all years. Individuals appearing
earlier in the season had significantly higher initial body mass, thorax
width, wing and proboscis length than those appeared later in several
years. Initial body mass varied little among years, while initial thorax
width showed high annual variation. Moderate annual variation and no
sexual difference were found in wing and proboscis length. 5. This is
the first study that revealed phenotypic senescence in a natural
butterfly population, using in vivo measurements. Despite their
different initial body sizes, the rate of senescence was similar in
males and females, indicating general life-history constraints. Annual
variation in body sizes and rate of senescence highlights the important
role of environmental factors, which should be further investigated.