Phenotypic senescence in a natural insect population

Abstract

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. We aimed (i) to 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 (ii) to assess the relationship of this change with sex and wing length. We studied a population between 2014 and 2020 using mark‐recapture during the whole flight period each year. Repeated measurements of body mass and thorax width and single measurements of wing length were performed on marked individuals. We analyzed body mass and thorax width change with age (days since marking), wing length, and the date of the first capture. Both body mass and thorax width declined nonlinearly with age. Individuals appearing earlier in the flight period had significantly higher initial body mass and thorax width and their body mass declined faster than later ones. Initial body sizes of females were higher, but males' body sizes decreased slower. Initial thorax width showed higher annual variation than body mass. To our best knowledge, this is the first study that revealed phenotypic senescence in a natural butterfly population, using in vivo measurements. We found sexual differences in the rate of phenotypic senescence. Despite the annual variation of initial body sizes, the rate of senescence did not vary considerably across the years.

Mate‐guarding success depends on male investment in a butterfly

“…CAP‐type and size) takes place on a more or less continuous scale of investment (Gór et al, 2023 ). The resources males can actually allocate in mate‐guarding may heavily impact success, and this would, in turn, depend on actual male quality, such as body size (Schöfl & Taborsky, 2002 ), age (Pásztor et al, 2022 ), as well as body reserves (Stjernholm & Karlsson, 2000 ). Male investment is supposed to be constrained through production capacity.…”

“…These imply strongly male-biased populations at the beginning, less male-biased or even female-biased at the end of the flight period, as were also found in another Clouded Apollo population . This, together with both sexes living in the flight period later being smaller than those living earlier (Pásztor et al, 2022), thus probably having less reserves, imply that female matelessness might occur in this population at the very end of the flight period.…”

“…Apollo thorax width declined with age in both sexes, and individuals emerging late in the flight period tended to be smaller (Pásztor et al, 2022). Both ageing and small body size are related to less reserves and probably incur low mobility.…”

“…In Clouded Apollos, thorax width shrinks over an individual's life with declining body mass (i.e. phenotypic senescence, Pásztor et al, 2022 ), thus both are related to the actual quality of an individual.…”

“…Other dimensions, such as shield height and width (Gór et al, 2023 ), body mass, wing length (Pásztor et al, 2022 ) and proboscis length (Szigeti et al, 2020 ) were also measured (see detailed methodology therein), however, these were omitted from the results due to their non‐significant effect. All measurements in the field were carried out by JK.…”