Anyways, here is a neat study about something I've long been curious about: how pollen-nonfeeding close relatives of Heliconius die of old age. According to previous papers Heliconius's close relative the julia (Dryas iulia) gradually suffers malnutrition as a result of the nectar-based* diet being poor in certain nutrients, and soon dies from this no matter how well fed it is; Heliconius reportedly avoids this fate by having evolved pollen feeding and is thus relatively longlived (a number of other longlived-adult lep taxa have been known to achieve their lifespans via adult consumption of fermenting fruit, which contains important microbial nutrients and presumably functions as an analogue of pollen feeding. Note that Danaus plexippus isn't exactly a fruitrot drinker and doesn't consume pollen either; I assume it achieves long adult life via some mechanism unrelated to adult diet).
*Julia is known to drink from puddles and teardrops in the wild. Tears of some if not all animals are protein-rich. I'm still under the overall impression that the diet seems to be mostly nectar tho. Also, some nectars have been reported richer in amino acids than previously believed, although whether amino acids derived solely from nectaring or teardrinking can be nutritionally complete is not known to me.
Long story short, this study disputes that classic story to some extent; some notable bits I found interesting:
- Dryas iulia fed a sugar solution with pollen in it exhibited a normal adult lifespan, but there were subtle behavioral changes
- D. iulia adult exhibits signs of DNA damage and weakened antioxidant activity with age, in other words its short adult lifespan appears to be in part or in whole due to "classical aging" as opposed to malnutrition death
- It was offhandedly mentioned that butterflies (paper was unclear about whether this happened to Dryas, Heliconius, or both) "were spotted flying vigorously around the cages just hours before being found dead"; this is in contrast to many insects, which tend to become listless and/or flightless a day or three before senescence-related death
But a different study I'm too tired to link said somewhere that putting pollen into sugar solution isn't "enough" or something like that (despite claims to the contrary), and that because of this Heliconius has proteases in its saliva. And the study I did link to didn't give the pollen in the sugar solution any fancy special treatment (nor did they test how much of the pollen-derived nutrients were actually being absorbed by the adult julias), so it's possible that D. iulia may have lacked the digestive enzymes to absorb most of what was in the pollen (although the subtle behavioral changes suggest that the D. iulia absorbed at least some pollen substance).
With that being said, though, given that D. iulia exhibits physiological signs consistent with "classical aging", even if some researcher made some magic pollen formula that was concretely proven to be absorbable by julias and other pollen nonfeeding butterflies I'm pretty sure they'd not live much longer than they normally do. Note that I am not implying nectar-only diet is nutritionally complete, only that it's hypothetically possible old age kills them before malnutrition. Adults of various butterflies (and of various nectar-feeding parasitoid wasps) that are outside the scope of this article have been documented to continuously lose weight over their adult lives by the way (if my memory is trustable, this is not just oviposition making them lighter) so in any case their body condition definitely deteriorates over time.
Somewhat relatedly, the jawed moth Micropterix calthella appears naturally shortlived despite being able to consume pollen (paywalled paper; relevant text reproduced below).
