Vitamin C is a dietary need for only certain types of birds - with implications for seed-dispersal mutualisms

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Please see http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3145266/figure/F6/ and https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3145266/.

The above gives preliminary findings on which birds have lost the ability to make ascorbic acid (https://en.wikipedia.org/wiki/Vitamin_C) in their own cells.

For background information, see:

https://pubmed.ncbi.nlm.nih.gov/5777214/
https://www.drlamcoaching.com/blog/passerines-vitamin-c/
https://www.jstor.org/stable/4089257
https://www.proquest.com/openview/834c7c66b0ca06f09a639b030f585cc3/1?pq-origsite=gscholar&cbl=42159
https://veteriankey.com/avian-nutrition/
https://www.nature.com/scitable/topicpage/the-mystery-of-vitamin-c-14167861/

The birds, shown in grey instead of black in http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3145266/figure/F6/, are those for which ascorbic acid qualifies a vitamin, somewhat as in humans.

They have lost the ability to synthesise this substance in their own cells. This means that ascorbic acid is a vitamin (=vitamin C) for them, based on the definition of vitamins as essential substances in the diet.

The birds shown in black, in the same figure, retain the ability to make their own ascorbic acid, regardless of their diet. This means that, for them, ascorbic acid does not qualify as a vitamin.

Now, please consider the likelihood that plants offer ascorbic acid to frugivores (https://en.wikipedia.org/wiki/Frugivore) and other fruit-eating animals, as an incentive/reward for the dispersal and sowing of seeds.

For example, ascorbic acid qualifies as vitamin C for all anthropoid primates (https://en.wikipedia.org/wiki/Simian).

Examples of genera of domestic fruits, the wild ancestors of which were intensely sour (https://www.bbvaopenmind.com/en/science/bioscience/the-genetic-mystery-of-citrus-fruits-how-did-we-manage-to-domesticate-them/), include Citrus (https://en.wikipedia.org/wiki/Citrus).

Citrus offers ascorbic acid. Therefore, one could hypothesise that its fruit-pulp and -juice originally provided animals with vitamin C (rather than sugar or any nutrient), as the main reward for dispersal and sowing of seeds.
 
We humans tend to think of fleshy fruits as sweet rather than sour (the various popular uses of lemon notwithstanding). So it is worthwhile to consider that various wild forms of fleshy fruits are sour, not becoming sweet even when fully ripe (e.g. Tamarindus indica, https://en.wikipedia.org/wiki/Tamarind, and Owenia acidula, https://en.wikipedia.org/wiki/Owenia_acidula#:~:text=Owenia%20acidula%2C%20commonly%20known%20as,grow%20to%20ten%20metres%20tall.).

So, how does this idea apply to those birds that eat fleshy fruits, and potentially disperse and sow the seeds?
 
Unfortunately, the coverage of bird taxa is far from complete.

However, it does show the following.
 
Examples of a birds able to synthesise its own ascorbic acid, and which therefore do not depend on vitamin C, include mynahs (Acridotheres, https://www.inaturalist.org/observations?place_id=any&taxon_id=14868&view=species). Does this apply to starlings (Sturnidae) in general?
 
Examples of birds for which ascorbic acid is tantamount to vitamin C, as in humans, are:

• bulbuls (Pycnonotus, https://www.inaturalist.org/observations?place_id=any&taxon_id=14587&view=species),
• orioles (Oriolus, https://www.inaturalist.org/observations?place_id=any&taxon_id=7863&view=species), and
• typical flowerpeckers (Dicaeum, https://www.inaturalist.org/observations?place_id=any&taxon_id=13375&view=species).

Readers, next time you see a bulbul or a starling, consider this:
The chances are that the bulbul is like humans in treating ascorbic acid as vitamin C. Meanwhile, the starling probably makes ascorbic acid in its body - and so for it there is no such thing as vitamin C.

This adds a dimension to our understanding of what it means to be a bulbul:
when the bird eats a fleshy fruit, and discards the seeds intact, part of its reward for this service to the plant is likely to be vitamin C. This reward may be inapplicable/irrelevant for the starling.

The implication is as follows:
If there is mutualism (https://en.wikipedia.org/wiki/Mutualism_(biology)) in the plant-starling relationship, then this is likely to be based on some other resource, usually sugar but in a few cases lipid, in the fruit-pulp (or aril, https://en.wikipedia.org/wiki/Aril) concerned.

So, is it generally true that the fruits preferred by starlings are sweeter (or oilier) than those forming the staple diets of bulbuls (e.g. https://www.researchgate.net/publication/287618831_Diet_of_the_Common_Bulbul_Pycnonotus_barbatus_in_the_Algiers_Sahel_Algeria and https://www.researchgate.net/publication/308929986_Food_and_Feeding_Ecology_of_Common_Bulbul_Pycnonotus_barbatus_in_Leventis_Foundation_Agricultural_School_Ilesa_South_Western_Nigeria and https://www.tandfonline.com/doi/abs/10.2989/OSTRICH.2008.79.1.9.365?journalCode=tost20 and https://www.researchgate.net/publication/352119651_Food_and_Feeding_Behavior_of_Red_Vented_Bulbul_Pycnonotus_cafer_and_Role_in_Seed_Dispersal_at_Urban_Area_Udaipur_Rajasthan_India)?

Furthermore, in the human species there seems to be a physiological antagonism between vitamin C and sugar (https://pubmed.ncbi.nlm.nih.gov/9550452/ and https://pubmed.ncbi.nlm.nih.gov/16118484/).

This implies that, the sweeter the fruits, the less effective they are in supplying vitamin C. This is because the nutritional value of a given dose of ascorbic acid will tend to be reduced by the accompanying sugar, via the effects of the latter on insulin and other hormones.

This, in turn, leads to the following predictive framework:
I hypothesise that the fruits preferred by - or seasonally important for - bulbuls will prove to be mainly acidic instead of sweet, whereas those taken as staples by starlings will prove to be sweet instead of acidic.

Posted on July 23, 2022 02:17 AM by milewski