# Why are butterflies drinking blood?

According to this video, butterflies sometimes drink blood. Do they have the organs necessary to digest blood? I thought they eat only salts and nectar, which are easy to digest compared to proteins.

@aandreev

100mg/dL is the human blood sugar content (I assume it is similar in other animals). Afaik. sugar energy density is about 4kcal/g. A butterfly is about 1g of weight, which means 1mL if we count with water. So it can drink around 0.5mL of blood. That means 0.5mL×100mg/dL×4kcal/g = 0.002kcal = 8.4J. The top butterfly speed is about 12 mph, so I think we can count with 6mph by an average butterfly, which is about 2.7m/s. So it needs 1/2×1g×(2.7m/s)2 = 0.0036J kinetic energy to move and 1g×9.81m/s2×1m = 0.0098J potential energy to keep the 1m height of flight. I think we can guess that it loses this kind of energy in 2secs, which means it needs 0.0134J/2s = 0.0067W power to fly. So the 0.5mL blood means 8.4J/0.0067W = 24925s = 21min of flight time, which is not much compared to nectar (30g/dL$,$2500J$,$100h). Ofc. this is just a very rough estimation (the real flight time is probably much lower) based on data coming from non-scientific sources, but I think you can see the difference.

• For salt content I think. Not sure enough to answer but blood is full of nice things besides calories. – Resonating Mar 15 '15 at 4:37
• calories in blood are glucose – aaaaa says reinstate Monica Mar 15 '15 at 7:54
• @aandreev I edited the question. – inf3rno Mar 15 '15 at 11:32
• what about salts though? – aaaaa says reinstate Monica Mar 15 '15 at 13:22
• @aandreev Find evidence and send an answer. This is how science works. :-) scholar.google.com – inf3rno Mar 15 '15 at 20:24

## 1 Answer

I didn't watch the video so I don't know if this was mentioned. Butterflies will exhibit a behavior called mud puddling which occurs mainly with male butterflies [1]. Puddling is when butterflies seek out moist surfaces in order to consume the fluid. Butterflies will puddle in soil, dung, carrion (rotting flesh/meat), etc. Boggs and Jackson claim that puddling falls into two categories which aren't mutually exclusive [1]:

1. scarce nutrients for example sodium
2. competitive exclusion of males or young individuals from flowers

But we can see blood isn't the only option for puddling. Basically anything that provides moisture and nutrients will do.

• This competition for flowers is very new to me, I did not know that butterflies can fight each other. :-) – inf3rno Mar 18 '15 at 6:57
• @in3rno I choose you Butterfree! Use Sleep Powder on the other Butterfree! Seriously though, I would assume they use the same tactics as birds...minus the beak stabbing, and vocalizations? – SmugDoodleBug Apr 12 '15 at 1:34