How many calories does a spider contain?

Dietary needs of giant spiders

Spiders don't have to eat that much. In terms of food needs, giant spiders would be pretty supportable in most environments.

According to Kleiber's law:

For the vast majority of animals, the metabolic rate of an animal is scaled to the ¾ strength of the animal's mass. Symbolically, if q0 is the animal's metabolic rate and M is the animal's mass, then Kleiber's law says that q0 ~ M ^ ¾. Thus, a cat with a mass 100 times that of a mouse has a metabolism about 32 times that of a mouse.

The reason for this is that an animal is smaller and the proportion of its body mass that is made up of structure rather than reserve is greater. The structural mass causes maintenance costs, the reserve mass does not. Ergo small animals breathe faster and need more calories per mass than larger ones.

Assuming that Kleiber's law applies to these giant spiders, we can calculate their metabolic needs by comparing them to a "real" spider with comparable levels of activity plus calorie needs for silk production.

A Goliath Bid-Eater , the largest tarantula species by mass, weighs around 70-80 g on average (although it can grow up to 170 g). According to spider care websites [2] is enough a diet of 6-8 crickets a week for "the larger species of tarantulas". That's only about 10 calories a week!

By this calculation:

70 g spider => 10 cal / week
∴ 70 kg spider => 10 * (1000 ^ ¾) Cal / week ~ 1780 Cal / week.

Let us take this number as "calorie requirement at rest" since a captured tarantula would have limited activity compared to a "wild spider" which would have to actively search for prey.

For spiders the production of silk is a significant source of metabolic costs.[1] . The more web / silk a giant spider produces, the more calories it will need. Spider silk is light - enough spider silk to go around the world would weigh only 500g, but let's say these giant spiders spin proportionally thicker silk so that the same length of silk weighs 100 times more.

Caloric Value of Silk ~ 4500 Cal / g

A spider weighing 70 kg and producing only 1 g of silk per day would require an additional calorie intake of 4500 cal need per day.

Because real spiders eat enough to leave them behind for days:

Accepted, 70 kg of spiders to produce 1 g of silk per day
And 100 kg deer that to 80% at 1.5 cal / g with a total of 120K Cal can be consumed
=> One (1) deer would four (4) spidersfor seven (7) days feed

ETA: Even with a thread 10 times thicker (10 micrometers => 100 micrometers) and 100 times heavier than normal spider silk, 1 g of spider silk would be more than 166 m (1.31 g / cm³ / (0.005 cm * 0.005) cm * pi) ), so it is highly unlikely that even a giant spider would produce 1g of silk per day. [Thanks to @Sempie for bringing this to my attention].

More realistically, if silk production is not significant enough (in terms of width / density / length) to weigh a lot, it would have an insignificant caloric cost.

In addition, spiders often recycle their web silk. If these giant spiders didn't expand their territory, they could eat old silk to mend their webs, which could further eliminate the external caloric needs (excluding silk).

Therefore:

Accepted, 70 kg spiders with negligible silk production
And 100 kg deer that to 80% at 1.5 cal / g with a total of 120K Cal can be consumed
=> One (1) deer would thirty-three (33) spidersfor fourteen (14) days feed

The paper linked above also provides an equation for the activity cost of web building.Total costs for building the web ~ (4.5 * weight of the web in mg) + [weight of the spider in grams * (2.79 * weight of the web in mg)] calories
I assume these activity costs are governed by Kleiber's law, but if so this is not the case , the nutritional needs would be:
=> (4,5 * 1.000) + [70.000 * (2,79 * 1.000)] = 4500 + [70.000 * 2790]
= full and unsustainable 195,304,500 calories!(For comparison, an adult African elephant needs about 70,000 Cal per day)

Danny Reagan

Interesting! I never noticed that spinning on the internet is such a strenuous activity for spiders. +1 for you

Sempie

The silk is an interesting point, but hunting spiders end up eating more often than the spiders that build the web. A normal spider's web lasts for days or weeks. The web of such a giant spider (70 kg) would be so strong that almost nothing can destroy it. So you probably don't need to fix it very often.

Shisa

@ Tempie That's right! But I also pondered the typical fictional narrative that these spiders' web would be much thicker than the normal web. Even if they didn't have to repair their webs very often, even a little fixation could bring them to 1 g in weight (spider silk density = 1.31 g / cm³).

Shisa

@Sempie Nevermind, thank you for bringing my attention to the actual silk calculations :) You're right, it wouldn't be necessary at all under general circumstances. Corrected my answer to reflect this.