Calcium metabolism in small mammals and exotics

Question 1

How do birds and reptiles absorb Ca?

Answer 1

• If you ↑ calcium in the diet it might not ↑ blood calcium levels.
• Vitamin D is needed to absorbed dietary calcium from the intestines.
• Vit D2 is found in plants, unlike mammals, birds and reptiles cannot easily use Vitamin D2. They need to use active vitamin D3.
• UV-B and heat are needed for some species to convert D2 into D3.

Question 2

When are dietary Ca supplements needed in reptiles?

Answer 2

Remember Ca:P ratio in the diet should be between 1:1 and 2:1.
Herbivorous reptiles: Always. Diets in captivity often have an inverse Ca:P ratio and/or contain phytates and oxalates which may further reduce calcium availability in the diet.
Insectivorous reptiles: Always. Captive-raised insects often have an inverse Ca:P ratio.
Carnivorous reptiles: Not if they are fed whole prey. If fed all-meat diet or neonatal prey results in inverse Ca:P ratio.

Question 3

When are dietary Ca supplements needed in birds?

Answer 3

Insectivorous birds: Always. Captive-farmed insects have an inverse Ca:P ratio.
Parrots: Not needed if diet is correct. Seed-based diets are deficient in calcium and vitamin D.
Carnivorous birds: Not if feed whole prey. All-meat diet, or over enthusiastic eviscerating will result in inverse Ca:P ratio.

Question 4

How is Ca metabolised in the rabbit?

Answer 4

• Same hormones involved (PTH, calcitonin and vitamin D3).
• Rabbit calcium metabolism differs to most other mammals because they can absorb dietary calcium from the gut using both an active and passive absorption methods:
  
  • Active absorption: Vit D2 to D3 in kidney by PTH if have low serum calcium, this stimulates intestinal absorption.
  • Passive absorption: absorbed across gut wall, vitamin D not required (unlike other mammals). No feedback mechanism.

Question 5

What will happen to rabbits if they have high calcium levels in their diet?

Answer 5

• They will absorb too much calcium
• In rabbits calcium excretion is mostly via kidney (44.9% fractional excretion vs 2% most other mammals). They can excrete up to 60% ingested calcium in their urine.
• So they have good tolerance of high levels of dietary Ca2+ (e.g. Ca:P as high as 12:1)

Question 6

How is calcium absorbed in guinea pigs?

Answer 6

• Like rabbits, guinea pigs do not regulate calcium absorption from the gut.
• Guinea pigs absorb round 50% of the calcium they ingest.
• If fed excess calcium, the excess absorbed calcium is excreted through the urinary tract.