Coprophagy: Feces Ingestion in Iguanas
Coprophagy (copro = excrement, dung; phagy = one that eats) in reptiles is not well documented. Neonate prehensile tails skinks (Corusia zebrata), who spend a considerable length of time in association with their mothers after they are born, have been seen eating the mother's feces (Burghardt et al. 1995). Hatchling green iguanas (Iguana iguana) have also been seen ingesting other iguana feces. While it has been shown that green iguana clutch mates recognize each other (Werner et al. 1987), it isn't known if they are eating the feces of their clutch mates, or that of other iguanas. It was previously held that iguanas need to eat the feces in order to establish their gut bacteria, and that they would not be able to eat food until they do so. However, hatchling iguanas, like other oviparous reptiles, continue to live off the remains of their yolk sac for at least a week after they hatch, during which time they will take little, if any, other food. In addition, despite some old books and care sheets out there, green iguana hatchlings are raised successfully in captivity without having been "inoculated" with adult feces (see Fecal Inoculation below).
Coprophagy is generally rare in reptiles. Frye (1991) reports it in some tortoises, such as the sulcata (Geochelone sulcata) and desert tortoises (Gopherus agassizii and G. Berlandieri), as well as in the cobra (Naja melanoleuca) and a garter snake (Thamnophis). The coprophagous tendencies of sulcatas is well known to sulcata keepers who also have dogs; if left to do so, some sulcatas would cheerfully clean up their yard of all dog feces. Other than a few references to coprophagy in hatchling iguanas, there are no references that I have found addressing to coprophagy in adolescent and adult iguanas.
Coprophagy in Green Iguanas
From personal observation and anecdotal reports of other iguana keepers, adolescent and young adult iguanas (~12-20 months) have been observed to eat their own or other iguana feces; when housed with other iguanas, the feces-eater generally eats the feces of one of the other iguanas, not its own. They generally ingest feces only a few times, and the inclination towards feces eating usually passes within a few weeks after it was observed to start. These events were not presaged by any changes in routine, habitat, or diet and no changes were noted in these factors when the coprophagy stopped.
My sense, which may or may not bear out, is that the feces eating of these green iguanas starts about the time that sexual maturity hits, and ends after the initial body chemistry changes. In my iguanas, it was the male who ate the female's feces; the female was never observed eating her own or any other iguana's feces. Other iguana keepers report episodes of iguanas of both sexes engaging in coprophagy, with an episode consisting of just one or only a few events of feces eating.
Figuring out when they are doing it still sheds no light on why they are doing it. If it is indeed occurring about the time of onset of sexual maturity, it may have to do with something related to the undetectable-to-us changes in hormones in the iguana whose feces is being eaten, or an attempt by the eater to detect such changes. In some mammalian species, males will flehman (a characteristic set of facial movements apparently reflecting the transfer of the scent to a vomeronasal organ in the roof of the mouth) when investigating the urine and other secretions of sexually mature female conspecifics, apparently in the attempt to determine if they are ready for mating. It may be that the coprophagy of the adolescent/adult iguana is something along the same lines. If so, one is left wondering why it only seems to happen during the first breeding season and not subsequent ones (based on observation and anecdotal reports).
A few iguana keepers have reported feces eating done seemingly as an alternative to poop painting or surfing. An iguana who is mad or upset with the keeper for some reason may pointedly eat feces when the keeper is watching, just as he or she may at other times smear feces all over the enclosure or area ("painting") or deliberately walk back and forth through it ("surfing") before trying to cuddle with or otherwise climb on the keeper.
The most obvious risk is that the feces ingested may contain parasites looking for a new host. Once the feces are ingested, the parasites will have found a new home. In reptiles eating the feces of other taxa, such as tortoises eating dog feces, the risk of disease transmission may be low, but there are still a good number of zoonotic diseases that may be transferred this way.
Salmonella, of course, can be transferred this way. One iguana keeper I know lost most of her hatchlings when she "inoculated" the hatchlings with the feces of one of her apparently healthy adults. It turned out that the adult had more Salmonella than the immature immune systems of the baby iguanas could cope with.
Salmonella could potentially be a problem when it comes to keepers who have a habit of kissing their iguanas on the mouth.