One of the minor downsides of our Biology and Taxonomy of Insects module on the MSc course is, that we do have to review a lot of families within some of the groups, Lepidoptera being a prime example. Current estimates range from 250 000 to 500 000 species in 124 families (Kristensen et al., 2007). Going through the basic biology of each family can be pretty dry stuff, even if I have a personal anecdote or two to help lighten information overload. I am, for example, able to wax lyrical for several minutes about small ermine moths and their incredible silk-production activities, but even after more than 40 years of playing around with insects I don’t have a personal story for every family of Lepidoptera 🙂 so I am always on the lookout for an extra interesting or mind-blowing fact to help leaven the student’s knowledge diet.
Imagine my delight then when I came across a clip* from a BBC One Wildlife programme, Ingenious Animals, describing an obligate association between sloths and moths and not just because of the rhyming opportunity** 🙂
Sloth with moths – BBC One Ingenious Animals
The earliest record of a moth associated with a sloth that I have been able to find is in 1877 (Westwood, 1877) which merely records that the unidentified moth was “parasitic on the three-toed sloth”. In 1908 a Mr August Busck on a visit to Panama saw a two-toed sloth, Choloepus hoffmanni fall from a tree and noticed several moths flying out of the sloth’s fur. He caught these and on his return to the United States presented them to Dr Harrison Dyar (Dyar, 1908a). If the name seems familiar to you that is because Harrison Dyar is better known in connection with Dyar’s Law, the observation that larval growth in arthropods is predictable and follows a geometric progression (Dyar, 1890). The moths were identified by Dyar as a new species which he named Cryptoses choloepi. Dyar hypothesised that the moths and their larvae lived in the fur of the sloth and it was this that caused the sloth’s matted hair.
Cryptoses choloepi (Lepidoptera, Chrysauginae)
Shortly after publishing the first note Dyar came across two more moth specimens, this time collected from a sloth in Costa Rica. He felt that these were another species, possibly Bradipodicola hahneli (Dyar, 1908b). The next mention of a sloth moth that I could fine is in a marvellously titled paper (Tate, 1931) who refers to a moth shot in western Ecuador whose fur was “literally alive with a small species of moth, whose larvae possibly fed on the greenish algae which grew in the hair”. The idea that sloth moths fed on the fur of living sloths was further reinforced by Brues (1936) although this was not based on any personal observations. It was only in 1976 that it was discovered that the larvae of the sloth moth Cryptoses choloepi were actually coprophagous (Waage & Montgomery, 1976), the female moths waiting for the three-toes sloth B. infuscatus to descend from the trees to relive their bowels, which they do about once a week. As an aside, I have known Jeff Waage for many years in his role as a biological control expert but until I discovered this paper about a month ago, had no idea that he had ever spent time inspecting sloth faeces 🙂 Jeff and his co-author Gene Montgomery, described the association between the moths and the sloths as phoretic, rather than parasitic, as they saw no harm being caused to the sloths, but a number of benefits accruing to the moths, namely oviposition-site location being simplified, the fur of the sloth acting as refuge from avian predators and diet enhancement from sloth secretions (Waage, 1980). It turns out however, that some species of sloth moth do spend their whole life cycle on the sloth, B. hahneli lose their wings once a sloth host is found and their eggs are laid in the fur of the sloth (Greenfield, 1981). The algae that these moths presumably feed on is considered to be in a symbiotic association with the sloths, providing camouflage and possibly nutrition in the form of trace elements (Gilmore et al., 2001). Hereby lies a tale. The two-toed sloths have a much wider diet and home range than three-toed sloths and also defecate from the trees, unlike the three-toed sloths which have a very narrow diet (entirely leaves) and narrow home ranges, yet descend from the relative safety of the forest canopy to defecate, albeit only once a week, but still a risky undertaking (Pauli et al., 2017). Rather than a phoretic relationship Pauli and colleagues see the relationship between sloths, algae and moths as a three-way mutualism, beautifully summarised in their Figure 3.
Postulated linked mutualisms (þ) among sloths, moths and algae: (a) sloths descend their tree to defecate, and deliver gravid female sloth moths (þ) to oviposition sites in their dung; (b) larval moths are copraphagous and as adults seek sloths in the canopy; (c) moths represent portals for nutrients, and via decomposition and mineralization by detritivores increase inorganic nitrogen levels in sloth fur, which fuels algal (þ) growth, and (d ) sloths (þ) then consume these algae-gardens, presumably to augment their limited diet. This figure brazenly ‘borrowed’ from Pauli et al. 2014).
The sloths take the risk of increased predation by descending to ground level, because by helping the moths they improve their own nutrition and hence their fitness. Yet another great example of the wonders of the natural world.
Although not as exotic as the sloth moth, we in the UK can also lay claim to a coprophagous moth, Aglossa pinguinalis, the Large Tabby which feeds on, among other things, sheep dung. In Spain it is recorded as a cave dweller feeding almost entirely on animal dung, apparently not being too fussy as to the source.
Bradley, J.D. (1982) Two new species of moths (Lepidoptera, Pyralidae, Chrysauginae) associated with the three-toed sloth (Bradypus spp.) in South America. Acta Amazonica, 12, 649-656.
Brues, C.T. (1936) Aberrant feeding behaviour among insects and its bearing on the development of specialized food habits. Quarterly Review of Biology, 11, 305-319.
Dyar, H.G. (1890) The number of molts of lepidopterous larvae. Psyche, 5, 420–422.
Dyar, H.G. (1908a) A pyralid inhabiting the fur of the living sloth. Proceedings of the Entomological Society of Washington, 9, 169-170.
Dyar, H.H. (1908b) A further note on the sloth moth. Proceedings of the Entomological Society of Washington, 10, 81-82.
Dyar, H.G. (1912) More about the sloth moth. Proceedings of the Entomological Society of Washington, 14, 142-144.
Gilmore, D.PP., Da Costa, C.P. & Duarte, D.P.F. (2001) Sloth biology: an update on their physiological ecology, behaviour and role as vectors of arthropods and arboviruses. Brazilian Journal of Medical and Biological Research, 34, 9-25.
Greenfield, M.D. (1981) Moth sex pheromones: an evolutionary perspective. The Florida Entomologist, 64, 4-17.
Kristensen, N., Scoble, M.J. & Karsholt, O. (2007) Lepidoptera phylogeny and systematics: the state of inventorying moth and butterfly diversity. Zootaxa, 1668, 699-747.
Pauli, J.N., Mendoza, J.E., Steffan, S.A., Carey, C.C., Weimer, P.J. & Peery, M.Z. (2014) A syndrome of mutualism reinfocrs the lifestyle of a sloth. Proceedings of the Royal Society B, 281, 20133006. http://dx.doi.org/10.1098/rspb.2013.3006.
Pinero, F.S. & Lopez, F.J.P. (1998) Coprophagy in Lepidoptera: observational and experimental evidence in the pyralid moth Aglossa pinguinalis. Journal of Zoology London, 244, 357-362.
Tate, G.H.H. (1931) Random observations on habits of South American mammals. Journal of Mammalogy, 12, 248-256.
Waage, J.K. (1980) Sloth moths and other zoophilous Lepidoptera. Proceedings of the British Entomological and Natural History Society, 13, 73-74.
Waage, J.K. & Montgomery, G.G. (1976) Crytopses choloepi: a coprophagous moth that lives on a sloth. Science, 193, 157-158.
Westwood, J.O. (1877) XXVIII. Entomological Notes. Transactions of the Entomological Society, 25, 431-439.
*For the clip about the sloth moth see here http://www.bbc.co.uk/programmes/p04840xn
**Now, when I see a sloth,
My first thought is for the moth,
That has to make that desperate jump
When the sloth decides to take a dump!