Toxic Birds Make For Sad Lice

Toxic Birds Make for Sad Lice.

Photo by John Dumbacher from
the California Academy of Sciences

SEM photograph from phthiraptera.info

Today's entry is a bit divergent from the norm.  I saw a giant biting fly nipping at one of my dogs this morning.  Besides pharmaceuticals, I wanted to know what natural defenses against insects were out there.  Alas, no toxic dog fur existed in my brief literature search, but I did come across toxic bird feathers that combat lice.

So without further delay, I present the birds with toxic (to insects) feathers.

Birds are host to an impressive diversity of external parasites, from insects (including lice, fleas, bugs and flies) to mites, ticks and even fungi and bacteria. These parasitic organisms can have severe negative effects on host fitness. Therefore, it is not surprising that birds invest a lot of time engaged in behaviors such as grooming, preening, dusting and sunning in attempts to rid themselves of their ectoparasites. A handful of unique birds from the genus Pitohui have an interesting physiological adaptation that may assist in the fight against parasite infestation: feather toxins.

Yes, toxic birds. The six species of Pitohui, which are endemic to New Guinea have been found to carry in their skin and plumage. These are the same potent toxins as those found in the skin of poison dart frogs (Phyllobates spp.) and are some of the most toxic natural substances known. The toxin present in the Pitohui is known as homobatrachotoxin and like all batrachotoxins is a neurotoxic steroidal alkaloid capable of de-polarising nerve and muscle cell membranes. The level of toxins present in Pitohui tissue varies between species and geographic location. The most toxic species is the hooded pitohui (Pitohui dichrous), from which merely handling an individual can cause numbness, sneezing, and irritation of the eyes and sensitive mucous membranes. It has been hypothesised that the high proportions of toxin present in the Pitohui skin and feathers could provide the bird with a barrier from ectoparasites that live and feed on skin, feathers and subdermal blood supplies.

batrachotoxins.

A curator and department chair of Ornithology and Mammology at the California Academy of Sciences was the first to test if the presence of toxins in Pitohui feathers and skin would deter or kill chewing lice (order Phthiraptera). In order to investigate this he conducted a series of choice and lifespan experiments. Dumbacher found that when individual lice in the laboratory were given a choice of two feathers (one toxic Pitohui feather and one non-toxic non-Pitohui feather) there was a statistically significant preference against feeding or resting on the toxic feathers. Lice exposed to the highly toxic feathers of P. dichrous rarely showed signs of eating, with many becoming immobile and inactive. In some cases the louse would simply drop off the toxic feather. In a natural setting, immobility and lower feeding rates reduces the damaging effect of the lice and may even allow the birds to more easily remove or dislodge the parasites mechanically by preening or flying. Since this part of the study showed that the lice exhibited an active choice against the naturally toxic Pitohui feathers we can conclude that homobatrachotoxin has the potential to act as a repellent against these parasites.

The Curator also determined that the natural levels of homobatrachotoxin in Pitohui feathers greatly increased louse mortality. The results of the lifespan experiments showed that the mean lifespan of lice exposed to feathers of either high or low level toxicity was half that of those on nontoxic feathers. Interestingly, the mean lifespan of the lice on the toxic feathers was similar even though the toxin levels in P. ferrugineus are ten times lower than P. dichrous. Therefore, Pitohui feathers with lower toxin levels may not have been potent enough to repel lice during the choice experiments but were as effective in increasing louse mortality as the highly toxic feathers. Increased mortality in lice could have many benefits for the host. Less time spent on the host will reduce the negative effect of each individual louse.

One observation from the study was that non-toxic feathers showed obvious damage from lice feeding. This may be due to the extended life span offering additional feeding time, or the lice simply find nontoxic feathers more palatable. Further investigations may provide insight into additional  benefits, for example whether or not the potent toxin is able to reduce louse fecundity. If mating in lice is decreased then subsequent generations of lice are also reduced. Smaller populations would cause less irritation to the host and also be less visible to potential mates. Additionally, less ectoparasites would reduce time spent mechanically removing them and more time to invest in other activities. The results of Dumbacher's study suggest that the naturally occurring homobatrachotoxin found in the skin and feathers of the Pitohui repels and kills lice. The presence of a powerful toxin in skin and feathers has the potential to create a formidable barrier and protect the bird against infestation from ectoparasites.

Reference:

Dumbacher, J. P. (1999). Evolution of toxicity in Pitohuis: I. effects of homobatrachotoxin on chewing lice (order: Phthiraptera). The Auk, 116: 957-963.