The zebra finch is arguably in the running to be the world’s next top model….organism, that is.  Zebra finches (Taeniopygia guttata), in this position, are poised to take over the globe. Of course, this set-up wouldn’t be possible without some critical human intervention – the geographic “range” of zebra finches can be liberally defined by the locations of the labs that house and use them as a study organism.  In the wild, zebra finches are limited to their native range of Australia and Indonesia, but their human-enabled spread has also led to recently-established populations in Puerto Rico and Portugal [1].  So what has made the zebra finch so popular to humans?

Zebra finches and other creatures deemed “model organisms” might be considered one of the luckiest species out there. This depends on your attitude, of course, but model organisms are species commonly used in research labs by scientists all over the world. From a fitness perspective, this is a great gig for these species! In captivity, these species have access to ad libitum (unlimited) food and water, are removed from predators and competitors in their captive settings, and are able (and often encouraged) to breed constantly.  Being a lab research animal seems like a cushy deal. 

The definition of a model organism, while a common term in science research, is actually quite difficult to pin down. Recently, renowned evolutionary biologist Dr. Hopi Hoekstra shared a tweet defining a model organism as “historically, an experimental organism used to understand human biology”[2].  Under this definition, the most common animals that come to mind are rodents like rats and mice, which scientists use as common mammalian models for disease and human health, or fruit flies and yeast, which are used to understand mechanisms of genetic inheritance.  However, Dr. Hoekstra went on to describe how the definition of a model organism has changed in recent years, now seen as a species “studied by a (sizable) community, which shares tools & resources, to learn general principles of biology”.  This might include things like the anole lizard, which, while not a clear model for some aspect of human health, is a common model in evolutionary biology and invasion ecology. (To illustrate, there is an entire blog run by anole researchers across the USA, called Anole Annals). Other scientists suggested edits to Hoekstra’s definitions, claiming that pioneering researchers did not always have human health in mind, but rather a basic understanding of biological principles (citing, as an example, the use of Arabadopsis as a common plant model). Often, model organisms are chosen because they need to be the “best tool for the job”, meaning they need to be easy to raise and study in the lab, with biology that addresses the question at hand.  I love how one reply put it in the original tweet:

“The idea of the ‘model organism’ was expressed by Jacques Monod: ‘Anything found to be true of E. coli must also be true of elephants.’ Today, we do well to be more modest: we study the local dialect – grammar & vocabulary – of 1 organism to get a glimpse at the language of life.”

Was asked to define “model organism.” My reply:

Historically, it was an experimental organism used to understand human biology.

Now, it is one studied by a (sizable) community, which shares tools & resources, to learn general principles of biology.”

What do you think? pic.twitter.com/xopYd8oqRC

— Hopi Hoekstra (@hopihoekstra) July 30, 2019

So where does the zebra finch fit in? If we use Dr. Hoekstra’s historical and current definitions, the zebra finch may fit both!  Following the historical definition, zebra finches originally rose as a model for human health and biology in the last 30 years [3]. Before this, zebra finches had already begun their campaign for world domination through the pet trade, and are still a pet store staple. Their move into the laboratory came with the development of neuroscience as a burgeoning field; zebra finches were used as models for the neurobiological basis of learning.  Anyone loosely familiar with current neuroscience research might be wondering: Birds?! Isn’t most neuroscience research done on rats and mice, and if really advanced, primates?  As a recent attendee of the national Neuroscience conference, I empathize: models beyond rodents were hard to find amongst the miles of posters.  But, unlike rodents, zebra finches (like most songbirds, and humans) are vocal communicators.  This means that they must learn and acquire “language” by listening to the dialects of other birds, just how baby humans learn language. This, plus their ability to thrive in captivity, made them the “best tool for the job” of studying vocal learning [4]. 

Model organisms, gotta study 'em all! pic.twitter.com/b3ikwLwUgV

— Biological Sciences (@ucdavisbiology) July 2, 2019

Since being used as a model organism in neuroscience, we have learned much about zebra finches (and their brains). This is where the second definition, “studied by a (sizable) community, which shares tools & resources”, comes in. There are published books full of detailed anatomy of the zebra finch brain [4], its genome was sequenced in 2010 (only 10 years after the human genome!) [5], and there are genetic tools to manipulate its DNA [6]; we can create genetically-engineered beeping birdies. This wealth of information about the inner workings of zebra finches makes it even easier for new labs to pick up a couple and start their own studies, without having to acquire all this information from scratch.  With these resources, labs now use the zebra finch to understand biological processes as diverse as parental care and social behavior, ecotoxicology of pollutants, flight biophysics, and animal coloration, as a quick literature search (from only 2019) shows.

We’ve discussed in detail the “modelling” career of the zebra finch in lab research across biomedical fields. Who was the zebra finch, before it became such a scientific star? What makes it so equipped to be the avian “lab rat”?  To answer that, we will need to return to the zebra finch’s native habitat Down Under, in Australia.

In recent years, researchers are getting to know the zebra finch for who it really is, not just as a model organism.  At first look, hearing accounts (like this one) of zebra finches in the wild Australian bush puts them in stark contrast to their expected caged setting.  In their natural habitat of the dry desert, water and resources are often scarce. While other passerines (song birds) use the changes in light that come with the shift in seasons to time their breeding, zebra finches can’t rely on these cues. In the desert, lengthening days can mean even hotter summers without any sign of reprieve. Starting breeding based on seasons alone would leave breeding zebra finches high and (quite) dry without food or water to provision themselves. Instead, zebra finches employ a strategy called opportunistic breeding [7] . Here, zebra finches time when they start building their nests using cues from the changes in their local environment, rather than changing day lengths alone. They wait for the monsoon rains to arrive in the desert, bringing with them water, green growth, and lots of bugs to feed hungry nestlings.  In addition to helping zebra finches sync with their native environment, this makes their reproductive organs and biology very flexible. They can breed in almost a moment’s notice, with little regard for time of year, if resources are available [8]. In other words, if you give them food and water (especially water), they will breed! This is important – as I know so well – for lab experiments that need to run all year round. It’s one of the reasons that rats and mice, known for their prolific breeding ability, are model organisms themselves. It is fascinating how a trait that evolved to serve zebra finch fitness in Australia has made them so well-prepared for life in the lab.

• 

• 

Besides being prolific breeders, zebra finches are highly gregarious and breed in large colonies. (Walk into any pet store selling them, and you will quickly understand why they are models for both vocalizations and social behavior – the dozens of them simply will not stop cheeping)! Like humans, and unlike rats, the social sense that zebra finches use is visual (recognizing each other) and auditory (song and vocal communications). This has made it important to keep them in large aviaries, or at least caged in pairs, in captivity. However, recent work by Damien Farine and colleagues looked at some of the roles these collective behaviors may play in wild zebra finch populations [9]. Using RFID tags to identify and scan individual finches in a colonial nesting site in Australia, Farine and team found that zebra finches form social networks of foraging and nesting that extend beyond the breeding season. The colonial, synced up breeding that zebra finches exhibit may actually represent complex social relationships, not just matching up by chance to access resources.  Future work will look at the similarities in zebra finches that sync up their breeding, behavior, and other aspects of their life history, such as when they begin breeding and how successful they are in raising young.

I could go on and on about the biology of such a beloved, well-understood bird. Science has learned so much about the brains, behavior, mate choice, nestling development, and more of zebra finches. However, as is often forgotten in biomedicine, it is key to appreciate our model in its natural habitat, and understand the back-woods backstory to such a world-conquering science super-star.

References:

1. “Zebra finch”. IUCN 2019. The IUCN Red List of Threatened Species. Version 2019-2. https://www.iucnredlist.org/species/103817982/132195948
2. Hoekstra, Hopi (hopihoekstra). (2019, July 30). [Twitter post]. https://twitter.com/hopihoekstra/status/1156289279045320705
3. McKinnon, J.S., Kitano, J., Aubin-Horth, N. (2019) Gasterosteus, Anolis, Mus, and more: the changing roles of vertebrate models in evolution and behavior. Evolutionary Ecology Research, 20:1-5. http://www.evolutionary-ecology.com/open/ccar3192.pdf
4. Mello, C. V. (2014) The Zebra Finch, Taeniopygia guttata: An Avian Model for Investigating the Neurobiological Basis of Vocal Learning. Cold Spring Harbor Protocols, online http://cshprotocols.cshlp.org/content/2014/12/pdb.emo084574.long
5. Warren, W. C. et al. (2010) The genome of a songbird. Nature, 464: 757-762. https://www.nature.com/articles/nature08819
6. Nixdorf-Bergweiler, B. E., Bischof, H.J. (2007). A Stereotaxic Atlas of the Brain of the Zebra Finch, Taeniopygia guttata. Bethesda, Maryland: National Center for Biotechnology Information.
7. Cornelius, J., Perfito, N., Zann, R., Breuner, C.W., Hahn, T.P. (2011) Physiological trade-offs in self-maintenance: plumage molt and stress physiology in birds. Journal of Experimental Biology, 214(6):2768-77. https://www.ncbi.nlm.nih.gov/pubmed/21795575
8. Perfito, N., Zann, R., Bentley, G.E., Hau, M. (2007) Opportunism at work: habitat predictability affects reproductive readiness in free-living zebra finches. Functional Ecology, 21(2):291-301.
9. Brandi, H. J., Griffith, S.C., Farine, D.R., Schuett, W. (2019) Wild zebra finches that nest syncronously have long-term social ties. Journal of Animal Ecology, in press. https://besjournals.onlinelibrary.wiley.com/doi/abs/10.1111/1365-2656.13082