Cracking the egg

31 October 2016 - By: Alex Evans

Cracking the egg

Cracking the egg



By Alex Evans, PhD Student, University of Leeds

In July 2016, Brighton played host to an invasion of eggheads. Held on the eve of the annual SEB Conference, the “Cracking the Egg” satellite meeting welcomed egg experts from around the world to gather and share their passion for one of nature’s most delectable phenomena. Organised by egg enthusiasts, Dr Steve Portugal (Royal Holloway, University of London) and Prof Mark Hauber (Cornell University), the aim of the meeting was to “bring together researchers from different scientific fields to talk about all things ‘egg’”. Here are some of the highlights of the day.

Hatching the idea

Eggs are incredibly diverse in terms of their physical form, physiology and their associated parental behavioural strategies, making them a fascinating showcase of evolutionary biology and a hotbed for multi-disciplinary research. Through the course of the day, symposium attendees were treated to a wealth of talks and poster presentations from PhD students and early career scientists through to long-standing academics, encompassing a wide range of animal species and fields of research. Prior to the conference, Steve was confident that the symposium would go beyond a ‘breakfast staple’: “You will be mind-blown by how amazing eggs truly are and what they have to offer – you will never look at them the same way again.”

War in the nest

A prominent theme throughout the symposium was the evolution of egg mimicry by avian brood parasites and how parasitized hosts have adapted to deal with egg imposters. In terms of parenting, brood parasites are the cheating champions of the bird world. Instead of incubating their eggs and rearing their chicks, they will lay their eggs into another bird’s nest and leave the owner to raise the imposter chick. Common examples are Europe’s common cuckoo and the cowbirds of North America, but brood parasitism has evolved independently a number of times in birds around the world and brood parasites are thought to account for 1% of all known bird species1.

Naturally, the consequences for the parasitized hosts can be devastating for their reproductive success, which has resulted in a strong evolutionary fightback from the targeted species. The tawny-flanked prinia (Prinia subflava), a small bird native to sub-Saharan Africa, is one such species parasitised by the cuckoo finch (Anomalospiza imberbis). Dr Claire Spottiswoode from the Universities of Cape Town and Cambridge has been studying these two species and told us why brood parasitism can be such a successful strategy.  “Brood parasitism sidesteps the most expensive parts of parenthood for altricial2 birds, which are the incubation of eggs and rearing of the young,” she said, further explaining that the energy and time costs of laying the egg are probably cheaper than the costs of brooding them and raising the chicks. “By foisting the costs of incubation and chick rearing onto another species, the female can instead invest in producing larger numbers of eggs.”

However, this strategy is only successful as long as the hosts can be fooled into rearing the cuckoo finch’s eggs. Species that have been parasitized for thousands of years3 have evolutionarily cottoned on to the brood parasites' devious machinations and have evolved physiological and behavioural mechanisms to try to prevent them from being duped. One of the ways that they can do this is by evolving new egg ‘signatures’ that are harder for the brood parasites to mimic. Female prinias are able to produce a variety of individually distinctive signatures when laying their eggs, which vary between females. These signatures may be any combination of spots, splodges and wavy lines that cover the egg’s surface4. Claire revealed that the forged signatures created by individual cuckoo finches are inherited from the mother, resulting in lineages of brood parasites that have adapted to specific hosts. “So, the more signatures there are, the more host-specific lineages we can expect to find in the cuckoo finches,” said Claire. “It’s co-evolution at large!”

Later in the day, Dr Daniel Hanley (Palacký University and Long Island University Post) provided a more detailed look at how the variety of egg colours we see in nature are produced. “There are two main pigments colouring bird eggs,” explained Daniel, “one is biliverdin, producing a blue-green colouration and the other is protoporphyrin, which creates red-brown colours. The egg colours of birds are the result of an admixture of these two pigments.” Daniel was particularly interested in how the colour of imposter eggs is interpreted by host birds, leading to novel questions about avian perception and cognition. “Brood parasitism is a really interesting study system because it helps us to better understand decision making processes,” added Daniel.

DINO EGGS ARE JURASSIC ARKS

Taking us back to the time of the dinosaurs, Dr Jasmina Weimann and Tzu-Ruei Yang from the University of Bonn presented findings from their work on fossilised oviraptor eggs and how they use the eggs of today’s extant animals to learn more about extinct ones. As living descendants of therapod dinosaurs, birds are often used as an analogue when trying to understand how dinosaur eggs may have been structured, or how they were cared for. “We see that some structures and their organisation are highly conserved between eggs from dinosaurs and modern birds,” said Jasmina, “making it possible to trace supposedly ‘avian features’ back to dinosaurs”.

Jasmina investigated the molecular information preserved within the 67-million-year old preserved oviraptor eggshells, finding evidence of a spongy layer and blood vessels similar to that found in the eggs of modern birds. Impressively, she and her team also found preserved pigment molecules, suggesting that the oviraptor eggs were once an olive green colour4. “Until today it was common sense to think that coloured eggs are limited to our modern birds,” said Jasmina. “Our study shows that eggshell colouration evolved much earlier.”

While Jasmina spends her days examining dinosaur eggs under the microscope, Dr Tzu-Ruei prefers to be out in the field examining the specimens in situ. During his talk, he described how he investigates the arrangement of dinosaur eggs within nests to learn more about the behaviour of the parents. The Chinese oviraptor specimen studied by Tzu-Ruei has long been thought to depict a parent engaging in egg brooding behaviour5. However, according to Tzu-Ruei, it is more likely that oviraptors probably weren’t egg brooders at all and that this raptor died whilst adding another egg to her clutch. Some bird species have adapted egg colouration as camouflage to conceal their eggs from predators, so parents can spend more time feeding away from their young, with a reduced risk of coming back to an empty nest. Tzu-Ruei explains that this may also have been the case for the oviraptors: “With the eggs protected, the parent could have invested less in parental care, so perhaps egg brooding wasn’t that important for these dinosaurs.”

SWAPPING SPARROWS

The final talk of the day focused not just on eggs, but also the people that dedicate their time to egg collections. Dr Caren Cooper, Assistant Director of the Biodiversity Research Lab at the North Carolina Museum of Natural Sciences, discussed the role of citizen science in large-scale research projects and how they are particularly important for her work with the museum’s bird egg specimens, including her ongoing ‘Sparrow Swap’ project6

In the USA, house sparrows are an invasive species and often considered to be a pest by occupying nestboxes intended for native species such as bluebirds. In order to mitigate the damage done by sparrows, birdwatchers and volunteer egg monitors will often remove any eggs laid by female sparrows. However, the efficacy of this management strategy is unknown. In addition to egg removal, Caren and her team have offered birdwatchers across the US an alternative option; to replace the sparrow eggs with replicas, whilst taking the real eggs to their lab for study. In this way, Caren and her team aim to gather information in partnership with these citizen scientists to learn more about how egg removal and egg swapping affects the nesting sparrows. So far, the Sparrow Swap project has been successful, with birdwatchers participating in almost every US state. “Hopefully we'll soon have enough data to tell which method is more effective at minimizing house sparrow damage,” said Caren.

As well as providing data on sparrow management, a long-term goal of the Sparrow Swap project is to build the museum’s egg collection for future research. Museum specimens act as preserved biological snapshots, making them useful for tracking historical and geographical changes in species and their habitats. In fact, many museum specimens are often used for new research projects long after they had fulfilled the original purpose for their collection. “With this collection, we want to assess how communities concerned about pollution might use house sparrow eggs as bio-indicators of environmental and, perhaps, even human health,” said Caren, adding “we also hope to use the eggshells for studies of geographic variation in selection pressures on egg characteristics.”

During her talk, Caren promoted the advantages of citizen scientists in aiding studies that can only be made possible by the observations recorded by interested individuals from all around the world. Often, the invaluable work of citizen scientists goes unacknowledged: “In 2014, I published a study7 with colleagues in which we examined the contributions of citizen science efforts to understanding migratory birds and climate change. We found that about half of what is known about migratory birds and climate change derives from citizen science efforts, yet none of the papers mentioned the term ‘citizen science’”. Countering this, Caren’s concluding remarks highlighted the fact that this trend is changing and publications are now starting to attribute citizen scientists, with increasing encouragement for the public to engage with researchers across the globe.

By the end of the day, in addition to the research showcased here, the delegates had been treated to insights into a wealth of egg topics, including the stiff-yet-fragile structure of eggshells8 and the complex role of hormones in bird eggs9 and across many other animal species including sharks, squid and sea turtles. Looking back, Steve feels the symposium succeeded in its goal of celebrating the wide world of egg research. “It was fascinating to see the diverse range of egg-based research currently going on,” said Steve. “Finding out about the different techniques and approaches being used to address fundamental questions regarding eggshell properties has changed my ideas for future research directions.” 

REFERENCES

1. Winfree R. (1999) Cuckoos, cowbirds and the persistence of brood parasitism. Trends in Ecology & Evolution, 14 (9), 338–343

2. Definition of Altricial birds: http://articles.extension.org/pages/65368/precocial-and-altricial-birds

3. Spottiswoode, CN & Stevens M. (2011) How to evade a coevolving brood parasite: Egg discrimination versus egg variability as host defences. Proceedings of the Royal Society B, 278 (1724), 3566-73

4. Wiemann J, Yang T, Sander PNN, Schneider M, Engeser M, Kath-Schorr S, Müller CE & Sander PM. (2015) The blue-green eggs of dinosaurs: How fossil metabolites provide insights into the evolution of bird reproduction. PeerJ PrePrints, 3:e1323 https://doi.org/10.7287/peerj.preprints.1080v1

5. Norell MA, Clark JA, Chiappe LM & Dashzeveg D. (1995) A nesting dinosaur. Nature, 378, 774-776

6. Sparrow Swap project on SciStarter: www.scistarter.com/sparrowswap

7. Cooper CB, Shirk J, Zuckerberg B. (2014) The Invisible Prevalence of Citizen Science in Global Research: Migratory Birds and Climate Change. PLoS ONE, 9(9): e106508 doi: 10.1371/journal.pone.0106508

8. Taylor D, Walsh M, Cullen A, & O’Reilly P. (2016). The fracture toughness of eggshell. Acta Biomaterialia, 37, 21-27
doi:http://dx.doi.org/10.1016/j.actbio.2016.04.028

9. Peluc SI, Reed WL, Gibbs P and McGraw KJ. (2014) Maternal dietary carotenoids mitigate detrimental effects of maternal GnRH on offspring immune function in Japanese quail Coturnix japonica. Journal of Avian Biology, 45: 334–344. doi:10.1111/jav.00360

 

 

 

 

 

Category: Animal Biology
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Alex Evans

Alex Evans is a PhD student at the University of Leeds investigating the energetics of bird flight. In his spare time, Alex enjoys writing about the natural world, contributing to the Bird Brained Science blog and exploring other avenues of science communication.