ANDE KA FUNDA (Does the shape of EGG matters: EVOLUTIONARY ADAPTATION)

Source: https://science.sciencemag.org/content/356/6344/1249
I found this article very interesting and presented for my Lab journal club (Off topic for my current Lab). Initially I never thought there would be scientists working so deeply in this field. I had a small discussion with my colleagues regarding different shapes of birds eggs. They all didn’t believe me, they all thought that all birds and reptiles have the same shaped egg, but with different sizes.
So, here I was simulated to present this paper to open their eyes. This paper talks about avian egg shape and how it has evolved or adapted in life history.Authors have done very beautiful work by making use of morphometric (depending on the size and shape of egg), mechanistic and macro evolutionary analysis on 14,000 different eggs i.e, around 1400 bird species were screened (AMAZING MAN POWER THEY MUST HAVE HAD THEN).
This analysis takes the best use of mathematics to answer many unexplained biological questions. Here, the authors have used 2 biologically relevant parameters to explain egg shape diversity: Firstly they talk about ASYMMETRY and then ELLIPTICITY (this is when I realized the actual shape of egg, is elliptical; till now whatever shape was elliptical, I always referred to those as egg shape).Using these 2 parameters, now one can plot in 2 Dimensional space; MORPHOSPACE.
Before we get immersed in the ocean of mathematics and its formulae, lets understand evolution of eggs (then we can talk about its shape).

Here the story goes.

360 million years (yupp..very long time ago) our ancestors decided to leave the ocean and moved towards land and started living there. This change of location/habitat caused evolutionary adaptation among living beings. Aquatic animals lay eggs which are membrane bound and transparent. But once there was change in habitat, these laid eggs needed more protection from the exposed environment, hence calcification of eggs (calcium carbonate shells) was adapted around the membrane. Now, we observe that these calcified eggs were of different sizes and shapes. For example, Owls lay spherical eggs (I didn’t know before, shocked emoji), humming birds lay the most elliptical eggs and shorebirds lay conical shaped egg (interesting).

Different EGG shapes

HYPOTHESIS kept forward:
a) Adaptive function of egg shape (foreg: a conical shape egg decreases its chance of falling from the cliff nest, by rolling in a tight circle. Just imagine the scenario if those eggs were spherical in shape, that might just roll over, fall and breaks).
b) Improving incubation efficiency in a clutch (it means number of eggs laid in a nest).
c) Sphere as in spherical eggs, has got minimum surface to volume ratio (mathematics geeks idea), having the least amount of shell covering for a given yolk volume present in a mature egg.
d) Increasing the surface area for gas exchange through pores (Conical eggs of shorebirds have maximum pores at the blunt end, acting as a specialized respiratory site for neural development in precocial (starts walking after hatching) birds.
e) Flight might also affect egg shape incidentally via the bird’s body structure (pelvis or oviduct).

SOLUTIONs, to resolve these queries, the authors have provided.

They prepared the MORPHOSPACE (2 dimensional representation using asymmetry (A) and ellipticity (E)).
Providing relationships between the egg shape and oviducts.

MORPHOSPACE
(Each dot represents a single species).

From this figure, it was observed that the shape of eggs is in continuity, with no clear cut distinct classes i.e, either completely spherical or symmetrical (I never knew such variations existed, may be that's how these birds could identify which is theirs). Asymmetry eggs might have evolved before the forefathers of extant (still alive) birds.

Authors have made BIOPHYSICAL MODEL to explain the shape of the EGG.
According to this model, the shape of the egg is predetermined in the oviduct of the bird and the shape is maintained by the membrane.To prove this, the shell was removed and observed that it still retains the shape. The X ray analysis of the oviducts also showed the egg shape without its shells.

Egg membrane without shell

To understand this, the authors have assumed that the egg is non spherical in shape with pressurized closed elastic membrane (membrane properties being in inhomogeneous leading to disparity in membrane thickness). Now what happens if you apply pressure on this membrane ? The membrane gets stretched and attains the shape. This is explained using mathematical modelling, by considering 2 parameters, (a) applied pressure difference across the egg membrane and
(b) differences observed in the membrane thickness and its material properties.
If you see the figure (4), there are 2 types of forces applied; external muscular forces (due to isthmus of oviduct) and internal pressure (due to yolk/albumin fluid). The difference in the two will be analysed. Secondly, differences in the membrane material properties (collagen, elastin etc) along the axial and azimuthal directions were also considered ( to understand the membrane thickness variations along the blunt and conical end of the egg).

Biophysical Model
Now, we understand that stretchability of egg membrane causes Ellipticity along the oviduct axis (blunt end to conical end), and Asymmetry is caused due to variations observed in the membrane material composition (yolk proteins, fibers, fluid etc). But, all eggs are AXIS-SYMMETRIC, which means that any alteration in the shape/ membrane shape or properties are symmetric along the longitudinal axis of the egg. This is described using PLANAR CURVE C (Oopss...Maths), with radial and angular coordinates as a curvilinear material coordinates (mathematical equation are avoided, please refer to the Research paper, Myself didn't get the point here. Why and what does the authors wanted to imply).

Evolutionary hypothesis:

Phylogenetic tree was prepared of 1209 species using molecular sequence data available and mapped according to the size, asymmetry, and ellipticity (Image 5).
From the image (5), it was clear that, with increase in egg size/length, there is an increase in ellipticity and asymmetry. And this found to be parallel across the different lineages, which means that highly elliptical and asymmetric eggs were evolved independently (for example, as in the case of Penguins and shorebirds).
To evaluate this hypothesis, authors and team compile all the data, including biometric, life history and surrounding habitat parameters of selected birds and included in the sample size. Parameters included features like food diet, body weight, clutch size, nest size and location and chick developmental growth. Environmental factors like temperature, latitude and rainfall precipitation and HWI (hand to wing index-flight efficiency and dispersal ability in birds).
So, for this Bayesian phylogenetic analysis was done using HWI, and it was found that high HWI birds have narrow and pointed wingtips, whereas low HWI birds have weaker, shorter and round wingtips. It was also observed that birds with high HWI have more asymmetric and more elliptical eggs. Since, it was shown that HWI is positively correlated to flight efficiency, this gave an idea to authors that adaptations to flight might have contributed to the variations in egg shapes.
Thus, birds with high HWI- have high powered flight – these birds have less body weight, small body size, a reduced abdominal cavity, therefore, less functional oviduct. Thus, effecting the egg shape. The birds with high HWI have increased ellipticity and asymmetry in eggs, this also eventually resulted in increase egg volume with same small girth size.

Volume vs Asymmetry/ellipticity
BUT, what about flightless birds???
Like Ostrich, Kiwi and Penguins??
Ostrich lay spherical shape and Kiwi lay hyperelliptical shape egg with high symmetry. In contradiction to this, Penguins lay slight elliptical with asymmetry shape egg. This might be because they are powerful underwater swimmers, making selection constrained for streamline body.

Flightless
Conclusions
Using MORPHOSPACE and biophysical model, various eggs were mapped based on its ellipticity and asymmetry. This model helped in understanding the egg diversity linking to membrane properties to shape. Macro evolutionary data analysis showed that birds with high HWI lay eggs with increased ellipticity and asymmetrical shaped ones.
My suggestions:
More of developmental biology work is needed to be done to understand the exact process and reason of shape formation. This might include studying egg metabolic activation analysis, orientation of neural tube formation etc.
I hope this article was interesting for readers. Any suggestions and comments are welcome.
The reference paper link is provided.
Avian egg shape: Research article
I will be happy if anyone could explain the mathematical part. We can have discussion.
Love Ruby