Sex and Size

Does size really matter between males and females?

Surely there must be an obvious answer to this, but to really understand the differences between the sexes in the animal kingdom, you have to bring out the big guns.

I had the pleasure of meeting Dr Daphne Fairbairn, an evolutionary biologist who has dedicated her life to studying sexual dimorphism – visual characteristics which separate males from females. Her experience in the field is awe-inspiring, with so many papers under her belt it’s difficult to imagine when she actually stops working! A previous editor-in-chief for the journal “Evolution”, she has spent the last five years working tirelessly on her latest book “Odd Couples: Extraordinary Differences between the Sexes in the Animal Kingdom”. Basing her seminar on the book, Dr Fairbairn gave a sneak preview into the importance of sexual dimorphism in understanding an animal’s life history ¹.

Let’s start with some statistics. Firstly, sexual dimorphism is present in 93% of all animal phyla which have separate males and females ². External reproductive systems differ in over 70% of these animals, while other external body differences (size, shape or colour) are found in 84%.  Even if you had little knowledge of biology, you could still be able spot the difference between a male and a female of many animals.

Sexual dimorphism exhibited in Riggenbach’s reed frogs; female (below) and male (above)

A large proportion of studies investigating sexual dimorphism have focused on colour, as birds and amphibians are the best representatives ³. Dr Fairbairn, however, is much more interested in body size as it is the most common form of dimorphism – represented by 65% of sexual dimorphic animals.

For instance, when you look at the differences in humans, men are usually bigger in body size than women. That seems to be the case with most large mammals; females are smaller and leaner in build, while the males are larger and more muscular. Competition between males has likely led to the selection for an increase body size ⁴. In order to win the females, the males have to be bigger and stronger than their rivals.

The sexual dimorphism exhibited in lions; males are larger in body size than females.

Yet this size ratio seems to be the minority. In fact, large terrestrial mammals are the odd bunch of the animal kingdom. Dr Fairbairn points out that when it comes to comparing body mass, over 69% of sexually dimorphic animals have larger females than males.

Males which are less than half the linear dimensions of females are termed “dwarfs” ⁵. This can been seen in a variety of species, particularly in fish and spiders. The greatest mass difference between males and females discovered is within the ray-finned fishes (Actinopterygii) ¹. However there is so much variation within this clade due to influences of female choice or male-male competition that dimorphism is not constrained to classes.

Since selection appears to be favouring sexual dimorphism, it must be beneficial and adaptive. As mass increases, the required energy input into reproduction and neonates decreases. For males this allows them to invest more energy into mating strategies or resource defence rather than in rearing offspring.

So why is it that there are so many species with larger females?

St. Andrews Cross spiders; female (left) is much larger than the male (right) who has cautiously approached her in order to mate

Dr Fairbairn explains it is actually female “gigantism” rather than male “dwarfism” ^{1, 6}. There is a strong correlation between prey size and female body size ^{1, 2}. This can be illustrated best in spiders. The females take longer to mature and require a higher food intake, so as to produce large quantities of eggs. Males however reach maturity earlier and spend a large portion of their adult life searching for a female. Smaller bodies means faster maturity and more time and energy can be spent in finding the females.

It is apparent that mating strategies have influenced the differences of body sizes between males and females in the animal kingdom. So it really does seem size matters after all.

References

1. FAIRBURN, D. J. 2007a. Introduction: the enigma of sexual size dimorphism. In: FAIRBAIRN, D. J., BLANCKENHORN, W. U. & SZEKELY, T. (eds.) Sex, size, and gender roles: evolutionary studies of sexual size dimorphism.: Oxford University Press.
2. FAIRBAIRN, D. J. & ROFF, D. A. 2006. The quantitative genetics of sexual dimorphism: assessing the importance of sex-linkage. Heredity, 97, 319-328.
3. SHINE, R. 1979. Sexual selection and sexual dimorphism in the amphibia. Copeia, 297-306.
4. BLANCKENHORN, W. U., PREZIOSI, R. F. & FAIRBAIRN, D. J. 1995. Time and energy constraints and the evolution of sexual size dimorphism – to eat or to mate. Evolutionary Ecology, 9, 369-381.
5. HERCZEG, G., GONDA, A. & MERILA, J. 2010. Rensch’s rule inverted – female-driven gigantism in nine-spined stickleback Pungitius pungitius. Journal of Animal Ecology, 79, 581-588.
6. OSTERGAARD, P., BOXSHALL, G. A. & QUICKE, D. L. J. 2005. Dwarfs or giants? Sexual size dimorphism in Chondracanthidae (Copepoda, Poecilostomatoida). Crustaceana, 78, 397-408.

Images

http://www.arkive.org/lion/panthera-leo/image-G15960.html

http://www.arkive.org/riggenbachs-reed-frog/hyperolius-riggenbachi/image-G21311.html

http://www.arkive.org/st-andrews-cross-spider/argiope-keyserlingi/image-G80416.html

Past and Future

Thinking about “history”, events such as wars and colonisations come to mind. Studying history – modern or ancient – is based on events dating back to the evolution of humans and taught as a part of the arts. But what if learning history covered much more than just human events?

To Prof David Christian, a historian at Macquarie University, history classes were about to have a make-over. Whilst lecturing in his early years, an idea began to formulate – what if there was a way to “build the bridge” between the humanities and the sciences? He realised that by having each discipline compartmentalised, society was becoming more disconnected. He found that within each science field, knowledge had grown rapidly and become so specialised that there was little communication occurring between them, which he thought to be tragic.

This led to the fruition of “big history”; which connects astronomy, geology, chemistry, biology, technology, and humanities to paint a bigger picture into understanding how we as human beings fit into the universe ^{1, 2}.

Prof Christian began teaching “big history” classes in the late 1980’s, through a series of guest lectures – utilising the universities resources of experts in various fields. Having no science background – his own expertise was Russian history – Prof Christian sat though the guest lectures and absorbed as much information as possible. Over the years, he gradually become more comfortable with the science content and began taking the lectures himself.

 Professor David Christian

After 25 years of teaching history, his time at Macquarie University drew to a close in 2000 and he left Australia to begin teaching in San Diego. It was during this time that an opportunity arose which would take “big history” to new heights. By chance, a meeting with Bill Gates took place and after hours of talking a new project had blossomed. Why not teach high school students about “big history”?

The “Big History Project” began in 2004 as an online teaching company comprised of video lectures and quizzes. Broken down into “thresholds of increasing complexity” each segment focuses on a different aspect of the history of the universe and where we fit in ³. By altering how the timescale is communicated, Prof Christian found students could understand the concepts better. For example making 14 billion years ago, when the early universe began, seem like only 14 years ago.

Of the nine thresholds, three focus on the universe from an astronomy and chemical point of view: the universe, stars, and planets ⁴. The first section deals with when the universe began to form ½ million years after the big bang, and the emergence of complex objects. The second section brings together chemistry and physics for the evolution of the stars. Most importantly, the origin of the table of elements becomes clear. Stars are composed of hydrogen and helium – the first elements in the period table – but as stars die their composition changes; the cores become higher in proton complexity and form into iron ⁵. The third section focuses on the formation of planets and the “goldilocks environment”, a concept for explaining how Earth was able to establish life.

  The formation of stars in the universe and the debris left behind.

The fourth threshold brings biology into focus – how did life on earth begin and how has it evolved? Life is complex and active, and exists in an unstable environment. Prof Christian emphasises the ability of living things to absorb information about their surroundings and utilise the information to make decisions. He conveys the idea that metabolism, homeostasis and natural selection are all forms of decision making.

Leading on from the evolution of humans, the next three sections are based on human history and the evolution of humans culturally. Prof Christian touches upon how energy has become a currency in civilisation today, and how much we rely on the mobility of energy for survival – the notion we are “energy slaves”. Humans have become a planet-changing species, but there are high costs for high energy consumption such as loss of biodiversity and altering processes such as the nitrogen cycle.

The last threshold is about the future. Prof Christian acknowledges that the near-future is full of uncertainty, but when looking into the distant future it is most likely that complexity will decrease as energy supplies become stretched too thinly across the universe.

Prof Christian’s classes on big history are likely to be taught across schools worldwide due to the current success of the project.

For more information:

1 – CHRISTIAN, D. 2011. Big history and the future of humanity. Journal of Global History, 6, 535-536.

2- CHRISTIAN, D. 2010. THE RETURN OF UNIVERSAL HISTORY. History and Theory, 49, 6-27.

3- ALVAREZ, W., CLAEYS, P. & MONTANARI, A. 2009. Time-scale construction and periodizing in Big History: From the Eocene-Oligocene boundary to all of the past. Late Eocene Earth: Hothouse Icehouse, and Impacts, 452, 1-15.

4- The big history project – https://www.bighistoryproject.com/home

5- BAUGH, C. M., COLE, S., FRENK, C. S. & LACEY, C. G. 1998. The epoch of galaxy formation. Astrophysical Journal, 498, 504-521.

Shifts and Change

Something strange has been happening in the sea. Something big. Something rapid… and Dr Will Figueira wants to get down to the bottom of it all.

A man with many questions, multiple ideas, and an incredible enthusiasm for fish, he gave everyone a titbit of information on his various ongoing studies. Currently calling Sydney “home” the American doctorate has been steadily working his way through the conundrum of the tropical fish migration. Not typically known for taking seasonal trips, tropical fish on the coast of Australia appear to be doing just that: catching the Eastern Australian Current – a.k.a the EAC – down to lower parts of NSW and even as far south as Victoria and Tasmania¹!

No they’re not riding turtles as seen in the motion picture “Finding Nemo”. The warming of the world’s oceans due to climate change is affecting the distance the juvenile tropical fish are travelling. During their pre-settlement larval stage, juvenile fish can travel vast distances using the currents to reach an appropriate region where food is in high supply and shelter can be found. From this point, the fish enter their post-settlement stage of development where the next step is to gather as much energy as possible for reproducing.

In the northern hemisphere, studies have been conducted on how climate change is impacting the oceans. The ocean’s surface temperature is warming significantly and there are noticeable changes in the movement of many marine species ^2,3. Tropical fish are changing their migratory patterns and appearing in regions where they were previously unrecorded ⁴.

Dr Figueira has been involved in various studies on tropical fish since his phD days in the Florida Keys and upon noticing the effects climate change was having on the northern hemisphere’s oceans, proceeded to investigate what was happening in Australian waters. The studies found the waters were warming and causing migration outside the normal range for tropical fish ^5,6. On further investigation, there were a few select species that were being caught in the upper regions of NSW as well as in the Sydney region, most commonly species of Damselfish.

The EAC route carries an increase in diversity of tropical fish to temperate waters

How was this happening?

Dr Figueria explained a few of the possible scenarios which could solve the mystery. From his research, it was found there were tropical species outside their migration range which appeared to be adapting to have wider latitudinal ranges. This would enable the fish to occupy the warming southern waters. Some tropical fish are even spawning earlier or entering their post-settlement stages further down the EAC. The next step into solving the mystery is for research into whether these select few tropical fish are adapting genetically or just behaviourally to the changes in water temperatures.

But there is one more issue with migrating further south – whom is preying on whom? The resident fish and resident predators of the temperate regions have been adapted to the presence of the other for some time. The new comers, the tropical migrants, do not have the same adaptation to avoid predation. So how are they able to survive after their journey down the EAC? Dr Figueria explains that fish are have different “burst swim speeds” which can also be interpreted as their active metabolic rates. His latest research is into whether the tropical fish are able to achieve their active metabolic rates in order to escape given the increase in temperature in their new temperate environment. The results, so far, have indicated that as the water temperature increases by a few degrees the tropic fish are reaching their active metabolic rates faster than the predators. Good news for the migrants, not so much for the residents! This research opens the doors for future studies into the effects of higher temperatures in temperate waters and the effects this will have on temperate-dwelling fish.

So if you’re heading down the EAC, keep an eye out for stray tropical fish – they’re heading to the big city!

1. BOOTH, D. J., FIGUEIRA, W. F., GREGSON, M. A., BROWN, L. & BERETTA, G. 2007. Occurrence of tropical fishes in temperate southeastern Australia: Role of the East Australian Current. Estuarine, Coastal and Shelf Science, 72, 102-114.

2. PERRY, A. L., LOW, P. J., ELLIS, J. R. & REYNOLDS, J. D. 2005. Climate Change and Distribution Shifts in Marine Fishes. Science, 308, 1912-5.

3. BURROWS, M. T., SCHOEMAN, D. S., BUCKLEY, L. B., MOORE, P., POLOCZANSKA, E. S., BRANDER, K. M., BROWN, C., BRUNO, J. F., DUARTE, C. M., HALPERN, B. S., HOLDING, J., KAPPEL, C. V., KIESSLING, W., O’CONNOR, M. I., PANDOLFI, J. M., PARMESAN, C., SCHWING, F. B., SYDEMAN, W. J. & RICHARDSON, A. J. 2011. The Pace of Shifting Climate in Marine and Terrestrial Ecosystems. Science, 334, 652-655.

4. LLOYD, P., PLAGÁNYI, É. E., WEEKS, S. J., MAGNO-CANTO, M. & PLAGÁNYI, G. 2012. Ocean warming alters species abundance patterns and increases species diversity in an African sub-tropical reef-fish community. Fisheries Oceanography, 21, 78-94.

5. FIGUEIRA, W. F. & BOOTH, D. J. 2010. Increasing ocean temperatures allow tropical fishes to survive overwinter in temperate waters. Global Change Biology, 16, 506-516.

6. GALAIDUK, R., FIGUEIRA, W., KINGSFORD, M. & CURLEY, B. 2013. Factors driving the biogeographic distribution of two temperate Australian damselfishes and ramifications for range shifts. Marine Ecology Progress Series, 484, 189-202.

Fig 1. “Finding Nemo” and the EAC http://www.polyvore.com/disneyland_finding_nemo_pixar_crush/thing?id=19586460

Predictions and Projections

Tags

Climate change, Climate models, ecology, science

Dr Rebecca Harris, from the University of Tasmania, is on a mission. Her research aims to “bridge the gap” between global climate models (GCMs) and reality, in the hope for climatologists and ecologists to communicate on common ground during studies on the effects of climate.

Climate scientists around the world are continually working on developing and improving 3D models which depict possible future projections of what the global climate may be like under different conditions such as temperature and rainfall ¹. These scientists are mathematicians and physicists who rely on numbers and have very little knowledge of biological sciences.

Captions from two different GCMs illustrating future projections of Australian climate with respect to rainfall ²

As an ecologist herself, Dr Harris has noticed the differences in terms and methods used between the two fields while working with climate scientists. Her research highlights the importance of understanding GCMs and how to interpret the vast amounts of data the outputs of the models generate in order to use the information for ecological studies ¹.

So how does an ecologist, or any other researcher, make use of global climate models?

The number one rule: never use one model. Each model has different degrees of uncertainty and focuses on different climatic scenarios. Some models project future climates in hot and dry atmospheres while others are based on colder, wetter atmospheres. Ecologists would benefit more from using a range of possible future projections. By using only one GCM, the results from the study become highly biased to that particular set of variables, which may hide the effects of other possible climate scenarios ². Another issue, which leads on to the next “rule-of-thumb” is that one model may not be suitable to the region of study.

Different models are constructed to suit different geographical regions, as climatic conditions vary across the world ¹. When using models in ecological studies, it is important to select models best suited to the region of interest. For instance, a study on butterfly distributions in the tropical regions of Australia would be better suited to using models constructed by CSIRO with Australian atmospheric data for future projections than ones constructed in Europe for projections using European data ³. Depending on the area of interest, models must be selected carefully to fit the variables available, according to Dr Harris during her seminar. With over 50 models to choose from, each producing many different future projections, it’s no wonder ecologists can get lost in translation!

Carrying on from this, Dr Harris emphasizes that the latest models can be downscaled to “regional” which is more suitable for studies as small-scale factors can influence projections at the regional level. “Dynamic downscaling” is a particularly useful tool when the research is interested in the seasonal changes or even daily changes to an area’s climate. For ecologists, projections at this scale are more appealing to work with and easier to integrate into studies such as investigating endemic species distributions ⁴.

The last key factor for ecologists is choosing a suitable “baseline” to have for the models. The latest IPCC report (International Panel of Climate Change) begins the 30-year period for projections using the data from 1986-2005 ¹. Dr Harris points out that historical factors such as drought can have significant influences of the output of the models for a particular region. If the model is not suitable to the region’s climate history, then the projections will not be relevant ⁵.

There are so many factors to think about for using GCMs in ecology that it can be too much information to handle. We, as biologists, just have to remember: there will never be only one right model – only many possible futures!

References:

¹ Harris, R. M. B., Grose, M. R., Lee, G., Bindoff, N. L., Porfirio, L. L., & Fox‐Hughes, P. 2014. Climate projections for ecologists. Wiley Interdisciplinary Reviews: Climate Change 5:621-637.

² Fordham, D. A., T. M. L. Wigley, and B. W. Brook. 2011. Multi-model climate projections for biodiversity risk assessments. Ecological Applications 21:3317-3331.

³ Beaumont, L. J., A. J. Pitman, M. Poulsen, and L. Hughes. 2007. Where will species go? Incorporating new advances in climate modelling into projections of species distributions. Global Change Biology 13:1368-1385.

⁴ Pierce, D. W., T. P. Barnett, B. D. Santer, and P. J. Gleckler. 2009. Selecting global climate models for regional climate change studies. Proceedings of the National Academy of Sciences of the United States of America 106:8441-8446.

⁵ Kapsch, M. L., M. Kunz, R. Vitolo, and T. Economou. 2012. Long-term trends of hail-related weather types in an ensemble of regional climate models using a Bayesian approach. Journal of Geophysical Research. Atmospheres 117.

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