Today, the city of Angkor in Cambodia lies in ruins. But a thousand years ago, life there was very different. Then, Angkor was the heart of the Khmer empire and the largest preindustrial city of its day. It had a population of a million and an area that rivalled modern Los Angeles. And the key to this vast urban sprawl was water.
Radar images of the city by the Greater Angkor Project (GAP) revealed that Angkor was carefully designed to collect, store and distribute water. The “Hydraulic City” included miles of canals and dikes, irrigation channels for supplying crops, overflow channels to cope with a monsoon, massive storage areas (the largest of which was 16km2 in area), and even a river diverted into a reservoir. Water was the city’s most precious resource, allowing it to thrive in the most unlikely of locations – the middle of a tropical forest.
But water, or rather a lack of it, may have been part of Angkor’s downfall. Brendan Buckley from Columbia University has reconstructed the climate of Angkor over the last 750 years, encompassing the final centuries of the Khmer Empire. The records show that Angkor was hit by two ferocious droughts in the mid-14th and early-15th century, each lasting for a few decades. Without a reliable source of water, the Hydraulic City’s aquatic network dried up. It may have been the coup de grace for a civilisation that was already in severe decline.
Many theories have been put forward for the downfall of Angkor, from war with the Siamese to erosion of the state religion. All of these ideas have proved difficult to back up, despite a century of research. Partly, that’s because the area hasn’t yielded much in the way of historical texts after the 13th century. But texts aren’t the only way of studying Angkor’s history. Buckley’s reconstruction relies on a very different but more telling source of information – Fujian cypress trees.
This article is reposted from the old WordPress incarnation of Not Exactly Rocket Science.
When it comes to sex, it makes sense to stick to your own species. Even putting aside our own innate revulsion, inter-species liaisons are a bad idea because they mostly fail to produce any young. In the few instances they do, the hybrid progeny aren’t exactly racing ahead in the survival stakes and are often sterile (think mules).
But having poor unfit young is still better than having no young at all and if an animal’s options are limited, siring a generation of hybrids may be a last resort. Karin Pfennig from the University of North Carolina found that the plains spadefoot toad uses just this strategy in times of need.
Female toads breed just once a year, so it pays for them to make the right choice. According to Pfennig’s work, they take their health and their environment into account when choosing mates. If their bodies are weak and their surroundings are precarious, the benefits that another species’ genes can provide to their young are enough to outweigh the risks.
The south-western United States is home to two species of spadefoot toads with overlapping ranges – the Mexican spadefoot, Spea multiplicata and the Plains spadefoot, Spea bombifrons (more Kermit-like, according to Pfennig). Where both species mingle, they can breed and, as usual, the hybrid young are worse at spawning the next generation than their pure-blooded peers. Hybrid males are often sterile, and hybrid females lay fewer eggs.
Nonetheless, up to 40% of toads in certain areas can be hybrids and this intrigued Pfennig. She wanted to work out whether this was just incidental, or if some circumstances nudged the toads towards mating with individuals from a different species.
Their breeding grounds provided the answer; spadefoots lay eggs in temporary ponds and it’s often a race for tadpoles to turn into frogs before the water dries out. Pfennig noticed that hybrids were more common in shallower ponds that dry out quicker, and that’s because the two toad species develop at different rates.
On average, Mexican spadefoot tadpoles take less time to make the transition into frog-hood than Plains spadefoot ones, and hybrid tadpoles lie somewhere in the middle. This means that a Plains spadefoot female that’s faced with a short-lived pond might do better if she mates with a Mexican spadefoot male, for her young will be more likely to grow up in time.
Pfennig tested this idea by placing Plains spadefoot females in tanks simulating shallow and deep ponds and letting them choose between recorded calls from males of both species. In deep water, they favoured their own kind about 65% of the time, but in the shallower pools, they had no such preferences.
In contrast, Mexican spadefoot females also showed no willingness for breed with other species. Since their tadpoles develop quickly anyway, they gain nothing by courting Plains spadefoot males. Pfennig also found that only Plains spadefoot females that lived in the same areas as Mexican spadefoots had the ability to switch their mate preferences. In parts of the States where the two species are geographically segregated, females never made this choice.
A Plains spadefoot female’s health also affects which species she fancies. If she is fitter, she could provision her eggs with more nutrients and her tadpoles would grow faster. That would obviate her reliance on Mexican spadefoot males, even in shallower ponds.
Pfennig’s experiments confirmed her idea; the unhealthiest females were the most likely to switch their preferences, from mating with their own kind in deep ones to preferring the other species in shallow ones.
Biologists are used to viewing a female’s choice of partners solely in terms of the physical traits of males. But Pfennig’s results show that it isn’t just about which male has the flashiest colours, the most melodious song or the most impressive antlers. For females, mate choice is a much subtler affair, influenced by environment, personal health and probably many other factors that we have only begun to consider.
Reference: Pfennig. 2007. Facultative mate choice drives adaptive hybridization. Science 318: 965-7.