The "Kappa duck" that lays eggs and produces milk is blocked by the dam

The platypus is probably one of the most bizarre animals. It is a rare egg-laying mammal with webbed feet and a mouth like a duck, but a flat tail like a beaver. It is no wonder that the scientists who first saw the platypus specimen suspected that this "stitched monster" was the product of a human prank.

However, not only are these cute little monsters declining in number due to crises such as climate change, they now have a new "enemy" - dams .

The first scientific drawing of the platypus in history, drawn in 1799, looks like an elongated Psyduck｜Frederick Polydore Nodder

The magical platypus is now a "near endangered" species

The platypus lives in the Australian continent. It appeared on Earth as early as 25 million years ago and is the only existing species in the platypus family.

The platypus has many characteristics that are different from other existing mammals . For example, it has not evolved breasts and nipples, and can only secrete milk through mammary glands like "sweating". Moreover, the platypus reproduces by laying eggs, which indirectly proves the evolutionary relationship between mammals and reptiles. It can be said to be a "living fossil". With milk and eggs, a platypus can make egg tarts by itself (wrong).

A platypus chick just hatched | Smithsonian Channel / Youtube

The platypus is far more peculiar than that. It has 10 sex chromosomes, five times as many as most mammals. Male platypuses have a poisonous spike on their hind legs, which can release venom to attack opponents when fighting for territory or mates. Their mouths are also covered with sensitive current sensors, which can accurately locate prey through the bioelectric field emitted by prey when hunting. And under ultraviolet light, their fur can even fluoresce.

Photos of the same platypus specimen under visible light and ultraviolet light, the top row shows the back, the bottom row shows the abdomen: Under ultraviolet light, the platypus fluoresces | Reference [2]

However, since Europeans settled in Australia more than two centuries ago, the number of platypuses has dropped by 50% . There are many reasons for the decline in the number of platypuses - initially, people hunted them for their fur; with climate change, more and more frequent extreme weather such as droughts and fires also directly threaten the survival of platypuses; environmental pollution, habitat loss, predation by invasive species, and accidental capture by fishermen have also made them precarious. On the Red List of the International Union for Conservation of Nature, the platypus is listed as a " near threatened " species.

Recently, a study published in Communications Biology showed that dams built by humans also seriously threaten the survival of the platypus .

The population blocked by the dam cannot flow its genes

Why would a dam affect the survival of platypuses? Would these little cuties run into a wall in a daze? In fact, the impact of dams on platypuses is not at the individual level, but affects the movement of the group , which in turn affects the survival of the entire group.

One of the characteristics of the platypus is that it has webbed feet similar to those of a duck. In the absence of dams, they can use their webbed feet to move freely between upstream and downstream of the river . With this pair of clumsy-looking webbed feet, the platypus can even climb over dams as high as ten meters. However, the height of Australia's major dams is more than 10 meters, and it has always been a mystery whether these small animals with webbed feet can climb over them.

The distribution range of the platypus (yellow part) and the main dam (red dot). The research is within the brown frame. Reference: Ask first [1]

To answer this question, scientists selected nine rivers in Australia, extracted DNA from 274 platypuses living near them, and compared the genetic composition of platypuses upstream and downstream of each river .

If platypuses can move freely, the genes of the upstream and downstream populations can flow and exchange through mating, and there will be no difference in genetic composition. On the contrary, if they cannot move freely, the upstream and downstream platypuses can only reproduce internally separately, and the genes cannot flow freely. Over time, some genetic mutations will accumulate within a single population, eventually causing obvious genetic differences between the upstream and downstream platypus populations.

The results of the study found that without a dam, the genes of the platypuses distributed upstream and downstream were not very different; but with a dam, their genetic differences were 4 to 20 times higher . This proves that the dam does prevent the movement of platypuses and also affects the genetic composition of different populations.

These duck-like webbed feet prevent them from swimming across a dam more than 10 meters high｜Field Museum / flickr

This blockage of genetic exchange does not seem to directly endanger the life of the platypus, but it will lead to a decrease in the number of platypuses in the future .

Trapped in fragmented habitats, unable to cope with risks

One of the reasons why the number of platypuses has decreased is the genetic phenomenon of "inbreeding depression". Simply put, inbreeding within a population leads to a decrease in the fitness of the population .

When inbreeding occurs, the genomes of both parents are similar, and some recessive traits are more likely to appear in the offspring; when these recessive traits are not conducive to the survival of the individual, the adaptability of the animals within the population to the environment will decrease, and their numbers will also decrease.

A population with a single genetic composition will also have a lower resistance to catastrophic events. If the genetic composition within a population is complex and diverse, when a random disaster strikes, some individual animals in the population may survive due to their unique traits, allowing the entire population to survive. If the genetic composition within a race is single, when a disaster strikes, the race may be "wiped out".

Platypus baby. If trapped by a dam, platypuses will experience inbreeding, and the offspring's adaptability to the environment may decrease｜cloudfront.net

The presence of dams also limits the ability of platypuses to find new habitats . In an environment without dams, platypuses can freely migrate along rivers to new places if the environment becomes uninhabitable. However, the presence of dams limits such migration, which in turn exacerbates the threat of natural disasters to platypuses.

This situation is actually similar to the " habitat fragmentation " faced by many endangered species. Dams cut off the habitat of the platypus, while other animals' homes may be cut off by roads built by humans or artificial forests that are not suitable for survival. In the end, each population can only be trapped in fragmented habitats, unable to exchange genes or find better habitats.

Platypus in its natural environment | Klaus / Wikimedia Commons

Fortunately, the platypus' plight is not yet beyond repair.

Although platypuses cannot cross high dams by themselves, people can help them. The authors of this study proposed two solutions: on the one hand, humans can build diversions or bypasses near the dams so that platypuses have a way to go; on the other hand, humans can also help platypuses exchange genes , sending platypuses from one population to another to increase the genetic diversity of the population. In each generation of animals, only one foreign individual is needed to minimize the risk of racial isolation.

Of course, a better way would be to avoid these problems in the first place, such as not building new dams near platypus habitats.

After all, platypuses are so unique that we can't afford to lose them.

The platypus in this study | Twitter: @UNSW

References

[1] Mijangos, JL, Bino, G., Hawke, T. et al. Fragmentation by major dams and implications for the future viability of platypus populations. Commun Biol 5, 1127 (2022). https://doi.org/10.1038/s42003-022-04038-9

[2] Anich, PS, Anthony, S., Carlson, M., Gunnelson, A., Kohler, AM, Martin, JG, & Olson, ER (2020). Biofluorescence in the platypus (Ornithorhynchus anatinus). Mammalia, 1(ahead-of-print).

[3] Australia's 'irreplaceable' platypus threatened by dams: study. https://phys.org/news/2022-11-australia-irreplaceable-platypus-threatened.html

Author:Hazel

Editor: Mai Mai

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