Image credit: Australian Museum
One of Australia's small animals, the brown bag shrew, is known for having the craziest and deadliest mating methods of mammals. This mating style comes at the expense of the death of all male brown bag shrews, but the species survives. Some studies have found that a warming climate and forest fires in Australia are seriously threatening the survival of the brown bag shrew.
In Cheating Time: Science, Sex and Aging, popular science writer Roger Gosden writes that in Australia, it's impossible to find a male possum after the August mating season. He described a mouse-like mammal, the Brown Bag Stuartii. Not only is it the smallest mammal, but it is also one of the very few species of possums including it, semelparity (once in a lifetime) of mammals.
< h1 class = "pgc-h-arrow-right" > male brown bag shrew forced to die</h1>
Every year, before spring arrives in Australia, all 11-month-old male brown bag shrews die. Previously, brown bag shrews had a mating period of up to 3 weeks. In order to have more offspring, the male brown bag shrew will mate with the females one after another during this period until they are exhausted. In the most extreme cases, male brown bag shrews die due to lack of energy to maintain a basic immune response. Their death was tragic: blind in both eyes, all the fur on their bodies fell off. The collapse of the immune system causes them to eventually die of gangrene (infection by secondary putrefactive bacteria after tissue necrosis) and tissue ulceration. In mammals, this deadly mating behavior is extremely rare and therefore impressed scientists.
The reason males are so desperate is that they only have a mating period once in a lifetime. This is common in insects and plants, such as the most primitive winged insect, the Ephemeroptera, a prime example. But what puzzles scientists is that the brown bag shrew can take a more complex and prolonged mating period like other mammals, so why would it choose a one-time breeding method similar to that of insects? In a previous study published, the researchers explored in detail why this behavior occurs in brown bag shrews and why this sexual competition only leads to male deaths.
Combining actual observations and models, they evaluated multiple theories previously proposed by scientists and believed that the reason for these phenomena was the combined effect of the environment and intra-population competition. First, the brown bag shrew (which feeds on a variety of insects) only has a surplus of food in the spring. It is exactly after the mating period in the spring, and when there is a lack of males in the population, food competition decreases and the female brown marsapper shrews and pups have more food. But the researchers believe that these male sacrifices may not be voluntary, more likely because the males themselves have a low chance of surviving mating, and the damage and age of mating also deprive them of some competitive advantage.
The brown bag shrew's habitat in Australia is derived from the IUCN Red List of Threatened Species. Image taken from Chermundy, Wikimedia Commons
In addition, they found an unexpected but crucial cause: the death of the male brown bag shrew was largely attributable to the females in the population. All female brown bag shrews have a uniform and fixed mating period, and they mate with multiple males during mating. In order to adapt to the changes in females, a single male brown bag shrew had to work to improve the survival rate of its own offspring, which in turn prompted the evolution of this special and intense mating mechanism.
< h1 class= "pgc-h-arrow-right" > strange physiological changes</h1>
The male brown bag shrew also evolved a physiological change that most mammals do not have. Before the mating period, their testicles are slowly degraded by the body and no longer produce sperm. That is to say, their sperm count will not increase, but will disappear little by little with the urine. When mating time comes, the instinct to reproduce drives them to mate with more females. But the competition between these male brown bag shrews is not a physical struggle, but rather a struggle to increase the likelihood that their own sperm will "win" in sperm competition (the competition of sperm from different individuals in the same female). In order for their sperm to be successfully fertilized, they need to try to mate as long as possible. Some scientists have observed that brown bag shrews can take up to 14 hours to mate in a single time.
Some researchers have also found that male brown marskets have extremely high levels of testosterone and cortisol (also known as "stress hormones") in their blood before they die. Among them, testosterone allows the brown bag shrew to mobilize all the energy in the body to power reproductive behavior, and the increase in cortisol gives them a stronger ability to endure exhaustion and pain. But the consequences can be imagined, in the late mating period, due to excessive consumption, the ensuing side effects make the male brown bag shrew die tragically.
Diana Fisher, a professor at the University of Queensland, has conducted a detailed study of the evolution of reproductive behavior in male brown bag shrews. She said that for these male brown bag shrews, there is only a short period of time a year with enough food, so they must be short-lived, and it may be the best strategy for them to devote all their energy to a crazy, deadly breeding period. The so-called fast life and early death are also an adaptive evolution.
<h1 class= "pgc-h-arrow-right" > a greater existential threat</h1>
While the total death of males for a while does not threaten the continuation of these marsupial populations, a larger environmental threat could bring them to the brink of extinction. According to a study published last year, scientists at the University of New England in Australia and others said that due to global warming, the ability of possum pups growing in higher temperatures to regulate body temperature and metabolism declines when the environment changes.
Image credit: Queensland Fire and Emergency Services
Since these possums are lifelong mammals, they may experience a full population turnover in a little over a year. The effects of temperature changes on body temperature regulation may be more serious threats to them. The researchers say the change may prevent these tasupers from going into hibernation when they encounter cold environments where food is scarce. In addition, global warming and drought are exacerbating forest fires in Australia. When a large number of insects die in the fire, the tasmanian shrew will also be precarious.
(Written by |.) Shi Yunlei)
Reference links: https://www.nationalgeographic.com/science/phenomena/2013/10/07/why-a-little-mammal-has-so-much-sex-that-it-disintegrates/
https://www.wired.com/story/antechinus-climate-change/
Article links:
https://www.pnas.org/content/110/44/17910
https://www.frontiersin.org/articles/10.3389/fphys.2020.00049/full