Meet 5 of Australia’s tiniest mammals, who tread a tightrope between life and death every night

Andrew Baker, Queensland University of TechnologyAustralia has a rich diversity of mammals, with around 320 native, land-based species, 87% of which are found here and nowhere else. Many of these mammals are secretive, only active at night, and small, weighing less than one kilogram.

Mammals are “endotherms”, which means they must generate their own heat and maintain the temperature within a narrow range. This requires a lot of food.

For small mammals, which have a high surface area to volume ratio, the energetic cost is even higher. This makes them particularly prone to heat gain and loss, putting them in peril every night.

The silver-headed antechinus, which weighs up to the equivalent of six $1 coins.
Gary Cranitch/QueenslandMuseum, Author provided

So how on earth do they cope?

Well, there are some advantages to being small. It’s harder to be seen by predators, and there are more places to hide. If the soil type is right, there’s no shortage of cracks and holes to slip into.

Such habitats not only keep small mammals concealed from predators during the day and parts of the night, but the temperature and humidity is also more stable underground, which means they expend less energy in maintaining body temperature.

To further conserve energy, many small mammals will also enter “torpor” — an inactive period that slows down their energy-burning metabolism. Torpor is like a mini hibernation that typically lasts for hours, rather than days.

A long-tailed planigale feasting on a grasshopper. In the corner, you can see it sitting on scientist Euan Ritchie’s finger for scale.
Euan Ritchie

For small mammals — prone to losing heat and needing to catch and eat up to half their body weight in food each night — having some periods of down-time during energy-conserving torpor can mean the difference between life and death.

In addition to the nightly challenge of finding enough food to maintain a stable body temperature, keep a complex brain functioning and have enough energy to move up to several kilometres, Australia’s small mammals face a host of human-caused threats. These include habitat clearing, climate change and feral predators.

The combined pressures have too often proven insurmountable. With 34 species lost forever, Australia has the worst modern-day mammal extinction record of any country on Earth.

So how can we turn this appalling situation around?

First, we humans must appreciate these unique animals and decide they need to be saved. That requires knowledge and understanding, so let’s get to know some of these mysterious mammals a little better.

1. Long-tailed planigale (Planigale ingrami)

Weight: 2.6-6.6 grams (up to two 10c coins)

Can you imagine a mammal that can weigh less than a ten-cent piece yet leaps five times its own height to bring down prey far larger than itself with persistent, savage biting to the head and neck?

This is the long-tailed planigale, the smallest Australian marsupial and one of the world’s smallest mammals.

Long-tailed planigale
Long-tailed planigales may be tiny, but they’re ferocious predators.
Anders Zimney, Author provided

They are ferocious predators, and anything that can be subdued is viciously attacked, including large centipedes, spiders, insects, small lizards, and even other small mammals.

They live in narrow crevices of cracking clays in blacksoil plains and move below and above the surface at night in search of food. Here, they run the risk of being eaten by predators, such as owls and feral cats.

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The conversion of grassland to agriculture and cattle grazing causes the soil to become compacted, which also poses a threat to this species.

2. Little forest-bat (Vespadelus vulturnus)

Weight: 2.6-5.5 grams (up to two 10c coins)

The little forest-bat is a denizen of various forest types found throughout southeastern Australia.

Its activity depends on temperature — in some parts of southern Australia, during cold periods, individuals may not emerge from roosts for several weeks.

Profile of the little forest-bat
When it’s cold, the little forest-bat won’t emerge from roosts.
Chris Lindorff CC-BY

This species feeds exclusively on flying insects, including moths and mosquitoes.

And they’re not considered threatened — unlike most Australian mammals, they appear to be tolerant of disturbance and will utilise agricultural or urban landscapes if no woodland habitat is available.

3. Eastern pebble-mouse (Pseudomys patrius)

Weight: 10-19 grams (up to seven 10c coins)

This is one of four species of tiny native mice that construct mounds of pebbles that comprise conical, volcano-like ramparts built around burrow entrances. This is unique behaviour among the world’s mammals.

The pebble mounds can be large, weighing more than 50 kilograms and encompassing 10 square metres — astonishing constructions given the architects weigh as little as 10 grams!

Eastern pebble mouse with a pebble in its mouth
Mouse-built pebble mounds can weigh more than 50kg.
Anders Zimny, Author provided

Mounds are energetically expensive to build. They are a critical limiting resource for eastern pebble-mice because females raise their litters in the mounds and their female offspring tend to disperse only as far as the next available mound. Some mounds may even remain in use for centuries, re-used by successive generations.

The erosion of hills and spread of dune fields in arid Australia are reducing the distributions of pebble-mice.

4. Mountain pygmy possum (Burramys parvus)

Weight: 30-82 grams (up to nine $1 coins)

The famously adorable mountain pygmy possum is the only Australian mammal limited to alpine and sub-alpine regions, where snow covers the ground for up to six months of the year.

The possums may move more than one kilometre each night in search of food, which includes seeds, fruits, spiders and insects. They have a preference for Bogong moths (Agrotis infusa).

They double their body weight prior to hibernation, which lasts between five and seven months. During this time, their body temperatures may drop down to 2℃ for up to 20 days at a time.

This species is endangered, and there may be as few as several thousand individuals in total across three isolated populations.

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Their biggest threats include droughts due to climate change, predation by feral cats and foxes, and habitat destruction, particularly after the devastating 2019-20 bushfires razed 15% of the species’ range.

5. Silver-headed antechinus (Antechinus argentus)

Weight: 16-52 grams (up to six $1 coins)

The 15 species in the genus Antechinus are “suicidal reproducers”. All males drop dead at the end of the breeding season, poisoned by their own raging hormones.

This is because the stress hormone cortisol rises during the two-week breeding period. At the same time, surging testosterone from the super-sized testes in males causes a failure in the biological switch that turns off the cortisol. The flood of unbound cortisol results in systemic organ failure and the inevitable death of every male.

But this happens only after they’ve unloaded their precious cargo of sperm, mating with as many promiscuous females as possible in marathon sessions lasting up to 14 hours.

Profile of the silver-headed antechinus
Antechinus species are famous for their marathon breeding sessions.
Gary Cranitch/Queensland Museum, Author provided

Silver-headed antechinuses are found only patchily in a few isolated populations of high-altitude wet forest in mid-eastern Australia. They eat mostly insects and spiders and are likely preyed upon by owls and feral cats.

The silver-headed antechinus is endangered and threatened by climate change. The species lost almost one-third of its core habitat in the 2019-20 megafires.

Yet, torpor can assist here as well, even after such extreme events. Antechinuses (and other small mammals) are known to use torpor more often after fire, when food is scarce and the risk of predation is higher, as there are fewer places to hide in a scorched landscape.

Read more:
Animal response to a bushfire is astounding. These are the tricks they use to survive

The Conversation

Andrew Baker, Senior Lecturer, Queensland University of Technology

This article is republished from The Conversation under a Creative Commons license. Read the original article.


Tiny Game of Thrones: the workers of yellow crazy ants can act like lazy wannabe queens. So we watched them fight

Wes Mountain/The Conversation, CC BY-ND

Pauline Lenancker, James Cook University and Lori Lach, James Cook UniversityThe invasive ant world is a competitive one, rife with territorial battles and colony raids. And yellow crazy ants (Anoplolepis gracilipes), one of the world’s worst invasive species, have an especially interesting trait: they’re the only invasive ant known to have workers who can reproduce.

Worker reproduction has big implications for a colony’s social dynamic. So we observed and experimented with more than 200 captive colonies of yellow crazy ants to understand what triggers worker reproduction and the potential costs and benefits for the colony.

We used a range of techniques, including removing queens and observing worker behaviour, and setting up ant gladiator rings to test how well reproductive workers fought other ants.

It wasn’t just for fun — learning about ants’ basic biology, including reproduction, may allow us to better understand their success and tailor management programs to help save the ecosystems they threaten.

Life in the queendom

Yellow crazy ants are thought to originate in southern or southeastern Asia but have spread across much of the Indo-Pacific, including several locations in Australia. They’re most well known for the cascade of ecological effects they’ve caused on Christmas Island by killing red land crabs and contributing to the damage, such as tree die-back, caused by scale insects.

Attempts to control or locally eradicate them are ongoing on Christmas Island, in Arnhem Land, and several locations in Queensland, including in and around the Wet Tropics World Heritage Area.

Yellow crazy ants, accidentally introduced by cargo ships, and subsequently multiplying to number in the billions, threaten the yearly crab migration on Christmas Island.

Like honey bees and wasps, yellow crazy ants are social insects. In these colonies, queens, workers and males all play distinct roles.

Queens and workers are all females. The queens reproduce, while the aptly named workers are the colony’s labourers, primarily responsible for bringing in food, caring for the queens’ offspring and defending the colony. The sole role of males is to mate with a queen before dying.

This elaborate task division is thought key to the success of social insects. However, in yellow crazy ant colonies, workers challenge the reproductive monopoly of the queen and produce males.

We could differentiate workers with active ovaries from regular workers by looking at their abdomen, which would be oversized as eggs take up space in the larger workers’ abdomen.

Five yellow crazy ants, four of which have large abdomens
In this picture, the worker ant on the far right has a regular-sized abdomen while the other workers have abdomen that looks swollen.
Dr Peter Yeeles, Author provided

When the queen was present, typically less than 20% of workers in our captive colonies had oversized abdomens. When we removed the queen, as much as half of the workers became oversized. We returned the queen after two months, and found the number of oversized workers decreased.

Our findings are consistent with the idea queens inhibit worker reproduction through pheromones, one of many chemical signals in ant colonies influencing ant worker behaviour and colony dynamics. Indeed, an ant queen’s failure to “smell” fertile may leave her subject to eviction or execution.

More lazy than crazy

So, did our yellow crazy ant queen wannabes behave more like workers or royalty? Our observations of oversized and normal workers revealed stark differences in behaviour.

Regular workers foraged during 85% of observations, whereas oversized workers were seen looking for food in only 5% of observations. Most of the time, oversized workers were immobile and remained sheltered inside their nests.

Three yellow crazy ants
Regular-sized workers are territorial and aggressive.
Peter Yeeles, Author provided

These oversized workers are slow to move when the nests are disturbed, not displaying the fast, erratic movement for which the species is named. Their behaviour was more similar to queens than workers.

Colony and resource defence is another important task for workers, as yellow crazy ant colonies often compete with native ants.

To test how these sluggish workers compare to normal workers in colony defence, we placed three oversized workers in one container, three regular workers in another, and paired each group with one gladiator, the charismatic green tree ant.

Green tree ants (Oecophylla smaragdina) are native and known for being very aggressive and territorial.

Our two videos show the typical response of oversized and regular workers.

In the first video, each encounter between a yellow crazy ant and green tree ant ends with the green tree ant rapidly retreating, often after having her legs bitten and pulled by the yellow crazy ant.

In the second video, you can see how oversized workers were more sedentary, less aggressive and less likely to start fighting with the green tree ant than normal workers in the first video. They were also less likely to kill their oppon-ant.

It seems oversized workers are lazy and would be ineffective at defending the colony. So why do they occur at all?

Like walking vending machines

Generally, ant colonies need workers to function and only the queen can produce this caste. In the ant world, the death of the queen signifies the death of the colony.

However, if the queen dies after laying eggs, including one destined to become a queen, then the virgin queen who eventually emerges can mate with a worker-produced male. This is important because males are unlikely to be present unless the colony is very large.

So while workers lack organs for receiving and storing sperm, their ability to produce males asexually may extend the life of the colony.

What’s more, oversized workers can produce sterile eggs as well, which serve as food for the queen and other colony members. We believe these workers may be like walking vending machines within the colony, providing food when conditions aren’t suitable for foraging.

A male yellow crazy ant with one female eye and one male eye.
A male yellow crazy ant with one female eye and one male eye.
Pauline Lenancker, Author provided

We also found males with mismatched eyes. These odd-looking individuals may possess a female eye on one side and a smaller male eye on the other side.

Such individuals are potentially sex mosaics, with male and female genes spread across their body in patches. Whether these individuals function as normal males is a question for further research.

What’s next?

Researchers don’t know the full story of yellow crazy ant reproduction, but it’s likely to be highly complex and potentially unique. Our study contributes to solving this mystery.

Eradication and control programs for yellow crazy ants will benefit from understanding their reproductive system and behaviour. It can shed light on how even a few workers and eggs — who may be inadvertently moved around by humans or persist after control treatment — could eventually build into large numbers.

Likewise, understanding foraging behaviour is useful for planning insecticidal baiting, because effective baiting relies on foraging ants bringing bait back to the colony to share with queens and larvae.

We have no doubt future genetic work and experiments will shed further light on the fascinating reproductive biology of yellow crazy ants.

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The Conversation

Pauline Lenancker, Research scientist, James Cook University and Lori Lach, Associate Professor, James Cook University

This article is republished from The Conversation under a Creative Commons license. Read the original article.