A Mammal That Dies After Mating Only Once

In biology, reproductive strategies can broadly be divided into two categories based on how many times a species reproduces during its lifetime: semelparity and iteroparity. Semelparous species reproduce only once before dying, whereas iteroparous species reproduce multiple times over the course of their lives. Many people learn early on that salmon migrate back to the rivers where they were born to spawn and then die shortly afterward. Because reproduction is followed immediately by death, this strategy is often perceived as inefficient or extreme. Yet in many groups of animals such as arthropods, molluscs, and fishes, this life history pattern is quite common. Among mammals, however, it has received far less attention, largely because it is genuinely rare.

Reproduction and parental care are energetically demanding processes for any organism. Every species must balance the number of offspring produced with the probability that those offspring will survive. Semelparous species take a particularly dramatic approach to this balance: rather than distributing reproductive effort across many seasons, they invest virtually all available energy into a single reproductive event.

Among mammals, semelparity has been documented in certain marsupials, including some species in the family Dasyuridae and a few in the family Didelphidae. One of the most striking examples occurs in the Australian genus Antechinus. In these animals, all males die within one to three weeks after the mating season, typically at around eleven and a half months of age. Females, in contrast, survive long enough to gestate and raise their young and may even reproduce again in subsequent seasons.

Members of the genus Antechinus are small carnivorous marsupials that resemble shrews or mice in appearance, though they are not closely related to either group. Fifteen species are currently recognized in the genus, and all are characterized by this single-breeding life history. Body size varies among species, with total length ranging roughly from 12 to 31 centimetres. Males are generally larger than females. One of the smallest species is Antechinus agilis, whereas one of the largest is Antechinus swainsonii. Most species build nests in tree hollows.

Antechinus agilis（Image source：Michael Sale， CC BY 2.0 ）

Antechinus swainsonii（Image source：Museums Victoria， CC BY 2.0 ）

Compared with many other marsupials, their pouch structure is relatively simple. Instead of a fully enclosed pouch, females possess only a fold of skin that partially covers the nipples. Even within a single species, the number of nipples can vary among females. Interestingly, females with fewer nipples are more likely to survive until the next breeding season. This pattern reflects the energetic burden of raising young: the number of nipples determines how many offspring can be nursed simultaneously, and raising fewer young reduces the physiological strain of reproduction.

These animals also exhibit a promiscuous mating system. Both males and females may mate with multiple partners, and in some species a single mating event can last for many hours.

Baby antechinuses inside the mother's pouch（Image source：John Baumgartner ， CC BY 2.0 ）

What exactly causes the death of males after mating? The mechanisms that lead to post-reproductive death differ greatly among species and cannot be generalized across all semelparous organisms. In male Antechinus, the process begins with dramatic hormonal changes during the breeding season. Testosterone levels rise sharply, and this surge interferes with the function of a protein called corticosteroid binding globulin (CBG), which normally binds and regulates stress hormones. As CBG becomes less effective, the level of free corticosteroids circulating in the blood increases substantially. Humans possess a similar hormonal interaction between testosterone and corticosteroids, but the physiological consequences are far less severe.

Corticosteroids are powerful molecules involved in regulating stress responses and suppressing inflammation. The body naturally produces them, but synthetic corticosteroids are also widely used as medications to treat inflammatory conditions. When corticosteroids are taken in excessive amounts, they can cause side effects such as acute vomiting, abdominal pain, and confusion. Long-term overexposure can lead to disorders such as Cushing's syndrome, muscle weakness, and severe suppression of the immune system, increasing susceptibility to infection.

Symptoms of Cushing's syndrome（Image source：Hariadhi， CC BY-SA 4.0 ）

In male Antechinus, however, the consequences of elevated corticosteroids are far more catastrophic. As breeding progresses, corticosteroid levels continue to rise, eventually triggering systemic physiological collapse. The immune system deteriorates, and the body becomes vulnerable to a wide range of infections. Studies have documented infections by bacteria such as Listeria monocytogenes, viruses including dasyurid herpesvirus-1, and various parasites in dying males. Because survival beyond the breeding season is extremely unlikely, the following year's mating season typically involves an entirely new generation of males.

Researchers have also discovered another remarkable effect of the hormonal surge. Elevated corticosteroid levels are associated with the accumulation of beta-amyloid plaques in the brain of male Antechinus. These plaques are one of the pathological hallmarks of Alzheimer's disease in humans. As a result, these animals may serve as valuable natural models for studying aspects of neurodegenerative disease.

The genus Antechinus first appeared roughly eleven million years ago, and its unusual reproductive strategy evolved independently within this lineage. Closely related animals such as those in the genus Murexia, which inhabit Papua New Guinea, follow a different strategy and reproduce multiple times throughout their lives.

Today, some Antechinus species face conservation challenges. The black-tailed dusky antechinus (Antechinus arktos) and the silver-headed antechinus (Antechinus argentus) are currently classified as endangered. Their decline is primarily driven by habitat destruction and the spread of invasive species. Logging and human-induced wildfires destroy the forest environments these animals depend on for nesting and shelter. Domestic cats and dogs may prey on them, while introduced rodents compete with them for food and other resources. These pressures have led to significant population reductions, making the conservation of these unusual mammals an increasingly urgent concern.

Antechinus arktos；Courtesy of Gary Cranitch, Queensland Museum

Antechinus argentus；Courtesy of Gary Cranitch, Queensland Museum

Author: Shui Ye-You

Reference:

Tian R et al. (2022). A chromosome-level genome of Antechinus flavipes provides a reference for an Australian marsupial genus with male death after mating. Mol Ecol Resour.

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