9 of the most dramatic examples of sexual dimorphism, MNN — Mother Nature Network

9 of the most dramatic examples of sexual dimorphism

Ever wonder why males and females of the same species can sometimes look radically different from each other? It’s all thanks to a condition known as sexual dimorphism, which is generally triggered by the process of sexual selection through competitive mating. Sexual dimorphism can manifest in many fascinating ways — size, coloration, behavior and the presence of secondary sex characteristics like tail feathers, breasts or antlers.

One of the best examples of this is the mandrill, which is widely considered to be the most sexually dimorphic mammal species. When you examine the images of the male and female mandrills above, one of the first things you’ll notice is that males exhibit a more vibrant coloration on their faces and behinds. However, if you were to encounter one of these majestic primates up-close, you’d quickly realize that the most dramatic difference between their sexes is their size. While the average female mandrill weighs about 27 pounds, some male mandrills can weigh up to 82 pounds!

Mandrills have nothing on the triplewart seadevil, though. Living as deep as 6,600 feet below the ocean’s surface, these creepy-looking anglerfish are arguably the world’s most extreme and downright bizarre manifestations of sexual dimorphism. As exhibited in the diagram below, the females of this species measure about a foot in length, while males barely reach half an inch.


Anthro Exam 2

Термины в модуле (74)

Males do not invest energy in courtship or mating

Males invest heavily in their offspring and in maintaining long-term bonds with their mates

Lactation ends abruptly; females may become sexually receptive almost immediately

Male dominance rank is associated with reproductive success in many studies

Female preferences can influence male mating success

Favors the evolution of traits that allow the limited sex to compete more effectively for access to the limiting sex (females)

Contest competition for mates -> traits that improve fighting success

Selection for large male body size -> body size sexual dimorphism

Selection for large male canine size -> canine dimorphism

greatest in one male, multifemale groups (Gorillas)

Honest signal (more info = increase in fitness of the receiver)

Heterodont dentition
-Incisors, canines, premolars and molars

Anterior teeth

-Cut food

-Tear food, also behavioral function

Posterior Teeth

Extremely diverse, representing 25% of all extant primate genera

Except for thumb and big toe, all digits have claws

b. Food distribution is important because females need enough food to nurse their offspring and nurse them in order to survive, therefore their behavior is influenced by food

most colobine monkeys eat leaves and have enlarged intestines

b. Fruits tend to be less predictable in supply and patchily distributed in space and time

-food sources are clumped in space and time

-there is enough food within defended patches to meet the needs of several individuals

-first come, first serve basis

-some individuals exclude others and obtain more resources

-Teeth can tells us about diet, behavior, and social structure

-Body proportions can tell us about Ecology

-Pelvis, spine can tell us about Obstetrics, locototions

-Hands can tell us about Manual dexterity

-Lower limb can tell us about Locomotion

-Lateral continuity
-If two layers of rock that are chemically similar, but are broken up by something else (think the red sndstoen picture) then they are the same layer but some outside evetns just broke it up. Same layer all the way through

-Recent layers on top, old layers on the bottom

-Cross-cutting relations
-Three layers in a lake basin, then a fourth layers cuts through the other three, a new layer will only occur after the other 3 layers have separated and form a new layer

-Uses the reversals and shifts in direction of earths magnetic field


Structure and distribution of dorsal unpaired median (DUM) neurones in the abdominal nerve cord of male and female locusts

Pflüger, H.-J. and Watson, Alan 1988.

—> Structure and distribution of dorsal unpaired median (DUM) neurones in the abdominal nerve cord of male and female locusts. Journal of Comparative Neurology 268 (3) , pp. 329-345. 10.1002/cne.902680304

Full text not available from this repository.


Dorsal unpaired median (DUM) neurones in insects have been shown to modulate the activity of both skeletal and visceral muscle. It has been suggested that as a population they carry out a role analogous to that of the sympathetic nervous system in vertebrates; however, the extent of their distribution throughout the ventral nerve cord has not been assessed. This paper aims to fill this gap by systematically describing the number and morphology of DUM neurones in each of the abdominal ganglia of male and female locusts. To achieve this, the lateral nerves of each abdominal ganglion were backfilled to reveal the position of the somata of DUM neurones. To confirm their identity and reveal their structure, DUM somata were then impaled with microelectrodes and, after physiological characterization, the neurones were stained by intracellular injection with cobalt ions. In each of the first six abdominal ganglia of both sexes, two DUM neurones, one with axons in the tergal nerve and one with axons in the sternal nerve, were found. In the seventh abdominal, and the terminal, ganglion (composed of the eighth to eleventh neuromeres), there was considerable sexual dimorphism in DUM neurone distribution, which was most marked in those associated with some of the nerves innervating the genitalia. In the male, four clusters of somata in the seventh, eighth, and tenth segments have axons in the genital nerve. In the female, which lacks a genital nerve, clusters of DUM neurones, absent in the male, have axons in the seventh and eighth sternal nerves and the cercal nerve

Item Type: Article
Date Type: Publication
Status: Published
Schools: Biosciences
Publisher: Wiley
ISSN: 0021-9967
Date of First Compliant Deposit: 31 July 2018
Date of Acceptance: 20 August 1987
Last Modified: 02 May 2019 11:28
URI: http://orca.cf.ac.uk/id/eprint/113772

Citation Data

Cited 57 times in Scopus. View in Scopus. Powered By Scopus® Data

Cited times time in Web of Science. View in Web of Science.


Chapter 1 Orientation Towards Hosts in Haematophagous Insects : An Integrative Perspective


A common problem for organisms is the localization, in space and time, of the resources necessary for survival using information from the environment. Haematophagous insects depend on vertebrate blood for their survival and reproduction. Their food sources are thus unique in that it is not in the host’s interests to be bitten (it can defend itself), the host is mobile, and the food circulates inside vessels beneath the host’s skin. An insect’s response to any stimulus depends on its sensory capacity and the way the gathered information is processed by their central nervous system. Here, I discuss how haematophagous bugs acquire and use information associated with their hosts, to efficiently feed on hosts’ blood. I will describe the role of physical and chemical cues in each step of the feeding process, in particular the role of thermal sensing how multimodal cues are integrated to facilitate the exploitation of a potential host. The parsimonious use of the same type of information to exploit different resources in different contexts is also discussed, as are the mechanisms underlying modulation of haematophagous behaviour by an insect’s physiological state.

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What is sexual dimorphism and how does it affect evolution in your opinion?

please explain thoroughly. : )

3 Answers

Darwin’s book The Descent of Man, and Selection in Relation to Sex (1871) contains a long review of sexual dimorphism in the animal kingdom. In humans, it seems, sexual dimorphism has decreased during our evolution. Some species, such as snakes, have almost no sexual dimorphism, but others, most spectacularly the peacock or birds of paradise, take it to remarkable extremes.

Darwin’s main argument for the importance of sexual selection was comparative: his principal evidence came from looking at a large number of species. The comparison showed that the species in which males and females are more similar were more often monogamous. Species with brightly colored, large, or dangerously armed males are more often polygynous: several females mate with one male (and other males do not breed at all).

In a polygynous species a single male can potentially breed with more females than under monogamy; selection in favor of adaptations that enable males to gain access to females (whether by male competition or female choice) is proportionally stronger. Darwin therefore reasoned that polygynous species should have stronger sexual dimorphism than monogamous species.

An extreme example of sexual dimorphism is found in the genus Osedax of polychaete worms, which lives on whale falls. The females feed on the bones of the dead whale; the males live inside the females and do not develop past their larval stage, except to produce large amounts of sperm. In the echiuran Bonellia viridis, exposure to adult females causes larvae to develop into tiny, semi-parasitic males which are swallowed and live inside the female’s genital sac. In the parasitic barnacles Sacculina, the males are tiny, free-ranging animals, whereas the females only exist as a web-like tissue inside their hosts. In the majority of scale insects, females are highly modified (eyeless and wingless, with non-functional appendages and reduced segmentation), attached permanently to their host plants, while males are rather ordinary though delicate insects, smaller and winged.


Here Are 6 Reasons To Be Happy That Humans Don’t Have Sex Like Sharks

So intense.

Sharks are amazing creatures that have existed on this planet for over 400 million years; which is 50 million years longer than trees.

These astonishing fish have a reputation for being much deadlier to humans than they actually are, but make no mistake: their evolutionary success is indeed due to the fact that they are incredible apex predators. These guys are pretty intense in a lot of ways, including reproduction.

1. For sharks, lovemaking is anything but gentle.

When a male shark finds a female he wants to mate with, he makes his intentions very obvious. The male nips the female on her body to let her know what is about to happen, and then flips her upside down, clamps down on her pectoral fin to hold her in place, and he goes about his business. Sometimes, the males can even draw blood.

Not exactly a gentle courtship.

No, as it turns out, the females actually don’t enjoy the process.

2. Some female sharks will avoid unwanted advances from males by hiding in shallow waters.

Because the mating process is really hard on the females, they don’t want to have sex any more than they have to. During the mating season, females can be found much closer to shore because the water is more shallow. This gives the males less of an opportunity to turn them over for sex, resulting in fewer injuries for the female.

When it does come time for the females to want to mate, they are better able to cope with the process because of certain evolutionary adaptations.

3. Females have literally evolved thicker skin to protect themselves from males.

The mating process does leave noticeable scars and pits in the female’s skin, which is much thicker than that of males. Females are generally much larger than the males, which minimizes the damage the male shark is capable of inflicting on them.

The size difference is an example of sexual dimorphism, and is fairly common throughout the animal kingdom. Because females require more resources to bear offspring than males who just crank out sperm cells, the ladies typically need to be bigger.

As if the way the male sharks get the attention of the females isn’t intense enough, their actual reproductive organs are pretty scary themselves.

4. Male sharks have claspers, which is kind of like a double penis.

Only one of the claspers is used at a time, and the males insert it into the female’s cloaca (vagina-like organ). However, that’s not the end of the story. The clasper has spurs that come out sort of like a fish hook to hold the organ in place until the male has successfully inseminated the female. Ouch.

Not all sharks go through this, though.

5. Some shark species are able to avoid this traumatic process by reproducing asexually.

Some captive female sharks have been known to lay unfertilized eggs, which are then able to develop into babies. This asexual process is known as parthenogenesis and isn’t just seen in sharks and other fish, but in reptiles, certain insects, and even birds as well.

It’s assumed that females resort to this when there isn’t access to a male (or one that the female deems as suitable). Researchers haven’t seen this one performed in the wild yet, so there’s no telling how common of practice it may be for sharks. At any rate, it’s a lot more gentle than the alternative.

So what happens after the sharks mate?

6. Shark pups have intense sibling rivalry and can even cannibalize each other in the womb.

Competition for food and resources is pretty fierce for shark pups, and for some, the struggle begins before they are even officially born.

While many sharks lay eggs, some shark species develop offspring inside the mothers and give birth to live pups. The pups that emerge first will eat his or her siblings, taking out the competition for the mother’s resources. However, one female shark can have offspring from several males at once, and those with the same father don’t generally eat each other. Instead, they’ll gang up on those with other fathers, and take them out.

This life cycle is not for the faint of heart.


The sex with the reduced sex chromosome dies earlier: a comparison across the tree of life

Zoe A. Xirocostas

Evolution & Ecology Research Centre, School of Biological, Earth and Environmental Sciences, UNSW Sydney, Sydney, New South Wales 2052, Australia

Susan E. Everingham

Evolution & Ecology Research Centre, School of Biological, Earth and Environmental Sciences, UNSW Sydney, Sydney, New South Wales 2052, Australia

The Australian PlantBank, Royal Botanic Gardens and Domain Trust, Australian Botanic Garden, Mount Annan, New South Wales 2567, Australia

Evolution & Ecology Research Centre, School of Biological, Earth and Environmental Sciences, UNSW Sydney, Sydney, New South Wales 2052, Australia

Zoe A. Xirocostas

Evolution & Ecology Research Centre, School of Biological, Earth and Environmental Sciences, UNSW Sydney, Sydney, New South Wales 2052, Australia

Susan E. Everingham

Evolution & Ecology Research Centre, School of Biological, Earth and Environmental Sciences, UNSW Sydney, Sydney, New South Wales 2052, Australia

The Australian PlantBank, Royal Botanic Gardens and Domain Trust, Australian Botanic Garden, Mount Annan, New South Wales 2567, Australia

Evolution & Ecology Research Centre, School of Biological, Earth and Environmental Sciences, UNSW Sydney, Sydney, New South Wales 2052, Australia


Many taxa show substantial differences in lifespan between the sexes. However, these differences are not always in the same direction. In mammals, females tend to live longer than males, while in birds, males tend to live longer than females. One possible explanation for these differences in lifespan is the unguarded X hypothesis, which suggests that the reduced or absent chromosome in the heterogametic sex (e.g. the Y chromosome in mammals and the W chromosome in birds) exposes recessive deleterious mutations on the other sex chromosome. While the unguarded X hypothesis is intuitively appealing, it had never been subject to a broad test. We compiled male and female longevity data for 229 species spanning 99 families, 38 orders and eight classes across the tree of life. Consistent with the unguarded X hypothesis, a meta-analysis showed that the homogametic sex, on average, lives 17.6% longer than the heterogametic sex. Surprisingly, we found substantial differences in lifespan dimorphism between female heterogametic species (in which the homogametic sex lives 7.1% longer) and male heterogametic species (in which the homogametic sex lives 20.9% longer). Our findings demonstrate the importance of considering chromosome morphology in addition to sexual selection and environment as potential drivers of sexual dimorphism, and advance our fundamental understanding of the mechanisms that shape an organism’s lifespan.

1. Introduction

Sexual dimorphism occurs in many traits and behaviours across the tree of life [1]. For example, in Krøyer’s deep sea angler fish (Ceratias holboelli), the extremely large female dwarves its male counterpart, which is so reduced that it appears as a scrotum anchored to the female’s skin [2]. In many birds (e.g. mandarin ducks, Aix galericulata), male feather patterns and colouration outshine the dull feathers of females [3]. Finally, most stick insects show strong sexual dimorphism, with males tending to be more slender, more likely to have the capacity to fly, and often possessing a remarkable drive to disperse [4]. Differences in morphology, colouration and behaviour can have substantial effects on an organism’s ecology. In this paper, we aim to advance understanding of the mechanisms underlying sexual dimorphism in one of the most fundamental life-history traits of all—lifespan. Specifically, we investigate the possibility that differences in lifespan between sexes are related to the difference in their sex chromosomes.

Sex determination in many organisms is controlled by sex chromosomes. Organisms possessing two identical sex chromosomes (e.g. XX in female humans or ZZ in male birds) are referred to as the homogametic sex. Organisms with differing sex chromosomes (e.g. XY in male humans or ZW in female birds) are known as the heterogametic sex [5,6].

The unguarded X hypothesis [7] suggests that the reduced or absent second sex chromosome in the heterogametic sex (e.g. the Y chromosome in mammals or the W chromosome in birds) might lead to heterogametic organisms being more likely to express undesirable morphological and physiological characteristics. This prediction is based on the fact that any recessive deleterious mutation on the X or Z chromosome is likely to be expressed in the heterogametic sex, while these mutations will generally be masked by the second copy of the X or Z chromosome in the homogametic sex [7–11]. The expression of these deleterious mutations is predicted to decrease longevity in the heterogametic sex.

There is some anecdotal support for the unguarded X hypothesis. For example, in mammals, males are the heterogametic sex and tend to have shorter lifespans than females [12,13]. Conversely, in birds, males are the homogametic sex and are usually longer lived than females [12]. Consistent with the trend of heterogametic mortality, Pipoly et al. [14], found that biases in the adult sex ratio of tetrapod populations were skewed towards the homogametic sex, as would be expected if the heterogametic sex had higher rates of early mortality and thus shorter lifespans. However, some researchers have questioned the influence of the unguarded X hypothesis, pointing out that lifespan differences between sexes are not solely genetically fuelled [13,15], but are also influenced by a combination of parental investment, exposure to predators, sexual selection and other biotic factors [13,15].

Alongside the unguarded X, other mechanisms such as the ‘toxic Y’ may shorten male lifespan as seen in Drosophila, in which the Y chromosome can impact the gene expression of other chromosomes and cause deleterious mutations to arise [16]. Cellular mosaicism, where somatic mutations cause cells to have differing genotypes, could also explain the role of sex chromosomes in heterogametic mortality [15]. This mosaicism could see entire chromosomes lost, gained and internally rearranged, and as an organism ages, its mosaicism increases, and at higher rates in sex chromosomes than in autosomes [17–21]. Complete chromosome loss may occur in both the X and Y chromosomes, but in females this usually occurs in the inactive X, lessening the genetic consequences as gene expression can still occur in the remaining active X chromosome [21]. Yet in males, both X and Y chromosome loss could garner more issues as males lack a second copy of either sex chromosome, thus increasing the risk of unfavourable gene expression and decreasing longevity [22].

In this study, we aim to extend previous work by determining whether the predictions of the unguarded X hypothesis are upheld not only amongst birds and mammals [12,13], which differ in many respects other than their sex chromosomes, but across the tree of life. Specifically, we ask whether the heterogametic sex tends to have reduced longevity relative to the homogametic sex. The increased phylogenetic span of this study relative to previous work is important, as it gives us greater power to disentangle the importance of sex chromosomes from the idiosyncrasies of particular clades. However, this is a correlative study, and as such cannot prove causation.

2. Material and methods

We began by downloading sex chromosome data from the Tree of Sex database (http://treeofsex.org/ accessed 1 February 2018) [23]. We excluded species that do not possess male or female heterogametic sex determination, including hermaphroditic species (as these organisms do not have two separate sexes for which we could compare longevity). Sex reversing organisms, and species that undergo environmental sex determination were also excluded because their chromosomes do not wholly determine their final sex [24].

We compiled lifespan data from peer-reviewed articles, books, encyclopaedias and databases (including the Animal Diversity Web; https://animaldiversity.org/ accessed 9 to 25 July 2019). An extensive search of online databases including the Web of Science and the Zoological record was conducted using keywords such as ‘lifespan’, ‘longevity’ and ‘age’ accompanied by species name and/or higher taxonomic ranking (using the included taxa from the Tree of Sex, e.g. ‘Cetonia aurata’ or ‘Coleoptera’). Lifespan data (measured in days, weeks, months or years) were recorded for each species. We included several different metrics for lifespan, including mean lifespan, maximum lifespan and median lifespan. We also included data recorded from both wild and captive individuals. Species were only included in our dataset if longevity data were available for both males and females, and the data type (mean/median/maximum; captive/wild; days/weeks/months/years) was consistent across the males and females within a comparison.

In some cases, suitable longevity data were found for taxa that were not included in the Tree of Sex database [23]. In these cases, we searched the Web of Science using the keywords ‘karyotype’ or ‘sex determination’ or ‘sex chromosomes’ accompanied by the species’ name. If the species was known to have either male or female heterogametic sex determination, it was included in our dataset, otherwise, it was excluded from the analysis.

The final dataset included longevity data for 229 animal species spanning 99 families, 38 orders and eight classes. The full dataset is available in the electronic supplementary material, appendix S1.

(a) Statistical analyses

To determine whether the homogametic sex lived longer, we calculated the log response ratio of the longevity of the homogametic sex versus the heterogametic sex for each species:

Before running our main analyses, we checked whether the type of data collected (mean, maximum or median age), or the circumstances under which individuals live (captive versus wild) affected the difference between the homogametic lifespan and heterogametic lifespan. We performed separate linear models with the log ratio of the difference between the homogametic lifespan and heterogametic lifespan (equation (2.1)) as the response variable and either the data type or lifestyle as the categorical predictor variable. Neither data type (p = 0.287) nor species’ growing conditions (p = 0.128) significantly affected the difference in lifespans between homogametic and heterogametic sexes. We, therefore, excluded these terms from further analyses. Where there were multiple data types (mean, maximum and/or median) available for a species, we used the mean data.

Next, we asked whether phylogenetic relationships between species might affect our results. A phylogenetic tree was sourced from PhyloT (https://phylot.biobyte.de/) [25], an online tree generator that uses the National Center for Biotechnology Information taxonomy database [26,27]. Using this tree with branch lengths calculated using the compute.brlen function in the ape package in R (see electronic supplementary material, appendix S2 for phylogenetic tree), we analysed the phylogenetic signal (i.e. the tendency of closely related species to have similar traits; [28]) of ln(homogametic lifespan/heterogametic lifespan) using the phylosig function in the phytools package in R [29]. We found mixed evidence for phylogenetic signal (Pagel’s λ = 0.413, p > 0.1; Blomberg’s K = 0.028, p 2

Figure 1. Lifespan dimorphism (ln(homogametic lifespan/heterogametic lifespan)) across all species analysed in our data. Phylogeny of species included to organize the species into categories including (from top to bottom): primates and Homo sapiens, Rodentia (rodents), pinnipeds (seals), carnivorous mammals, e.g. Panthera leo, ungulates (hoofed mammals), cetaceans (whales and dolphins), marsupial mammals, Aves (birds), reptiles and amphibians, Fish (both Chondrichthyes and Actinopterygii), beetles, Diptera (flies and mosquitoes), Lepidoptera (butterflies and moths) and other invertebrates. (Online version in colour.)

4. Discussion

Our study provides evidence that, across multiple taxa, the heterogametic sex tends to have a considerably shorter lifespan than the homogametic sex. That is, an organism’s chromosome morphology seems to have a substantial role in shaping this key life-history trait. The 17.6% difference between the lifespans of homogametic and heterogametic sexes revealed here is substantial enough to have major ecological and evolutionary implications. However, heterogametic sex chromosomes include everything from a complete absence of the second sex chromosome (X0 or Z0), to a highly reduced second sex chromosome (e.g. XY in humans), to X and Y or Z and W chromosomes of nearly equal length [5,32,33]. As not all heterogametic species have a degraded sex chromosome, our study likely represents a conservative test of the unguarded X hypothesis. A future direction will be to formally test the hypothesis that the difference in lifespan between sexes is proportional to the proportional difference in chromosome length between sexes. That is, to test the idea that species in which the second chromosome is absent or extremely reduced have a greater reduction in the lifespan of the heterogametic sex than do taxa in which the difference between sex chromosomes is relatively small. Ideally, this question should be addressed using a diverse range of taxa, both for generality, and to include species with as many different chromosome configurations, life histories and mating systems as possible. Another interesting direction for future research would be to begin to quantify the relative contributions of factors such as chromosome morphology, sexual selection, parental investment and exposure to predators.

Our second major finding was that when males are the heterogametic sex, they die 20.9% earlier than their female counterparts, but when females are the heterogametic sex, they die only 7.1% earlier than their male counterparts. Three possible explanations for this surprising trend include: (1) the degree of degradation of the Y chromosome, (2) telomere dynamics, and (3) side effects of sexual selection.

It is possible that the Y chromosome in male heterogametic species might tend to be more degraded than the W chromosome in female heterogametic species, potentially leading to a difference in heterogametic lifespan between XY and ZW systems. We know that many mammals (including humans) have highly reduced Y chromosomes [33–35]. There is also evidence that the relative length of the W and Z chromosomes can vary substantially even within clades (e.g. birds, snakes; [6,36–39]). However, a comparative analysis of the degradation of chromosomes across the tree of life has not yet been performed.

Telomeres are sections of non-coding DNA at the ends of chromosomes that protect coding DNA from deterioration during cell replication and other cellular processes [40,41]. Cell replication damages telomeres and studies suggest that the loss of telomere length over time causes the progression of ageing and shortening of lifespan [42]. However, oestrogen stimulates a promoter of the telomerase enzyme [43], which heals damaged telomeres by adding telomeric base pairs to its ends and indirectly activates other DNA repairing pathways [40]. Although we do not know whether oestrogen is important in all of our study species, it is possible that the effect of oestrogen on telomerase activation could help to explain the smaller decrease in lifespan when females are the heterogametic sex.

In many cases, males experience more intense sexual competition than females, as they are more reproductively efficient and so take more risks when pursuing a mating opportunity (e.g. males fighting for access to females or to establish their territory) [44,45]. Usually, females are not as efficient at reproducing, contribute more to their offspring than fathers, and so are predicted to engage in lower-risk behaviours [44–46]. Higher mortality in males owing to side effects of sexual selection, in combination with the effect of sex chromosomes on longevity, could also explain why there is a smaller lifespan difference between ZW females and ZZ males in comparison with XY males and XX females [11,15,44].

Understanding the mechanisms underpinning the substantial difference in lifespan dimorphism in male versus female heterogametic species is an important direction for future research, as this may improve our understanding of the factors that affect ageing. There is a multibillion-dollar industry in extending human lifespan [47], however, there is a crucial knowledge gap and we have much to learn about the basic biology underpinning longevity and the drivers of lifespan differences across sexes and species. Here, we have provided the first evidence that the heterogametic sex does, on average, die earlier than its homogametic counterpart across a range of taxa. We also found that lifespan dimorphism between the sexes is greater in male heterogametic species in comparison with female heterogametic species. These findings are a crucial step in uncovering the underlying mechanisms affecting longevity, which could point to pathways for extending life. We can only hope that more answers are found in our lifetime.

Data accessibility

The complete raw dataset is presented in the electronic supplementary material, Appendix S1. It is also available on Dryad (https://doi.org/10.5061/dryad.tmpg4f4vk) [48], and the code is available at https://github.com/SEveringham/UnguardedX.

Authors’ contributions

Z.A.X. led the project, including data compilation, data analysis, interpretation of results and manuscript preparation. S.E.E. contributed to data analysis, figure preparation and manuscript preparation. A.T.M. came up with the original idea, and contributed to interpretation of the results and manuscript preparation. All authors approved the final version of the manuscript and, agree to be held accountable for the content of this publication.

Competing interests

We declare we have no competing interests


We received no funding for this study.


Thanks to Ellen Jacobs for contributions to data compilation, Russell Bonduriansky for discussions of the ideas, Eve Slavich for help with analyses, and Claire Brandenburger, Karen Zeng and Alex Sentinella for comments on earlier drafts of this work. This research was originally conducted as part of Z.A.X.’s honours degree at UNSW Sydney.


Electronic supplementary material is available online at https://doi.org/10.6084/m9.figshare.c.4860624.

Published by the Royal Society. All rights reserved.


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