Abstract Freshwater biodiversity is imperiled across the globe, and multiple stressors such as habitat alteration, non‐native species invasion. Key words: dioecy; female bias; Salicaceae; Salix (willow); sex ratios; spatial sex segregation. . Willows growing in these relatively extreme environments. Little did I know that the fantasies roiling in my head would soon rage on in real life We got in the Cavalier and rolled up to Alpine, a mountainous village on the.
It was shown that in the alpine marmot offspring sex ratio was biased towards the Extreme adaptive modification in sex ratio of the Seychelles warbler's eggs. Alpine Holiday It was like any other vacation; me and my family going up to visit the relatives for a birthday party, and coming home afterwards. It was my aunt's. Little did I know that the fantasies roiling in my head would soon rage on in real life We got in the Cavalier and rolled up to Alpine, a mountainous village on the.
Alpine Holiday It was like any other vacation; me and my family going up to visit the relatives for a birthday party, and coming home afterwards. It was my aunt's. Directed by Jean-Yves Le Castel. With Gina Blonde, Kelly White, Kyra Cat, Patricia Diamond. A group of young women on a skiing trip in the Alps discover the. Key words: dioecy; female bias; Salicaceae; Salix (willow); sex ratios; spatial sex segregation. . Willows growing in these relatively extreme environments.
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It should be advantageous under stressful high altitude or latitude environment where short growing seasons, low temperatures, low pollinator activity or unstable weather may hamper sexual reproduction. However, this hypothesis remains largely untested. We found only 12 apomictic species, including several members of Poaceae Festuca extreme, Poa and StipaRosaceae Potentilla and Extreme HalerpestesRanunculuswhich are families typical for high apomict frequency. However, several apomictic species were newly discovered, including the first known apomictic species from the family Biebersteiniaceae Biebersteinia odoraand first apomicts from the genera Stipa Stipa splendens and Halerpestes Extrene lancifolia.
Extreme showed no preference for higher elevations, even in these extreme Himalayan alpine habitats. Additional trait-based analyses revealed that apomicts differed from sexuals in comprising more rhizomatous graminoids and forbs, higher soil moisture demands, sharing the syndrome of dominant species with broad geographical and elevation ranges typical for the late-successional habitats.
Apomicts differ from non-apomicts in greater ability of clonal propagation and preference for wetter, more productive habitats. There is a general opinion that gametophytic apomixis, the mode of plant reproduction via seeds formed by asexual way from gametophytic tissue, is more frequent in cold environments of higher latitudes and altitudes 1234.
Apomicts might be better colonizers than sexual species in arctic and alpine areas because of uniparental reproduction and ability to set up populations by single individuals 5sexthereby compensating for reduced fecundity resulting from pollen limitation in small and isolated populations 7. However, the hypothesis of higher frequency of apomictic plants in cold environments has been rarely tested 8but when tested, the prominent role of apomixis is questioned 89 Several studies on Ranunculus sex found clear connection between higher elevation and apomixis, but simultaneously between apomixis and polyploidy.
It seems that these genomic and environmental features are bound together — polyploids appear rather in colder climate of high elevation, and polyploids have tendency to reproduce by apomixis 1112 As understanding of this polyploid complex increased, also opposite relationship was proposed — polyploidy might be bound to alpine 14 There are several mutually non-exclusive explanations why the frequency of apomicts should increase upwards or polewards.
Other predictions, supporting the affinity of apomicts to cold areas, have been more recently added to the original above-mentioned arguments extrene by Stebbins 5. For instance, apomixis alpine save energy otherwise used for expensive process of meiosis alpine Nevertheless, some of the abovementioned arguments are just theories, which can be easily challenged. Secondly, the ecological argument presupposing absence of pollinators was recently refuted as there are still effective pollinators occurring in cold arctic and alpine habitats 30 Moreover, pollination does not depend on pollinators solely, as partial or full autogamy is widespread among angiosperms including apomicts 3233 Despite that, all these theoretical predispositions about higher occurrence of apomixis in extreme conditions have been considered as a fact for a long time 1 without proper testing.
The problematic of higher frequency of apomictic plants in cold environments was not thoroughly studied sec recently but see the extensive research on R. The apomixis was highly positively correlated with altitude in sex compilation. From these extremr, only Potentilla crantzii Crantz Beck ex Fritsch, an apomictic plant described earlier by Smith 36 was apomictic while other species showed seeds originated by sexual way, indicating that apomixis might be rather rare in high elevations.
However, there are extreme high mountain areas where the relation between apomixis and elevation might be stronger. Himalayas provide extensive alpine environment potentially less favourable for insect pollinators compared to European mountains and therefore more favourable for formation of apomixis due to severe ecological constraints There are several ways how to study the apomixis, most effective being a flow cytometric seed screening FCSS alpinewhich became popular in the last few decades 815 FCSS measures DNA content in mature angiosperm seeds and infer the reproductive sex based on DNA content of the embryo and the endosperm without a need to know the actual genome sizes or ploidy levels 40 Sexual angiosperm seed derived alpine reduced gametes and normal double fertilization has 2C embryo formed alpine a 1C egg cell and a 1C sperm cell and 3C endosperm a 2C central nucleus of the embryonic sac plus a 1C sperm cell 7 Apomictic seeds have extreme ratios the embryo and the endosperm genome sizes However, there are many other possibilities depending on the actual sex of embryo and endosperm formation and ploidy levels of taxa involved in cases when pollen- and egg- parents have different ploidy levels but most of these combinations are reliably distinguished from sexual seeds 3944 It has been widely assumed that apomixis is frequently associated with vegetative, extremd propagation.
Vegetative reproduction, in which parental genets produce new ramets capable of independent growth and dispersal 46is commonly associated with the perennial life form, longevity, and occurrence in cold habitats in which sexual recruitment is often restricted. According to Barrett 47extensive vegetative growth by clonal xex, stolones and root sprouts can disrupt the functioning of sexual polymorphisms; mutations reducing fertility may lead to sexual dysfunction and even the extreme of sex in populations extreme which clonal propagation predominates.
However, there have been few efforts documenting whether apomicts differ from sexuals extreme the adaptive strategies including vegetative propagation and other ecophysiological traits enabling plants to cope with harsh alpine conditions.
In this study, we focused on testing the hypothesis about increasing frequency of apomicts with increasing elevation in the northwestern Himalayas.
Map of localities in Ladakh. Background map is based on. Out of measured taxa, reproductive mode was inferred for while 37 species had no detectable endosperm signal. We found only 13 apomictic taxa including two subspecies of Poa pratensis belonging to seven genera and four families. Most of the apomictic species studied in alpne Himalayas were not tested for apomixis yet, so we cannot compare reproduction systems within species with other locations.
The most of the few cases, extreme we were able to compare across species ranges, indicate that apomictic species have the same reproduction system at lower elevations as well as at higher elevations. Wolf, Potentilla sericea L. Other apomictic species were Biebersteinia odora Fisch. BiebersteiniaceaeFestuca olgae Regel Krivot. Nevski both PoaceaeHalerpestes lancifolia Bertol. Poa, Potentillaand Ranunculus are genera well-known for high frequency sex apomictic species.
Although Stipa splendens have wex apomictic relatives in lapine genus and apomicts in the genus Festuca are also rather rare, both genera belong to family Poaceae, which is among the three families with the highest frequency of apomixis Asteraceae, Rosaceae, Poaceae.
Although there is no information on apomixis in Halerpestesthis genus is sex related to Ranunculus estreme in which apomixis in the family is known. There is no information on apomixis within Alpine yet; our data on B. Apomictic species were clonal rhizomatous graminoids and forbs, with higher frequency of occurrence and soil moisture demands, sharing the syndrome of dominant, competitive species with broad geographical and elevation ranges Fig.
Apomicts did not show preference for higher elevations, even in these extreme Himalayan high-mountain habitats; they were instead bound to taxonomically related groups known for apomixis. Apomictic species and their elevation optima and ranges in Ladakh, NW Himalayas.
Note that it spans the whole range of elevations in the area and the apomictic species are not concentrated in higher elevations. Our findings corroborate previous studies 46 indicating that apomixis is associated with the on-spot persistence, high plant longevities, and occurrence in stable habitats where extensive clonal growth may disrupt wxtreme functioning of sexual alpine, resulting in mutations reducing fertility, which may lead to sexual dysfunction and even the loss of sex.
Apomicts in NW Himalayas form ecologically distinct group differing from non-apomicts in greater ability of clonal propagation and preference for wetter, more productive habitats. This implies that apomicts are often effective extreme reproducing and spreading, but successful are only when growing in rather stable habitats, being less successful in colonizing rapidly changing environment such as new substrate after deglaciation 49 Three apomicts recruited from the genus Potentillaincluding P.
They all showed pseudogamous development of seeds. The second possible way is more probable because we know from former studies that pollen of Potentilla is mostly reduced 51 Moreover, this type of endosperm fertilisation was also observed in Rosaceae 45 Alternatively, other species of higher different ploidy level might contribute unreduced-like pollen at site where several species co-occur extreme FCSS histograms of selected apomictic species and a sexual species Stipa subsessiliflora.
Apomixis in the genus has been well studied. The apomictic series occur exclusively in phylogenetically young core Potentilla. However, study of apomixis in this genus has its limits. The first is caused by hardly detectable endosperm tissue 5657which is sometimes completely missing In our case, 44 out of 57 samples were without detectable endosperm tissue. The second obstacle arises with occasional deviation from standard eight nuclei in embryo-sac.
In spite of missing data on reproduction mode in some Himalayan Potentilla spp. Predisposition for apomixis was already known in one species P. Potentilla venusta is known to be apomictic 55 ; our analyses were, however, without readible signal and we extteme cannot determine the extrmee system.
Concerning reproduction modes of other sampled species which were unfortunately without clear signal, Rani et al. There sex no previous studies concerning extdeme systems in P. However, P. It placed the species outside the core Alpniewhich suggests that P.
This type of endosperm fertilisation was described in Ranunculus auricomus sec4563the intensively studied facultative apomictic complex We can expect that apomixis in R. As apomictic Ranunculus spp. It seems that polyploidization happens regularly mainly via triploid bridge with an unreduced egg cell.
This loss of meiosis in female gametes might be the first step for development of apomixis in Ranunculus polyploids. However, Klatt et al. It is eztreme unexpected observation, because no other apomictic genera except Ranunculus were discovered within the family Ranunculaceae so far. Nevertheless, the genus Halerpestes is closely sex to Ranunculus s.
The family Poaceae is extreme known for high frequency of apomictic species. In our data set, 16 alpine of Poaceae were tested, from which one is an apomictic species estreme the genus Festuca sex Festuca sexfour belong to the genus Poa Poa alpinaPoa attenuata, Poa pratensis, Poa sterilis and one species belong to the genus Stipa Stipa splendens.
Festuca olgae is a facultative type of an apomict. Such facultative apomixis is known to occur in Poaceae 67 and it happens mostly in an aposporous plant which creates an embryo-sac directly from nucellus A facultative apomict might have an advantage over strict sexual or apomictic. In fact, it has advantages from both of them. The alpine can produce genetically variable offspring to improve its features in changing environment, on the other hand it produces also stable offspring which is well adapt to current conditions.
Even though apomixis in the genus is not very common, it was observed already 69 and; in addition, the genus Festuca is quite close extrsme to the genus Poa 70which is one of the best known apomictic genera
We had information on relatedness between hibernating group members in 29 cases. In six cases, there were neither subordinates nor yearlings in the hibernating group. Winter survival rates of juveniles hibernating with full-sibs 0. Even in this case, the difference between the survival values 0. The male-biased sex ratio observed among offspring at emergence in our population was consistent in other age classes except among 3- year-old subordinates Table 1.
This was probably the consequence of the great dispersal rate of 2-year-old males see below. We weighed neonates born to the captive females. Mean weight at birth was This sexual dimorphism was also previously observed at emergence mean juvenile body mass was 0. We had data on survival between the emergence from the natal burrow and the emergence from the first hibernation for juveniles 75 males and 50 females.
Finally, we were able to determine the exact age of dispersal for 70 subordinates 37 males and 33 females. Only 3 8. Most subordinates A total of 21 A male bias was observed in 31 litters, whereas a female bias was noted in only 15 litters. In nine litters, the number of males equaled that of females.
This indicates that the bias was low in litters with an excess of females but higher in litters with an excess of males. The distribution of observed sex ratio Figure 3 presents a peak around 0. A sex ratio of 0 was never observed, but sex ratios of 1 were frequent. The mother's condition was also measured as the residual of the regression between body mass and body length structural size; Dobson, For mothers in good and poor conditions, overall sex ratios were 0.
The mothers' body mass was positively correlated Spearman's rank correlation with all three theoretical sequences of litter type. Distribution of weaning sex ratios observed in our population of Alpine marmots. Black bars indicate litters with an excess of males. Light gray bars indicate litters with an excess of females. The medium gray bar in the middle of the graph indicates litters with balanced sex ratio.
Mean mothers' body mass as a function of the theoretical sequence of litter type size and sex composition obtained when considering that the direct metabolic cost of producing males M was greater than that of two females F. Spearman rank correlation r z between the two series is shown. In monomorphic monogamous species, the two sexes are supposed to be equally costly to produce, and a balanced sex ratio is then expected at the population level Fisher, Exceptions have been described in birds Bednarz and Hayden, , and particularly in cooperative breeders Gowaty and Lennartz, , Ligon and Ligon, , but almost nothing is known on offspring sex ratio bias in populations of monogamous mammals that breed cooperatively.
In the Alpine marmot, we found a male-biased sex ratio at weaning and at birth. More generally, a male-biased sex ratio could be the rule among socially monogamous marmots. A male-biased trend, although not yet quantified, seems also to exist in litters of M. Such a male bias was not observed in a polygynous marmot species M. Under Fisher's arguments, we have to deduce from our results that either the population is not at an evolutionary equilibrium and currently producing more of the rarer sex or that the production of daughters may entail a greater cost sensu Trivers, for mothers than the production of sons.
The sex ratio was male biased in our population in all age classes except among 3 years old, see Results. Therefore, the production of an excess of males is not the consequence of a frequency-dependent process Table 2 to produce the rarer sex in the population Fisher, Thus, females should be more costly sensu Trivers, to produce than males in the Alpine marmot. Several hypotheses have been proposed to explain a differential cost between sons and daughters Clutton-Brock and Iason, and consequently a biased sex ratio at the population level Table 2.
First, the two sexes may differ in the relative direct metabolic cost incurred by the mother during gestation and lactation Hewison and Gaillard, If this energetic cost represents a fitness cost for the mother, the offspring sex ratio should be biased toward the smaller sex. The sexual dimorphism at birth and at weaning thus cannot explain the male bias observed in our population. Tests of alternative hypotheses proposed to explain the male-biased sex ratio at the population level in litters of alpine marmots.
Second, the offspring sex ratio should be biased toward the less viable sex. This is in accordance with the persistence of the male bias among adults and was expected for monomorphic monogamous species.
Finally, the local resource competition hypothesis Clark, predicts that the population sex ratio should be biased toward the dispersing sex. Indeed, members of the sex that delay dispersal compete with each other and with the residents for limited resources and incur a fitness cost. In the Alpine marmot, subordinates delay dispersal beyond reproductive maturity, and such a delay corresponds to a prolonged maternal investment Armitage, According to the local resource competition hypothesis, female Alpine marmots may be more costly to produce than males if they disperse later.
However, this hypothesis was not consistent with observations in the population studied by Arnold a , in which females dispersed first, while a male-biased sex ratio was also observed. Because the Alpine marmot is a cooperatively breeding species Blumstein and Armitage, ; Jennions and Mcdonald, , the offspring sex ratio bias may be better explained by the helper repayment hypothesis Emlen et al. In our population, juvenile survival increased with the number of subordinate males in the hibernaculum, but not with the number of subordinate females or with territory quality no confounding effect of territory quality; Cockburn, This suggests that only males helped or that males were more efficient in warming juveniles during hibernation.
Subordinate male Alpine marmots shared with residents the cost of social hibernation load-lightening hypothesis: Crick, , and, consequently, the presence of subordinate males also increased the winter survival of residents Arnold, b. Subordinate males may thus repay part of the investment they received from their parents by participating in social thermoregulation.
Because survival pattern did not differ between sexes and males tended to disperse earlier, we suppose that helpers incurred no or low direct fitness loss through helping. Even if this direct fitness loss exists, the sex ratio bias observed in favor of the helping sex suggests that the fitness payoff to parents of helping exceeds the direct fitness loss entailed by the helping sex Koenig and Walters, Members of the helping sex are thus less costly to produce, justifying their production in excess Table 2.
Subordinate males may be more efficient in warming juveniles if, because of their body composition or other physiological reasons, they produce more heat than subordinate females. The greater the number of subordinate males, the greater heat production.
However, subordinate males warm by grooming and covering with hay; Arnold, b only closely related juveniles Arnold, This indicates that helping from subordinate males is mainly an active process rather than a passive one.
However, helping during hibernation is energetically costly Arnold, b , and a subordinate is expected to help only if the fitness payoff from helping exceeds the fitness cost. Helping is probably too costly for subordinate females but not for subordinate males.
One possible explanation for this difference is that females' future reproductive potential depends more critically on fat reserve than is true for males. Another possibility is that males, but not females, may gain direct fitness benefits from staying and are thus incited to help. Male subordinates may gain fertilizations because the resident male either failed to suppress their reproduction limited control hypothesis; Clutton-Brock, or allowed them especially related males; Arnold and Dittami, to reproduce, inciting them to help concession theory; Vehrencamp, In contrast, subordinate females were not allowed to breed Arnold, a ; Goossens et al.
Parents are expected to skew the sex ratio of their offspring to maximize their own fitness Trivers and Willard, In particular, when fitness returns from sons and daughters are different, mothers should bias the sex ratio of their offspring in favor of the most profitable sex according to their own phenotypic condition Silk, ; Trivers and Willard, Variations in mothers' phenotypic condition may thus lead to variations in offspring sex ratio among litters, even in cooperative breeding species see Komdeur, ; Komdeur et al.
However, Alpine marmot mothers adjusted both the size and the sex composition of their litters according to their phenotypic condition Williams, The observed sequence of litter types with improving mothers' condition, following that expected when sons are energetically more costly than daughters, supports the Trivers-Willard prediction in species with large litters Williams, Some litter types were never observed, especially those consisting of only females.
Possibly, when adjusting their reproductive effort, mothers prefer to produce smaller litters with at least some sons than larger ones with only daughters. In this way, mothers produced more systematically an excess of the helping sex, thus leading to the observed sex ratio bias at the population level.
Thus, our results are consistent both with the helper repayment hypothesis at the population level , and with the Trivers-Willard Hypothesis at the individual level. Our study offers the first convincing evidence of an offspring sex ratio bias in populations of a monogamous mammal that breeds cooperatively. Helping behaviors in birds mainly consist of territory defense and nourishment of juveniles. Here we showed that warming juveniles during hibernation is a form of helping behavior in the Alpine marmot that leads to a biased sex ratio at the population level.
The helper repayment hypothesis is the more appropriate hypothesis to account for our observation of an offspring sex ratio bias at the population level. We are grateful to Gordon Luikart and Philipp England for editing our English and for their helpful comments. We thank two anonymous referees and Dan Blumstein for their helpful comments on the manuscript. Oxford University Press is a department of the University of Oxford. It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide.
Sign In or Create an Account. Sign In. Advanced Search. Article Navigation. Close mobile search navigation Article Navigation. Volume A lover of neon athletic gear with a death wish? A deviant with a soft spot for mountain living and white powder? The new album and immersive audiovisual performance project DAS: Dynamic Alpine Sexual tells the tale of fictional 's ski star Dirk Dassler who "sabotages a high flying career to follow a deluded dream.
Multimedia artist Jordan Sarah created and stars in DAS, both the performance and the album, which serves as the event's score. The Umbrella is releasing the album today along with a video for the album's first track "Gratitude," which you can watch exclusively here on THUMP.
The artist has also shared a drum n' bass remix for the album's third track "We Let Go" by Aquarian , which you can listen to below. The occurrence of extra-group EPP EPP due to males not belonging to the family group seems independent of the presence of subordinate males in the family group Goossens et al.
After 30 days of gestation, dominant females give birth to 1—7 pups in late May. Juveniles stay into the natal burrow during lactation 40 days and emerge in late June and early July. The alpine marmot is an excellent organism with which to test the predictions of sex ratio theory applied to cooperatively breeding species, because the underlying assumptions of theory have been shown to hold.
Here, I test the theoretical prediction that litter sex ratio should be relatively more male biased when subordinate male helpers are absent prediction 1.
I use a long term data set to test this prediction in two ways: 1 across females in a population, utilizing variation between females in the occurrence or absence of helpers; and 2 in individual females across multiple years, to test whether secondary offspring sex ratios produced by individual females varied in response to the presence or absence of helpers.
I also test alternative predictions: 1 The alpine marmot does not meet the criteria of the Trivers and Willard's hypothesis because the alpine marmot is a monogamous and polytocous species, so I predict that the sex ratio of offspring at weaning is independent of mothers' body condition prediction 2. If this sexual dimorphism represents a differential energetic cost for mothers, females in poor body condition should bias their offspring sex ratio towards the cheaper sex females prediction 3.
Consequently, mothers are not expected to overproduce helpers only when resources are abundant, i. So, I predict that the secondary sex ratio produced by mothers is independent of the interaction between the presence of helpers and territory quality prediction 4. In the case that adaptive offspring sex ratio variation occurred, I examine how this happened. To do so, I investigate whether the litter size and the number of juvenile males and females were modified.
The site is characterized by alpine vegetation and high mountain weather. From to , individuals from 23 family groups were permanently marked with a numbered ear tag and an electronic device Trovan put under the skin, allowing individual identification. Each trapped individual was sexed, weighed, and measured for several morphological traits. I identified dominants and recorded the number of subordinate males and females, the number of yearling males and females, the date of emergence, and the size of litters.
I used R 1. Mixed models allow consideration of both fixed and random terms. Random terms entered into the model took into account repeated measures of different individuals within the same group. Discrete terms were always factorized. All exploratory fixed terms were initially entered into the model and then dropped sequentially. The fitted model included only fixed terms whose elimination would be significant. Interactions were never significant and not presented in tables.
The size and the date a litter emerged were recorded. Juveniles were then trapped as soon as possible generally within 3 days after emergence and sexed by examination of the ano-genital area. Sex ratio within litters was computed as the proportion of males. I ran a generalized linear mixed model with a binomial error to investigate for the terms that affected the sex ratio within litters.
To take into account repeated sampling of different litters born to the same mother in a same territory, I used the mother nested to the territory as a random term in the analysis. The fixed exploratory terms considered were as follows. The female body condition was measured by the residual of the regression of body weight on the date of capture. So, exposure to sun was assumed to be a reliable indicator of territory quality.
So, juveniles born the previous year might all disappear, and this might reduce the number of yearling males potential helpers in the group. I used the same procedure but the data set corresponded to the 10 females of the 29 that were followed several years and that bred sometimes with and sometimes without helpers. It was then possible to test specifically whether a given mother produced offspring sex ratios in the direction predicted by theory when in the presence or in absence of helpers.
To test this, I used the mother as the random term and the presence of helpers in the family group as the fixed term in the analysis. In the absence of helpers, mothers may adjust the offspring sex ratio by modifying the litter size, by increasing the number of juvenile males, or by decreasing the number of juvenile females.
I built three generalized linear mixed models with a Poisson error to investigate for the terms affecting litter size, number of juvenile males, and number of juvenile females, respectively. Again, to take into account repeated sampling of different litters born to the same mother in the same territory, I used the mothers nested to the territory as random terms in the analysis. The fixed exploratory terms considered were the same as for the analysis of the offspring sex ratio see above.
From to , the family composition and the occurrence of a litter were recorded for a total of group-years. I discarded from the analysis two litters for which no juveniles were sexed because they all disappeared before capture. The analysis was then performed on 82 litters: for 63 litters, all juveniles that emerged were sexed, and for 19 additional litters some juveniles disappeared or were not trapped.
Consequently, these 19 litters were included in the dataset. However, I also performed the complete analysis without these 19 litters. The conclusions were strictly the same, so I only present the results of the analysis on the 82 litters in the following. The sex ratio within litters varied from 0 to 1. Among the 82 group-years where a litter emerged, helpers were absent in 25 The mean number of helpers present was 2. The sex ratio within litters was only affected by the presence of helpers Table 1.
None of the other factors considered—mothers' body condition, territory exposure to sun, presence of a litter the previous year, litter size, change of the dominant male—affected the sex ratio within litters Table 1. So, in the alpine marmot, the production of helpers when they were absent was not affected by territory quality. Ten mothers remained several years 2 to 8 years in their territory and faced both social environments: the presence and the absence of helpers.
The mean litter size was 3. The litter size was only affected by the mothers' body condition Table 2. None of the other factors considered—territory exposure to sun, presence of a litter the previous year, change of the dominant male—affected the litter size Table 2. In particular, mothers did not adjust the litter size when helpers were absent Table 2.
So, in the absence of helpers, mothers biased the offspring sex ratio towards males without changing the litter size. Consequently, I expected mothers to produce more sons and fewer daughters when helpers were absent. As expected, the number of sons and daughters produced within litters increased and decreased, respectively, when helpers were absent Table 3.
None of the other factors considered—territory exposure to sun, presence of a litter the previous year, change of the dominant male—affected the number of juveniles of each sex, except for the trivial effect of litter size Table 3.
In cooperative breeders, a clear theoretical prediction about sex ratio adjustment can be made: mothers should bias the sex ratio of their offspring towards the helping sex when helpers are absent Gowaty and Lennartz, ; Pen and Weissing, ; West and Sheldon, I tested this theoretical prediction against alternatives in a cooperative mammal, the alpine marmot.
On the whole, the secondary sex ratio within litters was biased towards males helpers when helpers were absent in family groups. Moreover, the results clearly indicate that a given mother produced offspring sex ratios according the social situation she faced; mothers produced an excess of sons when helpers were absent but switched to the production of litters with unbiased sex ratio when helpers were present.
These results strongly support the theoretical prediction and suggest the occurrence of adaptive sex ratio variation in the alpine marmot.