What limits the size of nature’s most extreme structures? For weapons like beetle horns, one possibility is a tradeoff associated with mechanical levers: as the output arm of the lever system—the ...beetle horn—gets longer, it also gets weaker. This “paradox of the weakening combatant” could offset reproductive advantages of additional increases in weapon size. However, in contemporary populations of most heavily weaponed species, males with the longest weapons also tend to be the strongest, presumably because selection drove the evolution of compensatory changes to these lever systems that ameliorated the force reductions of increased weapon size. Therefore, we test for biomechanical limits by reconstructing the stages of weapon evolution, exploring whether initial increases in weapon length first led to reductions in weapon force generation that were later ameliorated through the evolution of mechanisms of mechanical compensation. We describe phylogeographic relationships among populations of a rhinoceros beetle and show that the “pitchfork” shaped head horn likely increased in length independently in the northern and southern radiations of beetles. Both increases in horn length were associated with dramatic reductions to horn lifting strength—compelling evidence for the paradox of the weakening combatant—and these initial reductions to horn strength were later ameliorated in some populations through reductions to horn length or through increases in head height (the input arm for the horn lever system). Our results reveal an exciting geographic mosaic of weapon size, weapon force, and mechanical compensation, shedding light on larger questions pertaining to the evolution of extreme structures.
•Biomechanical costs of extreme structures are often hidden by compensatory traits•We trace the evolution of long horns in a beetle and measure the effect on strength•Horns increased in length twice, yielding weapons with weaker lifting forces•Some populations later grew taller heads or shorter horns, restoring lifting force
Weber et al. document a biomechanical tradeoff associated with the evolution of an exaggerated animal weapon. When beetle horns got long, they also got weaker—a cost that was later ameliorated in some populations. Reconstructing the sequence of evolution of this weapon provides a rare glimpse of an ephemeral and elusive cost to extreme structures.
mating system of the brown bear Ursus arctos STEYAERT, Sam M. J. G; ENDRESTØL, Anders; HACKLÄNDER, Klaus ...
Mammal review,
2012, 2012-01, January 2012, 2012-01-00, 20120101, Letnik:
42, Številka:
1
Journal Article
Recenzirano
1 Research on mating systems and reproductive strategies is valuable for providing ethological knowledge, important for the management and conservation of a species, and in a broader sense, important ...for biodiversity conservation. 2 We reviewed the literature to document the mating system of the brown bear Ursus arctos. We determined that many aspects of the reproduction of the brown bear remain unclear, including (i) biological aspects, such as hormone and oestrous cycling, sperm competition, mate choice, sexually selected infanticide, etc. and (ii) human impacts on the mating system, occurring when humans alter population size and structure, through, for example, hunting or habitat degradation. 3 We considered three mating system classification frameworks from the literature (Emlen & Oring 1977, Clutton‐Brock 1989, Shuster & Wade 2003) and applied various brown bear populations to them. We did this (i) to document the plasticity of the mating system of the brown bear, and (ii) to find commonalities among the reported mating system classifications in order to provide a general and common classification of the brown bear's mating system. 4 The mating system of the brown bear can, in general, be classed as ‘polygamous’. Subclassifications can nevertheless be valuable on smaller spatial scales. 5 Within the polygamous mating system of the brown bear, biological aspects and human impacts can influence reproductive strategies at the individual and population level. Mating system classification frameworks often lack a common terminology, which contributes to the variety of published classifications of the mating system of the brown bear.
Male adaptations that limit sperm competition include guarding females, applying mating plugs and chemically reducing the attractiveness or receptivity of females. In many web-building spider ...species, females attract males with silk-borne volatile pheromones. In widow spiders (Latrodectus, 30 species), the courting male often engages in web reduction behaviour during which he excises and bundles sections of the female's web and wraps them with his own silk. Hypothesized functions of this widespread behaviour include sexual communication (e.g. through dissemination of male sex pheromone) and/or decreasing the female's attractiveness to rivals. The latter function was previously demonstrated in a single spider species, Neriene litigiosa, but the extent to which web reduction may decrease male–male competition has never been quantified in the field. In a dense population of western black widows, Latrodectus hesperus, we ran mate attraction experiments to test the hypothesis that web reduction and/or male silk addition decrease web attractiveness to potential rivals. Webs reduced by males attracted three times fewer males than intact webs; webs with a similar proportion of silk experimentally removed attracted as many males as intact webs. However, the experimental addition of male silk did not affect the attractiveness of intact webs. We conclude that web reduction in black widows limits male–male competition by reducing the attraction of rival males to females' webs. This effect is probably mediated through targeted excision of pheromone-laden silk by courting males, possibly in combination with the male's silk forming a physical barrier to pheromone emission.
•We investigated the function of web reduction by courting male black widows.•We compared the attractiveness of intact and reduced female webs to males.•Male-reduced webs attracted three times fewer males than intact webs in the field.•Web reduction alters or prevents emission of silk-borne female pheromones.•Web reduction helps males monopolize females during lengthy courtship.
Scent marking is often assumed to be a secondary sexual trait that increases males' mating and reproductive success, although direct evidence for this hypothesis is lacking. We conducted a study with ...wild-derived house mice, Mus musculus musculus, to test whether scent marking increases males' reproductive success when females can freely choose between two territorial males. We also experimentally manipulated males' competitive scent marking by exchanging scent-marked tiles between the neighbouring males' territories (intrusion treatment) or relocating males' tiles within their own territory (control). Experimental animals were tested twice and we examined whether individual males were consistent in their marking. We found that males marked more in the intrusion treatment than controls and more at shared territorial borders than elsewhere. We found high day-to-day variation in most individuals' scent marking, and yet the sum of individuals' scent marking was consistent over time and across different social conditions. Genetic paternity analyses revealed that males' scent marking significantly increased their reproductive success in both the intrusion treatment and the controls. Surprisingly, however, female social preference was not positively correlated with male scent marking. These results provide direct evidence that scent marking increases males' reproductive success when females can choose their mates, even though it did not increase females' social preferences.
•Sum of male scent marking was positively correlated with male reproductive success.•High day-to-day variation in individual male's scent marking.•Consistency in the sum of marking over time and across different social conditions.•Increased scent marking at borders adjacent to neighbouring males.•Female social preference influenced by male-male interactions but not scent marking.
Male stag beetles (Coleoptera: Lucanidae) use their mandibles as weapons to compete for resources and reproduction. Mandible size in stag beetles can be associated with different behaviours and the ...outcome of male contests. We investigated the allometric relationship between mandible and body size in males of the stag beetle
to uncover distinct morphs. The results divided male
into majors and minors with the switch point of mandible length at 14.01 mm. The allometric slope of mandibles was positive for both morphs but was steeper for the minors. We also characterised the fighting behaviour of the different morphs in size-matched contests using sequential analyses. Males matched each other's behaviour in contests with many physical contacts, no injury and a progression from low towards high aggression and rare de-escalation. Major and minor males employed the same behavioural elements in contests, but major males were more likely to escalate directly into more aggressive phases and minor males tended to stay within phases. This finding suggests that major males may compete more aggressively than minor males in contests.
Across vertebrates, high social status affords preferential access to resources, and is expected to correlate positively with health and longevity. Increasing evidence, however, suggests that ...although dominant females generally enjoy reduced exposure to physiological and psychosocial stressors, dominant males do not. Here we test the hypothesis that costly mating competition by high-ranking males results in chronic, potentially harmful elevations in glucocorticoid production. We examined urinary glucocorticoids (n = 8029 samples) in a 20-year longitudinal study of wild male chimpanzees (n = 20 adults) in the Kanyawara community of Kibale National Park, Uganda. We tested whether glucocorticoid production was associated with dominance rank in the long term, and with mating competition and dominance instability in the short term. Using mixed models, we found that both male aggression and glucocorticoid excretion increased when the dominance hierarchy was unstable, and when parous females were sexually available. Glucocorticoid excretion was positively associated with male rank in stable and unstable hierarchies, and in mating and non-mating contexts. Glucorticoids increased with both giving and receiving aggression, but giving aggression was the primary mechanism linking elevated glucocorticoids with high rank. Glucocorticoids also increased with age. Together these results show that investment in male-male competition increases cumulative exposure to glucocorticoids, suggesting a long-term tradeoff with health that may constrain the ability to maintain high status across the life course. Our data suggest that the relationship between social rank and glucocorticoid production often differs in males and females owing to sex differences in the operation of sexual selection.
•Male chimpanzees had elevated glucocorticoids when competing for status.•Male chimpanzees had elevated glucocorticoids when competing over mates.•Rank correlated positively with glucocorticoids in both mating and non-mating contexts.•Rank correlated positively with glucocorticoids in both stable and unstable hierarchies.•Giving aggression was the main behavior linking glucocorticoids with rank.
The evolution of sexually dimorphic, elaborate male traits that are seemingly maladaptive may be driven by sexual selection (male–male competition and or female mate choice). Tusk possession in the ...Asian elephant is sexually dimorphic and exaggerated but its role in male–male competition has not yet been determined. We examined the role of the tusks in establishing dominance along with two other known male–male signals, namely, body size and musth (a temporary physiologically heightened sexual state) in an Asian elephant population in northeastern India with equal proportions of tusked and tuskless males. We observed 116 agonistic interactions with clear dominance outcomes between adult (>15 years) males during 458 field days in the dry season months of 2008–2011. A generalized linear mixed-effects model was used to predict the probability of winning as a function of body size, tusk possession and musth status relative to the opponent. A hierarchy of the three male–male signals emerged from this analysis, with musth overriding body size and body size overriding tusk possession. In this elephant population tusk possession thus plays a relatively minor role in male–male competition. An important implication of musth and body size being stronger determinants of dominance than tusk possession is that it could facilitate rapid evolution of tuskless males in the population under artificial selection against tusked individuals, which are poached for ivory.
•Tusks in Asian elephants are sexually dimorphic and larger in males than females.•We examined the role of tusks, musth and body size in male dominance.•Tuskless males in musth were dominant over tusked nonmusth males.•Musth and body size override tusk as a male–male signal of dominance.
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