Thomas J. Hayden

David's score: a more appropriate dominance ranking method than Clutton-Brock et al.'s index

here are many procedures, of varying complexity, forranking the members of a social group in a domi-nance hierarchy (reviewed by de Vries 1998; also Jamesonet al. 1999; de Vries & Appleby 2000; Albers & de Vries2001). Roughly, two types of method can be distin-guished, one in which the dominance matrix is reorgan-ized such that some numerical criterion, calculated forthe matrix as a whole, is minimized or maximized, andone that aims to provide a suitable measure of individualoverall success, from which a rank order can be directlyderived. Two relatively simple, and somewhat similar,ranking methods belonging to the latter type are Clutton-Brock et al.’s index (Clutton-Brock et al. 1979, 1982) andDavid’s score (David 1987, 1988). Both methods can beused to calculate dominance ranks for individuals in agroup, based on the outcomes of their agonistic inter-actions with other group members, while taking therelative strengths of their opponents into account.Clutton-Brock et al.’s index (CBI) was originally devel-oped as a measure of fighting success for red deer,

Sexual size dimorphism in fallow deer (Dama dama): do larger, heavier males gain greater mating success?

Abstract Sexual size dimorphism may evolve as a result of both natural and sexual selection. In polygynous mammals, the main factor resulting in the evolution of large body size in males is the advantage conferred during competition for mates. In this study, we examined whether sexual selection acts on body size in mature fallow bucks (Dama dama) by examining how the following traits are inter-related: age, body (skeletal) size, body mass, prerut dominance rank, rut dominance rank and mating success. This is the first study to examine how all these factors are together related to the mating success of a large sexually dimorphic and polygynous mammal. We found that male mating success was directly related to body size, but not to body mass. However body mass was related to prerut dominance rank which was in turn strongly related to rut dominance rank, and thus there was an indirect relationship between mating success and body mass. Rut dominance rank was the variable most strongly related to mating success. Mating success among mature males was unrelated to age. We conclude that larger mature fallow bucks have advantages over other males when competing for matings, and sexual selection therefore continues to act on sexual size dimorphism in this species. Heavier fallow bucks also have advantages, but these are mediated through the dominance ranks attained by males before the rut.

Age-specific survival and reproductive probabilities: evidence for senescence in male fallow deer (Dama dama)

Survival and reproduction are key features in the evolution of life–history strategies. In this study, we use capture–mark–resighting and multi–state models to examine survival senescence and reproductive senescence in six successive cohorts of fallow bucks that were studied for 16 years. We found that the overall age-specific survival probabilities of males were highly variable and the best–fitting model revealed that fallow bucks have four life–history stages: yearling, pre–reproductive, prime–age and senescent. Pre–reproductive males (2 and 3 years old) had the highest survival. Survival declined sharply after the age of 9 years, indicating that senescence had begun. When we considered reproducing and non-reproducing males separately, there was no evidence of senescence in the former, and steadily decreasing survival after the onset of social maturity in the latter. Reproduction probability also declined in older males, and thus we provide very strong evidence of senescence. Reproducers had a greater chance of reproducing again in the following year than non–reproducers. Furthermore, there were differences in the survival probabilities, with reproducers consistently surviving better than non–reproducers. In our study population, reproducers allocate more to the effort to reproduce than non–reproducers. Therefore our results indicate the generally higher phenotypic quality of reproducing males. These results, along with earlier studies on the same population, could indicate positive relationships between fitness correlates.

Lifetime mating success, sexual selection and life history of fallow bucks (Dama dama)

Abstract We used data from a long-term study (15 years) of fallow deer to report for the first time the lifetime mating success, overall variance in lifetime mating success, and age-specific mortality levels of males. Fallow bucks that gain matings have higher social dominance rank, higher rates of fighting, and invest more in vocal display during the breeding season than unsuccessful males. Therefore, we examined if mating was associated with trade-offs in terms of survival, lifespan, and mating potential. We found that the variance in lifetime mating success was very high: 34 (10.7%) males mated, and of those, the 10 most successful males gained 73% of all matings (n=934). Mortality rates were generally high and only 22.3% (71/318) of males reached social maturity, i.e., 4 years. The oldest male was 13 years old. We found that fallow bucks that mated were not more likely to die during the following year, did not suffer from a reduction in lifespan, and did not incur lower mating success later in life as a result of mating during the early years of social maturity. Our results show that mating males at age 5 years (and possibly 9 years) may be more likely to survive than non-mating males. Additionally, the number of matings gained by males during the first years of social maturity was positively correlated with lifespan. We suggest that mating males are of higher quality than non-mating males because they are not more likely to incur trade-offs as a result of their increased reproductive efforts.

Cumulative long-term investment in vocalization and mating success of fallow bucks, Dama dama

We carried out behavioural observations to investigate the function of long-term investment in vocal display by fallow bucks during the breeding season. The measures of long-term investment used were the date of initiation of vocal activity, the number of days vocal during the breeding season, and the proportion of time spent vocalizing. We analysed data from 3 years (1993-1995) to assess the relationship between the date of initiation of vocal activity, and the number of days vocal, and age, dominance rank and mating success. Observations from a sample of focal males in 1996 were used to determine the effect of the proportion of time vocal during the breeding season on dominance rank and mating success. The majority of socially immature males (</=3 years old) did not vocalize; among socially mature males (>/= 4 years old), dominance rank was more important than age in explaining variation in vocal activity. The onset of vocal activity by fallow bucks was not a direct consequence of the presence of mating opportunities since the first males became vocal more than 3 weeks before any matings occurred. Long-term investment in vocal activity did not alter the dominance relationships that had been established between males before they became vocal. When we considered all mature males from 3 years of observations, the majority of matings were achieved by those that had high rank, initiated vocal activity early during the breeding season and remained vocal on most days. For the 1-year sample of mature males, the factor most highly correlated with mating success was the proportion of time that males spent vocalizing during the rut. Thus we have shown a strong relationship between the time invested in vocal display by fallow bucks and their mating success. Copyright 1999 The Association for the Study of Animal Behaviour.

Genetic and behavioural estimates of reproductive skew in male fallow deer

Populations of fallow deer, in general, have low genetic diversity. Nevertheless, we screened 39 microsatellite loci and identified 20 that were polymorphic and suitable to determine paternity of fallow deer. To date, paternity has been studied for 87, 110 and 152 fallow deer fawns born between 2000 and 2002. Our results confirm the existence of a strong polygynous mating system in our population and confirm that the number of copulations performed by males is globally a good estimator of their reproductive success: males which performed the largest proportion of matings fathered the largest proportion of fawns. Nevertheless, we report some differences between the two measurements of the males’ reproductive success: measures of copulatory success underestimated the variance of the males’ reproductive success. On average, males whose copulatory score exceeded their paternity had mated with a higher proportion of younger females. Young females may be more likely to lose the conceptus, or their offspring may suffer high postnatal mortality.

Colonization of Ireland: revisiting ‘the pygmy shrew syndrome’ using mitochondrial, Y chromosomal and microsatellite markers

There is great uncertainty about how Ireland attained its current fauna and flora. Long-distance human-mediated colonization from southwestern Europe has been seen as a possible way that Ireland obtained many of its species; however, Britain has (surprisingly) been neglected as a source area for Ireland. The pygmy shrew has long been considered an illustrative model species, such that the uncertainty of the Irish colonization process has been dubbed ‘the pygmy shrew syndrome’. Here, we used new genetic data consisting of 218 cytochrome (cyt) b sequences, 153 control region sequences, 17 Y-intron sequences and 335 microsatellite multilocus genotypes to distinguish between four possible hypotheses for the colonization of the British Isles, formulated in the context of previously published data. Cyt b sequences from western Europe were basal to those found in Ireland, but also to those found in the periphery of Britain and several offshore islands. Although the central cyt b haplotype in Ireland was found in northern Spain, we argue that it most likely occurred in Britain also, from where the pygmy shrew colonized Ireland as a human introduction during the Holocene. Y-intron and microsatellite data are consistent with this hypothesis, and the biological traits and distributional data of pygmy shrews argue against long-distance colonization from Spain. The compact starburst of the Irish cyt b expansion and the low genetic diversity across all markers strongly suggests a recent colonization. This detailed molecular study of the pygmy shrew provides a new perspective on an old colonization question.

Investment in fighting in relation to body condition, age and dominance rank in the male fallow deer, Dama dama

According to life history theory, males of iteroparous species are expected to trade off investment between current and future reproduction based on age (mating strategy or terminal investment hypotheses) or body condition (individual quality hypothesis). However, although central to this latter model, the question concerning whether and to what extent condition regulates competitive investment in polygynous species is unknown. Consequently, we investigated this issue with reference to fight structure in fallow deer contests. Support for the individual quality hypothesis was limited: males with larger necks as determined by prerut neck girth fought for longer than males with smaller necks. However, prime-aged males had higher investment in fighting than preprime- or postprime-aged males indicating that investment in fighting might be age related. Other aspects of our results also failed to support condition-related predictions; although we found that jump clashing and vocal rate were related to weight loss and decline in neck girth, respectively, there was no relationship between investment in fighting and prerut measures of body size. Moreover, we also found that rank was predicted by investment in fighting (backward pushing) rather than body condition. Our results show that, in addition to body condition and age, variation in competitive investment between individuals also influences reproductive effort in the fallow deer.

Is the parallel walk between competing male fallow deer, Dama dama, a lateral display of individual quality?

During competitive encounters protagonists are expected to use signals of individual quality particularly if there is a risk of injury or death. Lateral presentation of body profile, by which information regarding phenotypic characteristics associated with individual quality are displayed, may represent such a strategy. During aggressive interactions, male fallow deer frequently engage in parallel walking which is assumed to represent a mutual display of quality, as mediated by exposure of the maximal profile of the body or antlers. We examined the context and role of the parallel walk during competitive encounters to investigate whether there was evidence that dyads of competing males were assessing differences in phenotypic characteristics. There was no evidence to support the hypotheses that the parallel walk is a lateral display of body size or weaponry or that its use is associated with a reduced level of escalated or risky behaviours during fighting. Total time spent fighting was not shorter when a parallel walk was present than when there was no parallel walk. The parallel walk was highly associated with fighting and it was more likely to be initiated by the subsequent loser. Furthermore, parallel walking frequently followed bouts of fighting and as such may represent a strategy that permits an animal the opportunity to decide whether to continue fighting. Parallel walking was also associated with a failure to resolve contests in favour of one animal indicating that it may be a means of withdrawing from further fighting without incurring a loss in dominance status. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.

Range expansion in an invasive small mammal: influence of life-history and habitat quality

Invasive species pose a major threat to biodiversity but provide an opportunity to describe the processes that lead to changes in a species’ range. The bank vole (Myodes glareolus) is an invasive rodent that was introduced to Ireland in the early twentieth century. Given its continuing range expansion, the substantial empirical data on its spread thus far, and the absence of any eradication program, the bank vole in Ireland represents a unique model system for studying the mechanisms influencing the rate of range expansion in invasive small mammals. We described the invasion using a reaction–diffusion model informed by empirical data on life history traits and demographic parameters. We subsequently modelled the processes involved in its range expansion using a rule-based spatially explicit simulation. Habitat suitability interacted with density-dependent parameters to influence dispersal, most notably the density at which local populations started to donate emigrating individuals, the number of dispersing individuals and the direction of dispersal. Whilst local habitat variability influenced the rate of spread, on a larger scale the invasion resembled a simple reaction–diffusion process. Our results suggest a Type 1 range expansion where the rate of expansion is generally constant over time, but with some evidence for a lag period following introduction. We demonstrate that a two-parameter empirical model and a rule-based spatially explicit simulation are sufficient to accurately describe the invasion history of a species that exhibits a complex, density-dependent pattern of dispersal.