Simona Sanvito

Vocal signalling of male southern elephant seals is honest but imprecise

In the most common models of communication, it is assumed that animals provide reliable information about phenotype, and hence can settle competitive contests without physical interactions like fights. This assumption has rarely been tested for wild mammals. Recent studies of mammals have revealed relationships of vocal attributes to age and body size. Here, we analyse relationships of frequency attributes of agonistic vocalizations to phenotype (age, body size, proboscis size and agonistic behaviour) in males of the southern elephant seal, Mirounga leonina, a species with intense male competition for access to females, and in which vocalizations are used frequently to settle male–male contests. We analysed formant structure and vocal tract size, and found that nasal and oral components of the vocal tract contribute separately to vocal formants; hence, the males proboscis serves to elongate the vocal tract. We also found that formants in the upper part of the frequency spectrum (fourth and fifth in particular) and formant dispersion convey significant information about age, size and resource holding potential at large, and, therefore, can be honest signals of a vocalizers phenotype. Explained variance was statistically significant in our study and in similar studies but was not high, so formant structure cannot serve as the sole basis of acoustic assessment. Other possible sources of information exchanged in elephant seal contests are nonvocal acoustic signals (e.g. vibrations) and optical displays.

The cost of success: reproductive effort in male southern elephant seals (Mirounga leonina)

Reproductive effort is a key parameter of life history because it measures the resources allocated to reproduction at the expense of growth and maintenance. Male reproductive effort always had a minor role with respect to female effort both in the development of theories and in field research. Elephant seals are an ideal subject for reproductive effort studies because they fast during the breeding season, splitting the phase of energy acquisition from the phase of energy use for breeding. In this paper, we present results on male reproductive effort (weight loss estimated by photogrammetry) in southern elephant seals (Mirounga leonina), the most dimorphic and polygynous of all mammal species. We show that total reproductive effort increases with age, with no sign of late decrease or senescence. Male reproductive effort in this species depends mostly on behavioral factors, i.e., the success in competition with other males, and the intensity of interaction with females. A large effort results in large gains in both mating success and fertilizations. The large reproductive success that a few males are able to achieve come at a big cost in terms of energy expenditure, but this cost does not seem to affect the likelihood of survival to the following breeding season.

Relatedness and site fidelity at the southern elephant seal, Mirounga leonina, breeding colony in the Falkland Islands

Southern elephant seals are highly gregarious during the breeding season, and both sexes show fidelity to the colony. We used microsatellite DNA analysis to assess kinship among seals in the main colony on the Falkland Islands. Specifically, we investigated whether females tend to cluster with close kin and avoid mating with male kin. We also tested expectations for kinship patterns based on sex differences in site fidelity and philopatry. Relatedness within a harem was significantly greater than between harems for only two of seven harems and was not related to harem size. Some long-term associations of female kin were found within harems, including associations of up to 5 years, but kinship among these females was not significantly higher on average than among dyads of other returning females. There was no pattern suggesting that females tended to choose harems with harem holders that were either more or less related to them than alternative harem holders. Overall, pairwise comparisons of females showed significantly greater kinship than pairwise comparisons of males, consistent with previous studies suggesting greater male dispersal.

Viability of the southern elephant seal population of the Falkland Islands

Accurate long-term series of demographic data are available for most populations of southern elephant seals. However, research on the elephant seals of the Falkland Islands began only recently, and information for an accurate forecasting of the future of this population is lacking. In this paper we present data on the current status of the population and its trend in size during the last 11 years. We built an age-structured model of the population and analyzed the effect of variation in demographic parameters on population growth. Elasticity analysis demonstrated that variation in mortality has a more pronounced effect on instantaneous growth rate than an equivalent variation in fecundity. We examined the effects of environmental variability, inbreeding and catastrophes on population viability by a computer simulation using the VORTEX PVA programme. In the stochastic model, the most important factor affecting extinction risk was variability in mortality rates, in particular of the adult classes. We concluded that, although the population does not appear to be at immediate risk of extinction, its small size and isolation make it necessary to accurately monitor the population trends and to acquire additional information on life history and feeding strategies.

SOURCE LEVEL OF MALE VOCALISATIONS IN THE GENUS MIROUNGA: REPEATABILITY AND CORRELATES

ABSTRACT Male vocalisations have an important role in mating tactics, breeding strategies and sexual selection. Most studies of vocalisations are concentrated on the time and frequency domains, while the intensity of sound, an important acoustic parameter that should be related to body size, is almost completely ignored as a possible honest signal of resource holding potential (RHP) and cue for mate choice. In this paper, we analyse the repeatability, the correlations with age and size, and the relationship with breeding status of source level (SL) of male vocalisations in the two species of elephant seals (Mirounga leonina and M. angustirostris). We found a high repeatability of SL, equal or higher than the repeatability of frequency domain parameters estimated in a previous study. Southern elephant seal males were significantly larger and produce significantly more powerful vocalisations than northern males. Moreover, in each species SL was related to age, body size, and breeding status of males, but relationships were weak and accounted for just a small proportion of SL variance. We conclude that, although SL may be an honest signal of gross differences of RHP, it is not, by itself, a good candidate for the transmission of high-resolution information on individual phenotype. A combination of SL and frequency components could be, on the contrary, an effective way to communicate RHP.

Development of aggressive vocalizations in male southern elephant seals (Mirounga leonina): maturation or learning?

Summary Vocalizations are an important component of male elephant seal agonistic behaviour. Acoustic and behavioural components of vocalizations emitted during agonistic contests show gross differences between young and old males, but the variation with age depends on the specific feature. Vocalizations become more frequent and effective at later ages. Acoustic features that are constrained by structural phenotype, which changes with age, also should change with age, while acoustic features that are independent from structural phenotype should show no relationship with age. We demonstrate that, in southern elephant seals, formant frequencies, which are constrained by the vocal tract length and, therefore, by body size, show a clear decrease with age, whereas temporal and structural features of sounds, which potentially are unconstrained, show no correlation with age. Formants ontogeny seems, therefore, to be mostly the result of body maturation, and hence formants may be reliable signals of age. In contrast, acoustic features such as temporal features and syllable structure, are free to change, and hence may serve as the raw material for vocal learning and individual recognition.

Opportunity for selection in southern elephant seals ( Mirounga leonina ): the effect of spatial scale of analysis

The opportunity for selection, I, calculated as the variance in relative fitness, sets an upper limit to the amount of adaptive change that selection may produce. Therefore, it is a potentially valuable, and frequently used, measure of the potential of action of phenotypic selection. Although many different aspects of I calculation and analysis have been explored, the effect of the spatial scale chosen for calculation received little attention, notwithstanding the growing evidence that natural populations are not homogeneous and present a hierarchical spatial structure. The effect of scale on the estimation of I was examined from data collected in two populations of southern elephant seals (Mirounga leonina), an easily observable and strongly polygynous species. A significant effect of spatial scale on three important aspects of I calculation and analysis was found: dependence of I on mean fitness, between population variation of I, and effect of local demography on I.

Blood Spots in Pinnipedia Hormone Studies: Measure of Cortisol Levels in Southern Elephant Seals (Mirounga leonina)

The collection of blood spots on filter paper for hormone analysis has become quite popular in human and primate studies, mostly because of the ease of handling, storage, and transportation of samples, but has never been tested in wild marine mammals. In this paper, we describe a protocol for the collection of blood spots and the analysis of cortisol in southern elephant seal (Mirounga leonina) weanlings. We demonstrated that cortisol measured in blood spots is very well correlated to cortisol measured in serum samples, and we calculated equations to convert between the two. We describe the possible pitfalls of the blood spot analysis protocol, and suggest solutions. The use of blood spots for hormone analysis presents many advantages for field research, may open interesting opportunities like the serial sampling of unrestrained adult individuals, and represents a step towards the reduction of invasiveness of hormone studies.

Behavioural and morphometric measurements of parental investment in southern elephant seals at the Falkland Islands

Abstract. Parental investment is a key variable in the study of breeding strategies and life-histories evolution. In Pinnipedia, parental investment is usually calculated from direct measurements of pup weight gain or energy transfer between the mother and the pup. These direct methods always involve handling and restraining procedures that pose practical, logistical and ethical problems. To evaluate if weighing can be substituted by indirect observational estimates of parental investment, we analysed the relationship among various behavioural measures of suckling and post-natal growth in the southern elephant seal population of Sea Lion Island (Falkland Islands). Behavioural measures were in all cases a poor predictor of true investment as estimated by weighing. We concluded that there are currently no effective alternatives to direct handling, and that the best way to reduce the potential adverse impact of investment studies is the improvement of the handling protocol, which should include an estimation of the long-term effects on the health of handled animals. Further research is needed to test the validity of non-behavioural indirect methods (e.g. 3D photogrammetry).