Fritz Trillmich

Drastic effects of El Niño on Galapagos pinnipeds

SummaryPopulation dynamics of pinnipeds living in the tropical upwelling ecosystem of the Galapagos were strongly influenced by the 1982–83 Southern Oscillation-El Niño (EN) event which was the strongest recorded in this century. The Galapagos fur seal (Arctocephalus galapagoensis) population lost the four youngest year classes (1980–1983) almost entirely and approximately 30% of the adult females and non-territorial males. Mortality of large territorial males was almost 100%. Most of the 1982 year class of Galapagos sea lions (Zalophus californianus wollebaeki) died and there was a much lower pup production in the breeding season following EN. Recurrent EN events must strongly influence age structure and average population size of these and other otariid species depending on tropical upwelling ecosystems.

Social structure in a colonial mammal : unravelling hidden structural layers and their foundations by network analysis

The processes governing social interactions and the resulting relationships among members of colonially organized animals are largely unknown. Using network analytical tools we investigated fundamental components of social structure in a highly fluid fission–fusion society. We constructed a social network and studied the relative importance of sex and age class, fine-scale site fidelity and male territory distribution in a breeding colony of the Galapagos sealion, Zalophus wollebaeki. The social system was partitioned into a hierarchy with at least three levels. The outermost level of organization was given by a single social network, where all individuals of the population were interconnected. This level split into communities and further into cliques. Sex and age class influenced social structure at the population level, whereas fine-scale site fidelity explained most of the structure found at the community level. None of these assortment variables could account for clique structure, which might therefore be explained in terms of individual preferences, genetic relatedness or a combination of both. Male territories did not form the basic unit of social structure, but appeared simply to be superimposed on the structural backbone formed by females and young.

Maternal investment and sex-allocation in the Galapagos fur seal, Arctocephalus galapagoensis

SummaryMaternal investment and sex-allocation were measured in a large, sexually dimorphic mammal, the Galapagos fur seal (Arctocephalus galapagoensis). The sex ratio at birth was 1.06. Males were always heavier than females and, at least initially, grew faster. Growth was variable from year to year suggesting energetic constraints on maternal investment. Sucking time conrrelated with milk intake. Mothers suckled yearling and 2-year-old sons more than daughters of the same age. Age at weaning appeared to be the same in both sexes or even slightly greater in males. No sex differences was found in mortality prior to weaning or in post-weaning dispersal. Birth rates of females with yearlings or 2-year-olds were significantly lower than those of females with no dependent young. Mothers invested more in sons than in daughters until weaning. It is unlikely that higher post-weaning investment in daughters balances the higher pre-weaning investment in sons. Data on sex ratio at birth, different growth rates, and weaning age of the sexes are typical of otariid seals as a group. The results of this study fit Maynard Smiths (1980) model of the evolution of sex allocation better than Fishers (1930).

Lekking in marine iguanas: Female grouping and male reproductive strategies

Abstract The breeding and non-breeding distributions of male and female marine iguanas, Amblyrhynchus cristatus , and their mating behaviour were analysed. The population size was low because of a long lasting El Nino. The distribution of female-sized iguanas was more clumped during the mating season than before and afterwards. Model iguanas placed outside territories attracted more females during the breeding season than during the non-breeding season. The majority (75%) of large males established clustered territories while the remainder established single territories. The locations of the clusters were not related to physical characteristics of the habitat and were only partially related to non-breeding female densities. Non-territorial ‘sneaker’ males, similar in size to females, attempted copulations inside territories when territorial males were absent. Moderate sized, non-territorial males remained in the vicinity of territories and attempted to copulate forcefully with female-sized iguanas. This form of harassment was much higher outside territories, but levels did not differ between lek and single territories. Therefore, while harassment may lead to an increase in grouping among female-sized iguanas, it probably does not influence the type of territory visited. Females preferentially mated with the largest territorial males and reproductive success for these males was independent of lek size. The reproductive success of smaller territorial males, however, increased with lek size. Lekking in marine iguanas, therefore, may represent a ‘hotshot’ phenomenon where small territorial males associate with large males to increase reproductive success. The likely mechanism behind clustering appeared to be a greater propensity for females to mate in leks where stimulation rates were higher.

Ontogeny of diving behaviour in the Galapagos fur seal

We studied the ontogeny of diving behaviour in the Galapagos fur seal (Arctocephalus galapagoensis, Heller 1904). Six-month-old seals spent less than 12% of observation time at sea and were entirely dependent on maternal milk for nutrition. Maximum dive depths for this age group averaged 5.6 m, maximum durations 50 s (N = 12). Modal swim speeds averaged 0.55 m/s; maximum observed swim speeds ranged from 0.9-2 m/s (N = 5). Six-month-old seals swam distances of 2.9 km/24 hrs on average (N = 5). No diel patterns were apparent in their diving behaviour. One-year-old fur seals were the youngest age group to show substantial diving activity, reaching maximum depths of 47.5 m and durations of 2.4 min on average (N = 21). Modal swim speeds averaged 0.9 m/s, maximum speeds ranged from 1.4-2.3 m/s (N = 6). Total distances traveled averaged 15.4 km/trip to sea. Yearlings spent 23.9% of observation time at sea, the majority (> 95%) of that at night. This corresponds to the activity pattern of adult females who dive exclusively at night. Yearlings were the youngest age group to contribute to their own nutrition through independent foraging, but were still dependent on their mothers: no yearlings were successfully weaned in this study. 18-month-old seals were better divers, spending 27.6% of observation time at sea, mostly during the night. Maximum dive depths for this age group averaged 61.1 m; durations 3.1 min (N = 11). Modal swim speeds averaged 1.4 m/s (N = 4), and were comparable to modal speeds of two adult females of 1.2 and 1.6 m/s. Maximum speeds for these juveniles ranged from 2.2-2.7 m/s (N = 4); the two adult females reached 2.8 and 4 m/s respectively. Total travel distances averaged 42 km/trip to sea (N = 4 juveniles). These distances corresponded to those covered by two adult females, 43 and 45 km/trip. Two-year-old fur seals included the youngest that were successfully weaned in this study. They spent 35% of observation time at sea, primarily at night. Maximum dive depths averaged 69.7 m, durations 3.4 min (N = 20). Adult females were the best divers in this study, spending 49.5% of observation time at sea, almost exclusively at night. Maximum dive depths averaged 106.5 m, durations 4.5 min (N = 32). During ontogeny, age was the best predictor of time spent at sea. For animals older than six months, body mass was the best predictor for maximum dive depth as well as maximum and median durations. The physiological maturation process precludes weaning before the age of one year. After that, the diving capacity of young fur seals takes a central role in the weaning process, in conjunction with the temporal patterns of relative prey accessibility.

Sibling competition and cooperation in mammals: challenges, developments and prospects

Many vertebrates grow up in the company of same or different-age siblings, and relations among them can be expected to significantly influence individual life histories and the development of individual morphological, physiological, and behavioral phenotypes. Although studies in birds still dominate and have stimulated most theoretical considerations, the increasing number of mammalian studies promises to broaden our understanding of this complex field by enabling interesting comparisons with the rather different bird system. It therefore seems timely to bring together recent studies of sibling relations in mammals and to demonstrate what these can offer in the way of fresh insights. In this brief review, intended to accompany a series of papers on a diverse range of mammals, we outline the current state of sibling research in mammals, comparing it to the better studied birds. Most obviously, in mammals, mother and young are in much closer contact during early life than in birds, and siblings can influence each other’s development as well as the mother’s physiology while still in utero. During nursing, mammalian young also encounter a very different feeding situation to bird siblings. These contrasts should help stimulate further debate, as well as provide further opportunities to study the relative importance of maternal versus sibling effects on individual development. Finally, we discuss the need to balance studies of sibling competition and conflict with a consideration of the benefits accruing to individuals from sibling presence and the need for long-term studies of the influence of early sibling relations on individual development and life histories.

Parental Investment in Pinnipeds

Publisher Summary This chapter gives information on the natural history of maternal strategies with focus on data about fitness effects of maternal care in pinnipeds. Pinnipeds are a group of species for which excellent data on maternal strategies exist. Energetic expenditure of mothers for nursing their young is high and in phocids is also highly concentrated in time. Energetics of free-ranging animals have been measured in unusual detail in a number of species. Moreover, sexual size dimorphism is extreme in the best-investigated species. For Northern elephant seals, lifetime reproductive success and its correlation with size have been studied in detail for females as well as for males. Thus, at first sight, the group seemed to offer striking patterns of maternal care as well as strong evidence for extreme sexual selection on males and, therefore, promised relatively large and easier to analyze effects of parental investment than many other mammalian groups. The chapter also focuses on evidence for maternal investment and the relationship between energy expenditure and investment and the evidence for and against differential investment in male and female offspring. It presents the observation of post weaning care in pinnipeds for which it is not clear if investment is involved at all.

Unfolding Personalities : The Importance of Studying Ontogeny

We aim to stimulate an ontogenetic approach to personalities. We explain the importance of studying development for understanding proximate and ultimate aspects of personality and critically discuss, partly by perhaps provocative statements, our current lack of knowledge and potential approaches to the study of personality development. We first clarify some terminology and argue for a difference between behavioral profiles (BP; at the descriptive level) and personality (at the explanatory level). We then focus on the issue of temporal stability of personality, arguing that based on evolutionary theory, neurophysiological knowledge, and recent findings, personality is probably less stable than often thought. Next we consider the potential influence of genes, discussing gene by environment correlations and interactions and argue that developmental changes in the regulation of DNA expression are probably more relevant than individual differences in DNA sequence. We end by suggesting perspectives for future research.

Lunar cycles in diel prey migrations exert a stronger effect on the diving of juveniles than adult Galapagos fur seals

In our study of the development of diving in Gal pagos fur seals, we analysed changes in diving activity and body mass trends over the lunar cycle. Based on previously observed lunar cycles in colony attendance patterns, we hypothesized a greater impact of prey migrations of deep scattering layer organisms on younger fur seals. Using electronic dive recorders, we determined that seals dived less and deeper on moonlit nights than at new moon, and incurred body mass losses. These changes in foraging over the lunar cycle correlate with the suppression of the vertical migration of prey by lunar light. All effects were more pronounced in juveniles than adult females, with greater relative mass loss during full moon, which must (i) negatively affect long-term juvenile growth rates, (ii) lengthen periods of maternal dependence, and (iii) contribute to the lowest reproductive rate reported for seals. This underlines the importance of studying ontogeny in order to understand life histories, and for determining the susceptibility of animal populations to fluctuations in food availability.

ENERGY LIMITS TO BODY SIZE IN A GRAZING REPTILE, THE GALAPAGOS MARINE IGUANA

Galapagos marine iguana (Amblyrhynchus cristatus) populations show con- siderable differences in body size. Adult body mass varies by more than 10-fold, and body length (snout-vent length, SVL) varies by -2.2-fold. Predation and interspecific food com- petition are largely absent and can be excluded as potential forces explaining differences in body size among populations. This provides an ideal system in which to determine how proximate environmental factors affect adult body size. We compared the small iguanas from Genovesa Island (mean adult male SVL 250 mm) to the larger Santa Fe iguanas (mean adult male SVL 400 mm). Marine iguanas forage on intertidal algae pastures in scramble competition. Energy availability was lower on Genovesa than on Santa Fe, because of lower marine productivity on Genovesa. The length of grazable algal turf decreased with increas- ing sea surface temperature (SST). SST was -20C lower on Santa Fe than on Genovesa, implying 1.5 mm lower algae pastures on the latter. Genovesa showed a fivefold lower standing algal biomass and a twofold lower productivity of algae pastures than did Santa Fe. The smallest iguanas of each island had 1.5-fold higher bite rates during foraging, and their absolute food intake per day was 35% that of the largest iguanas. However, food intake per bite per gram of body mass was about twice as high for small iguanas as for large iguanas. Large iguanas of both islands showed a marked decline in body mass during the two study years, whereas small iguanas (SVL 200 mm SVL on Genovesa and >310 mm SVL on Santa Fe significantly lost mass; in the more productive year (1992/1993), thresholds were 230 mm and 350 mm SVL, respectively. Thus, food abundance (length and turnover of algal swards) explained differences in adult body length and mass between islands as a result of energetic limitation. On a given island, foraging efficiency (intake/bite) explained the negative energy balance of large compared to small iguanas. This also explained why, on both islands, the largest animals suffered higher mortality rates than did medium-sized ones when food was scarce. The finding that small animals outcompeted larger ones because of their higher foraging efficiency resembles the grazing succession in ungulate herbivores.