Female reproductive success and calf survival in a North Sea coastal bottlenose dolphin (Tursiops truncatus) population

Using ecotourism boats for estimating the abundance of a bottlenose dolphin population in south-eastern Australia

It is challenging to collect robust, long-term datasets to properly monitor the viability and social structure of large, long-lived animals, especially marine mammals. The present study used a unique long-term dataset to investigate the population parameters and social structure of a poorly studied population of bottlenose dolphins (Tursiops sp.) in southern Port Phillip Bay, south-eastern Australia. Photo-identification images have been collected between 2012-2022 both opportunistically and following a protocol by patrons, staff, and volunteers of ecotourism companies using their vessels as platforms. The resulting large dataset was available to be processed through the online platform Flukebook and used in capture recapture models to estimate abundance and demographic parameters. In addition, the social structure of the population and the reproductive parameters were investigated. The marked adult population abundance (45.2 ± 2.7 individuals) was found to be stable over the last decade and the calving rate ranged between 0.06-0.19 new calves per identified individuals per year, while the inter-birth interval was 3.7 ± 0.8 years. Social analysis suggested the population has a fission-fusion structure with no apparent clusters. The stability of the population over the study period suggests no deleterious effect of anthropogenic or environmental factors during the last decade. This study is the outcome of the effort of the ecotourism organisations and the results obtained, along with their similarity to those of other dolphin populations worldwide, highlight the importance of such data sources for long-term information that would otherwise be too expensive or logistically difficult to obtain.

Multi-year passive acoustic monitoring of coastal dolphins along the Israeli Mediterranean shallow shelf reveals the impact of marine fish farms and trawling patterns on their habitat utilization

Dolphin preference and usage of various habitats along the Israeli shallow coastal shelf were investigated between 2019 and 2021 with passive acoustic monitoring devices. A hurdle model was used to examine the dolphins' visiting probability (chance of detection) and visit duration (length of stay once detected) across habitats, with diel cycle and season as explanatory variables. The influence of spatiotemporal prohibitions placed on trawler activity was also examined. It was found that dolphins exhibited higher presence in the vicinity of fish farms, up to three orders of magnitude, and even more so during periods when trawler activity was halted. The study also found a higher presence during the winter season and nighttime. Modeling did not find significant differences in the visiting probability or the visit duration between any non-farm-associated sites, including areas where trawling is prohibited. Further restrictions on the fishing industry may induce recovery of the benthic ecosystem and lower competition for resources, thus promoting higher dolphin presence in natural habitats along the shelf.

Total energy expenditure of bottlenose dolphins (Tursiops truncatus) of different ages

Marine mammals are thought to have an energetically expensive lifestyle because endothermy is costly in marine environments. However, measurements of total energy expenditure (TEE; kcal day-1) are available only for a limited number of marine mammals, because large body size and inaccessible habitats make TEE measurements expensive and difficult to obtain for many taxa. We measured TEE in 10 adult common bottlenose dolphins (Tursiops truncatus) living in natural seawater lagoons at two facilities (Dolphin Research Center and Dolphin Quest) using the doubly labeled water method. We assessed the relative effects of body mass, age and physical activity on TEE. We also examined whether TEE of bottlenose dolphins, and more generally of marine mammals, differs from that expected for their body mass compared with other eutherian mammals, using phylogenetic least squares (PGLS) regressions. There were no differences in body mass or TEE (unadjusted TEE and TEE adjusted for fat-free mass) between dolphins from the two facilities. Our results show that adjusted TEE decreased and fat mass increased with age. Different measures of activity were not related to age, body fat or adjusted TEE. Both PGLS and the non-phylogenetic linear regression indicate that marine mammals have an elevated TEE compared with that of terrestrial mammals. However, bottlenose dolphins expended 17.1% less energy than other marine mammals of similar body mass. The two oldest dolphins (>40 years) showed a lower TEE, similar to the decline in TEE seen in older humans. To our knowledge, this is the first study to show an age-related metabolic decline in a large non-human mammal.

Application of endocrine biomarkers to update information on reproductive physiology in gray whale (Eschrichtius robustus)

Most of our knowledge on reproductive biology of gray whales dates back to scientific research conducted during commercial whaling in the late 1950s and 1960s. The goal of the present study was to provide updated insights on reproductive physiology of gray whales, using progesterone and testosterone as biomarkers. We measured hormone concentrations using enzyme immunoassay (EIA) techniques in blubber biopsies collected from 106 individual whales from March to November over a span of 12 years (2004-2016) between California and Alaska. We found testosterone concentrations in males to increase significantly with age (P = 0.03). Adult males showed significantly elevated testosterone concentrations when sampled in the fall compared to the summer (P = 0.01), likely indicating physiological preparation for mating. We measured testosterone concentrations in females of different age classes, but no statistical differences were found. We found significantly higher progesterone concentrations in pregnant females compared to non-pregnant females and adult males (P< 0.001), indicating progesterone is a valid biomarker for pregnancy in gray whales. Both female and male calves had elevated progesterone concentrations, suggesting maternal transfer via lactation. We fit a mixture of two normal distributions to progesterone data from all non-calf females to identify clusters of high and low progesterone and estimated the probability of being pregnant for whales of unknown reproductive status. With this approach we identified likely pregnant and non-pregnant animals. This study represents an important milestone on reproductive profiles in this population, that can be used to estimate more accurate and precise reproductive parameters to be used for better understanding population dynamics of gray whales.

Behavioural Diversity Study in Bottlenose Dolphin (Tursiops truncatus) Groups and Its Implications for Welfare Assessments

Simple Summary

For the first time, a behavioural diversity study was conducted on several bottlenose dolphins (Tursiops truncatus) groups within European Association of Aquatic Mammals (EAAM) accredited facilities. This study was carried out by professional animal staff on 54 dolphins, and the goal was to analyse behavioural diversity in bottlenose dolphins at the group level to investigate how particular factors might impact the diversity of behaviours within the group and to discuss its implications for dolphin welfare assessments. This study showed its feasibility and revealed impacting factors that would need to be considered in future dolphin welfare assessments. We strongly believe that behavioural evaluations and measurements could be applied routinely on cetaceans under professional care to assess their welfare.

Abstract

In the recent past, animal welfare studies have tried to determine the best animal welfare measures and indicators. Expression of behavioural diversity is considered a potential positive welfare indicator, and to the authors’ knowledge, it has not been validated nor studied in cetaceans. For the first time, a behavioural diversity study on bottlenose dolphins (Tursiops truncatus) groups was conducted at six European facilities. The study was carried out by the animal care staff, biologists and veterinarians and included 54 dolphins housed in several group compositions at the different participating facilities. The goal of our study was to analyse behavioural diversity in bottlenose dolphins at the group level to investigate how particular factors might impact the diversity of behaviours within the group and to discuss its implications for dolphin welfare assessments. Eight factors (i.e., “observer location”, “number of individuals”, “age class”, “sex”, “social grouping”, “presence/absence of leading male”, “presence/absence of visitors” and “enrichment provision”) impacted the behavioural diversity of the observed groups, while no significant impact of the factors “time of day” and “activity before/after observation” could be found. Our study showed the feasibility of this kind of approach for cetaceans under professional care and the relevance to considering this parameter in dolphin welfare studies, despite certain limitations that warrant further research.

A deepening understanding of animal culture suggests lessons for conservation

A key goal of conservation is to protect biodiversity by supporting the long-term persistence of viable, natural populations of wild species. Conservation practice has long been guided by genetic, ecological and demographic indicators of risk. Emerging evidence of animal culture across diverse taxa and its role as a driver of evolutionary diversification, population structure and demographic processes may be essential for augmenting these conventional conservation approaches and decision-making. Animal culture was the focus of a ground-breaking resolution under the Convention on the Conservation of Migratory Species of Wild Animals (CMS), an international treaty operating under the UN Environment Programme. Here, we synthesize existing evidence to demonstrate how social learning and animal culture interact with processes important to conservation management. Specifically, we explore how social learning might influence population viability and be an important resource in response to anthropogenic change, and provide examples of how it can result in phenotypically distinct units with different, socially learnt behavioural strategies. While identifying culture and social learning can be challenging, indirect identification and parsimonious inferences may be informative. Finally, we identify relevant methodologies and provide a framework for viewing behavioural data through a cultural lens which might provide new insights for conservation management.

Presenting vertebral deformities in bottlenose dolphin Tursiops truncatus calves from a protected population in northeast Scotland

Photographs collected during a 23 yr photo-identification study in the Moray Firth were examined to assess the prevalence, type and severity of vertebral deformations present in bottlenose dolphin Tursiops truncatus calves. Fifteen cases of presenting spinal anomalies (scoliosis, kyphosis, lordosis and combinations thereof) of variable severity were identified in 7.4% of all known calves from the population. Thirteen of the 15 anomalies were either manifest from birth or acquired from an early age, as ascertained from longitudinal sightings histories of their mothers. Most afflicted calves died during early development or shortly after maternal separation. However, 3 survived to adulthood and persist in the population to date, in addition to 2 dependent infants whose fate remains to be established. At 15+ yr of age, the oldest surviving individual was remarkably one of the most severe cases identified, highlighting the ability of these delphinids for adaptation to such gross structural deformities. The aetiology of the observed conditions could be attributed to a range of causative factors that may have implications for the well-being and health of this North Sea coastal dolphin population, a topic which merits further investigation.

Joint estimation of survival and breeding probability in female dolphins and calves with uncertainty in state assignment

While the population growth rate in long-lived species is highly sensitive to adult survival, reproduction can also significantly drive population dynamics. Reproductive parameters can be challenging to estimate as breeders and nonbreeders may vary in resighting probability and reproductive status may be difficult to assess. We extended capture-recapture (CR) models previously fitted for data on other long-lived marine mammals to estimate demographic parameters while accounting for detection heterogeneity between individuals and state uncertainty regarding reproductive status. We applied this model to data on 106 adult female bottlenose dolphins observed over 13 years. The detection probability differed depending on breeding status. Concerning state uncertainty, offspring were not always sighted with their mother, and older calves were easier to detect than young-of-the-year (YOY), respectively, 0.79 (95% CI 0.59-0.90) and 0.58 (95% CI 0.46-0.68). This possibly led to inaccurate reproductive status assignment of females. Adult female survival probability was high (0.97 CI 95% 0.96-0.98) and did not differ according to breeding status. Young-of-the-year and 1-year-old calves had a significantly higher survival rate than 2-year-old (respectively, 0.66 CI 95% 0.50-0.78 and 0.45 CI 95% 0.29-0.61). This reduced survival is probably related to weaning, a period during which young are exposed to more risks since they lose protection and feeding from the mother. The probability of having a new YOY was high for breeding females that had raised a calf to the age of 3 or lost a 2-year-old calf (0.71, CI 95% 0.45-0.88). Yet, this probability was much lower for nonbreeding females and breeding females that had lost a YOY or a 1-year-old calf (0.33, 95% CI 0.26-0.42). The multievent CR framework we used is highly flexible and could be easily modified for other study questions or taxa (marine or terrestrial) aimed at modeling reproductive parameters.

Food-provisioning negatively affects calf survival and female reproductive success in bottlenose dolphins

Food-provisioning of wildlife can facilitate reliable up-close encounters desirable by tourists and, consequently, tour operators. Food-provisioning can alter the natural behavior of an animal, encouraging adverse behavior (e.g. begging for food handouts), and affect the reproductive success and the viability of a population. Studies linking food-provisioning to reproductive success are limited due to the lack of long-term datasets available, especially for long-lived species such as marine mammals. In Bunbury, Western Australia, a state-licensed food-provisioning program offers fish handouts to a limited number of free-ranging bottlenose dolphins (Tursiops aduncus). Coupled with long-term historical data, this small (<200 individuals), resident dolphin population has been extensively studied for over ten years, offering an opportunity to examine the effect of food-provisioning on the reproductive success of females (ntotal = 63; nprovisioned females = 8). Female reproductive success was estimated as the number of weaned calves produced per reproductive years and calf survival at year one and three years old was investigated. The mean reproductive success of provisioned and non-provisioned females was compared using Bayes factor. We also used generalized linear models (GLMs) to examine female reproductive success in relation to the occurrence of food-provisioning, begging behavior and location (within the study area). Furthermore, we examined the influence of these variables and birth order and climatic fluctuations (e.g. El Niño Southern Oscillation) on calf survival. Bayes factor analyses (Bayes factor = 6.12) and results from the best fitting GLMs showed that female reproductive success and calf survival were negatively influenced by food-provisioning. The negative effects of food-provisioning, although only affecting a small proportion of the adult females' population (13.2%), are of concern, especially given previous work showing that this population is declining.

Increasing trends in fecundity and calf survival of bottlenose dolphins in a marine protected area

Estimates of temporal variation in demographic rates are critical for identifying drivers of population change and supporting conservation. However, for inconspicuous wide-ranging species, births may be missed and fecundity rates underestimated. We address this issue using photo-identification data and a novel robust design multistate model to investigate changes in bottlenose dolphin fecundity and calf survival. The model allows for uncertainty in breeding status, and seasonal effects. The best model estimated an increase in the proportion of females with newborn calves from 0.16 (95% CI = 0.11-0.24) in 2001 to 0.28 (95% CI = 0.22-0.36) in 2016. First year calf survival also increased over this period from 0.78 (95% CI = 0.53-0.92) to 0.93 (95% CI = 0.82-0.98). Second year calf survival remained lower, but also showed an increase from 0.32 (95% CI = 0.19-0.48) to 0.55 (95% CI = 0.44-0.65). Females with newborn calves had a slightly higher mortality than those with older calves, but further work is required to evaluate potential costs of reproduction. This study presents a rare example of empirical evidence of a positive trend in reproduction and survival for a cetacean population using a Marine Protected Area.

Senescence impacts reproduction and maternal investment in bottlenose dolphins

Reproductive senescence is evident across many mammalian species. An emerging perspective considers components of reproductive senescence as evolutionarily distinct phenomena: fertility senescence and maternal-effect senescence. While fertility senescence is regarded as the ageing of reproductive physiology, maternal-effect senescence pertains to the declining capacity to provision and rear surviving offspring due to age. Both contribute to reproductive failure in utero making it difficult to differentiate between the two prenatally in the wild. We investigated both components in a long-lived mammal with prolonged maternal care through three parameters: calf survival, interbirth interval (IBI) and lactation period. We provide clear evidence for reproductive senescence in a wild population of bottlenose dolphins (Tursiops aduncus) using 34+ years of longitudinal data on 229 adult females and 562 calves. Calf survival decreased with maternal age, and calves with older mothers had lower survival than predicted by birth order, suggesting maternal-effect senescence. Both lactation period and IBIs increased with maternal age, and IBIs increased regardless of calf mortality, indicating interactions between fertility and maternal-effect senescence. Of calves that survived to weaning, last-born calves weaned later than earlier-born calves, evidence of terminal investment, a mitigating strategy given reduced reproductive value caused by either components of reproductive senescence.