Costly lifetime maternal investment in killer whales

The expensive son hypothesis

In its initial form, the expensive son hypothesis postulates that sons from male-biased sexually dimorphic species require more food during growth than daughters, which ultimately incur fitness costs for mothers predominantly producing and rearing sons. We first dissect the evolutionary framework in which the expensive son hypothesis is rooted, and we provide a critical reappraisal of its differences from other evolutionary theories proposed in the field of sex allocation. Then, we synthesize the current (and absence of) support for the costs of producing and rearing sons on maternal fitness components (future reproduction and survival). Regarding the consequences in terms of future reproduction, we highlight that species with pronounced sexual size dimorphism display a higher cost of sons than of daughters on subsequent reproductive performance, at least in mammals. However, in most studies, the relative fitness costs of producing and rearing sons and daughters can be due to sex-biased maternal allocation strategies rather than differences in energetic demands of offspring, which constitutes an alternative mechanism to the expensive son hypothesis stricto sensu. We observe that empirical studies investigating the differential costs of sons and daughters on maternal survival in non-human animals remain rare, especially for long-term survival. Indeed, most studies have investigated the influence of offspring sex (or litter sex ratio) at year T on survival at year T + 1, and they rarely provide a support to the expensive son hypothesis. On the contrary, in humans, most studies have focused on the relationship between proportion of sons and maternal lifespan, but these results are inconsistent. Our study highlights new avenues for future research that should provide a comprehensive view of the expensive son hypothesis, by notably disentangling the effects of offspring behaviour from the effect of sex-specific maternal allocation. Moreover, we emphasize that future studies should also embrace the mechanistic side of the expensive son hypothesis, largely neglected so far, by deciphering the physiological pathways linking son's production to maternal health and fitness.

Individual variation in activity budgets of a stable population of killer whales in managed care across a year

Activity budget investigations are necessary to understand how individuals within a group manage their daily activities, thus providing insights into the social dynamics of a species. Our objective was to explore and describe the behavioral activities within a group of managed care killer whales. From 261 hours of coded surface video collected from April 2022 to January 2023, we scan-sampled day-time hours to examine eight behavioral categories exhibited by 8-9 killer whales bimonthly. Minimal sex differences were found within each behavioral category across month and hour block. Females exhibited significantly more attention to trainers and rubbing on the environment than males. Individual variability was documented for all behavioral categories. The youngest male showed the most (43 % of scans) proximity and social interactions with conspecifics whereas all adults, both females and males, showed similar levels of social behavior. Three adult females and one adult male spent about a third of all scans observing their trainer(s) as they moved around the habitat. This observational behavior has been described in other delphinids as anticipatory behavior indicating a willingness to participate in future interactions with their trainers whether food rewards were available or not, and has been confirmed to represent a state of positive well-being. All killer whales actively engaged with peers and their environment in about 40 % of the documented scans. These findings are similar in degree to that observed in various other delphinid populations and support the interpretation that this killer whale group is in receive of positive welfare, displaying normal levels of social and individual behaviors for this species. Our results add to the growing body of knowledge about how killer whales manage their actions, both with conspecifics and their environment, across different social settings given their surroundings.

Hormonal Actions in the Medial Preoptic Area Governing Parental Behavior: Novel Insights From New Tools

The importance of hormones in mediating a behavioral transition in mammals from a virgin or nonparenting state to parental state was established around 50 years ago. Extensive research has since revealed a highly conserved neural circuit that underlies parental behavior both between sexes and between mammalian species. Within this circuit, hormonal action in the medial preoptic area of the hypothalamus (MPOA) has been shown to be key in timing the onset of parental behavior with the birth of offspring. However, the mechanism underlying how hormones act in the MPOA to facilitate this change in behavior has been unclear. Technical advances in neuroscience, including single cell sequencing, novel transgenic approaches, calcium imaging, and optogenetics, have recently been harnessed to reveal new insights into maternal behavior. This review aims to highlight how the use of these tools has shaped our understanding about which aspects of maternal behavior are regulated by specific hormone activity within the MPOA, how hormone-sensitive MPOA neurons integrate within the wider neural circuit that governs maternal behavior, and how maternal hormones drive changes in MPOA neuronal function during different reproductive states. Finally, we review our current understanding of hormonal modulation of MPOA-mediated paternal behavior in males.

An early-life challenge: becoming an older sibling in wild mandrills

In monotocous mammals, most individuals experience the birth of a younger sibling. This period may induce losses in maternal care and can be physiologically, energetically and emotionally challenging for the older sibling, yet has rarely been studied in wild primates. We used behavioural data collected from a natural population of mandrills to investigate changes in maternal care and mother-juvenile relationship throughout the transition to siblinghood (TTS), by comparing juveniles who recently experienced the birth of a younger sibling, to juveniles who did not. We found that the TTS was associated with an abrupt cessation of the weaning process for the juvenile, and to a decrease in maternal affiliation. Juveniles' reactions were sex-specific, as males associated less with their mother, while females tended to groom their mother more often after the birth of their sibling. Despite the substantial loss of maternal care, juveniles did not show an increase in conflict or anxiety-related behaviours. This study contributes to explain why short interbirth intervals often pose a risk to juveniles' survival in monotocous primates. Our results contrast existing studies and further highlight the importance of examining the TTS in species and populations with various life histories and ecologies.

Social and genetic connectivity despite ecological variation in a killer whale network

Philopatric kin-based societies encourage a narrow breadth of conservative behaviours owing to individuals primarily learning from close kin, promoting behavioural homogeneity. However, weaker social ties beyond kin, and across a behaviourally diverse social landscape, could be sufficient to induce variation and a greater ecological niche breadth. We investigated a network of 457 photo-identified killer whales from Norway (548 encounters in 2008-2021) with diet data available (46 mixed-diet individuals feeding on both fish and mammals, and 411 exclusive fish-eaters) to quantify patterns of association within and between diet groups, and to identify underlying correlates. We genotyped a subset of 106 whales to assess patterns of genetic differentiation. Our results suggested kinship as main driver of social bonds within and among cohesive social units, while diet was most likely a consequence reflective of cultural diffusion, rather than a driver. Flexible associations within and between ecologically diverse social units led to a highly connected network, reducing social and genetic differentiation between diet groups. Our study points to a role of social connectivity, in combination with individual behavioural variation, in influencing population ecology in killer whales.

Quantifying maternal investment in mammals using allometry

Maternal investment influences the survival and reproduction of both mothers and their progeny and plays a crucial role in understanding individuals' life-history and population ecology. To reveal the complex mechanisms associated with reproduction and investment, it is necessary to examine variations in maternal investment across species. Comparisons across species call for a standardised method to quantify maternal investment, which remained to be developed. This paper addresses this limitation by introducing the maternal investment metric - MI - for mammalian species, established through the allometric scaling of the litter mass at weaning age by the adult mass and investment duration (i.e. gestation + lactation duration) of a species. Using a database encompassing hundreds of mammalian species, we show that the metric is not highly sensitive to the regression method used to fit the allometric relationship or to the proxy used for adult body mass. The comparison of the maternal investment metric between mammalian subclasses and orders reveals strong differences across taxa. For example, our metric confirms that Eutheria have a higher maternal investment than Metatheria. We discuss how further research could use the maternal investment metric as a valuable tool to understand variation in reproductive strategies.

The evolution of menopause in toothed whales

Understanding how and why menopause has evolved is a long-standing challenge across disciplines. Females can typically maximize their reproductive success by reproducing for the whole of their adult life. In humans, however, women cease reproduction several decades before the end of their natural lifespan1,2. Although progress has been made in understanding the adaptive value of menopause in humans3,4, the generality of these findings remains unclear. Toothed whales are the only mammal taxon in which menopause has evolved several times5, providing a unique opportunity to test the theories of how and why menopause evolves in a comparative context. Here, we assemble and analyse a comparative database to test competing evolutionary hypotheses. We find that menopause evolved in toothed whales by females extending their lifespan without increasing their reproductive lifespan, as predicted by the 'live-long' hypotheses. We further show that menopause results in females increasing their opportunity for intergenerational help by increasing their lifespan overlap with their grandoffspring and offspring without increasing their reproductive overlap with their daughters. Our results provide an informative comparison for the evolution of human life history and demonstrate that the same pathway that led to menopause in humans can also explain the evolution of menopause in toothed whales.

The social formation of fitness: lifetime consequences of prenatal nutrition and postnatal care in a wild mammal population

Research in medicine and evolutionary biology suggests that the sequencing of parental investment has a crucial impact on offspring life history and health. Here, we take advantage of the synchronous birth system of wild banded mongooses to test experimentally the lifetime consequences to offspring of receiving extra investment prenatally versus postnatally. We provided extra food to half of the breeding females in each group during pregnancy, leaving the other half as matched controls. This manipulation resulted in two categories of experimental offspring in synchronously born litters: (i) 'prenatal boost' offspring whose mothers had been fed during pregnancy, and (ii) 'postnatal boost' offspring whose mothers were not fed during pregnancy but who received extra alloparental care in the postnatal period. Prenatal boost offspring lived substantially longer as adults, but postnatal boost offspring had higher lifetime reproductive success (LRS) and higher glucocorticoid levels across the lifespan. Both types of experimental offspring had higher LRS than offspring from unmanipulated litters. We found no difference between the two experimental categories of offspring in adult weight, age at first reproduction, oxidative stress or telomere lengths. These findings are rare experimental evidence that prenatal and postnatal investments have distinct effects in moulding individual life history and fitness in wild mammals. This article is part of the theme issue 'Evolutionary ecology of inequality'.

Postreproductive female killer whales reduce socially inflicted injuries in their male offspring

Understanding the evolution of menopause presents a long-standing scientific challenge1,2,3-why should females cease ovulation prior to the end of their natural lifespan? In human societies, intergenerational resource transfers, for example, food sharing and caregiving, are thought to have played a key role in the evolution of menopause, providing a pathway by which postreproductive females can boost the fitness of their kin.4,5,6 To date however, other late-life contributions that postreproductive females may provide their kin have not been well studied. Here, we test the hypothesis that postreproductive female resident killer whales (Orcinus orca) provide social support to their offspring by reducing the socially inflicted injuries they experience. We found that socially inflicted injuries, as quantified by tooth rake marks, are lower for male offspring in the presence of their postreproductive mother. In contrast, we find no evidence that postreproductive mothers reduce rake marking in their daughters. Similarly, we find no evidence that either reproductive mothers or grandmothers (reproductive or postreproductive) reduce socially inflicted injuries in their offspring and grandoffspring, respectively. Moreover, we find that postreproductive females have no effect on reducing the rake marks for whales in their social unit who are not their offspring. Taken together, our results highlight that directing late-life support may be a key pathway by which postreproductive females transfer social benefits to their male offspring.

Animal behavior: Killer whale mamas' boys

Long-term study of killer whales reveals that sons stay with their mothers for life, and although this benefits sons, it negatively impacts the mother's reproductive output.