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1. The cost of associating with males for Bornean and Sumatran female orangutans: a hidden form of sexual conflict?

   https://doi.org/10.1007/s00265-020-02948-4  

   Kunz, Julia A.; Duvot, Guilhem J.; van Noordwijk, Maria A.; Willems, Erik P.; Townsend, Manuela; Mardianah, Neneng; Utami Atmoko, Sri Suci; Vogel, Erin R.; Nugraha, Taufiq Purna; Heistermann, Michael; et al (January 2021, Behavioral Ecology and Sociobiology)

   null (Ed.)

   Abstract Sexual coercion, in the form of forced copulations, is relatively frequently observed in orangutans and generally attributed to their semi-solitary lifestyle. High ecological costs of association for females may be responsible for this lifestyle and may have prevented the evolution of morphological fertility indicators (e.g., sexual swellings), which would attract (male) associates. Therefore, sexual conflict may arise not only about mating per se but also about associations, because males may benefit from associations with females to monitor their reproductive state and attempt to monopolize their sexual activities. Here, we evaluate association patterns and costs for females when associating with both males and females of two different orangutan species at two study sites: Suaq, Sumatra ( Pongo abelii ), and Tuanan, Borneo ( Pongo pygmaeus wurmbii ). Female association frequency with both males and females was higher in the Sumatran population, living in more productive habitat. Accordingly, we found that the cost of association, in terms of reduced feeding to moving ratio and increased time being active, is higher in the less sociable Bornean population. Males generally initiated and maintained such costly associations with females, and prolonged associations with males led to increased female fecal cortisol metabolite (FCM) levels at Tuanan, the Bornean population. We conclude that male-maintained associations are an expression of sexual conflict in orangutans, at least at Tuanan. For females, this cost of association may be responsible for the lack of sexual signaling, while needing to confuse paternity. Significance statement Socioecological theory predicts a trade-off between the benefits of sociality and the ecological costs of increased feeding competition. Orangutans’ semi-solitary lifestyle has been attributed to the combination of high association costs and low predation risk. Previous work revealed a positive correlation between association frequencies and habitat productivity, but did not measure the costs of association. In this comparative study, we show that females likely incur costs from involuntary, male-maintained associations, especially when they last for several days and particularly in the population characterized by lower association frequencies. Association maintenance therefore qualifies as another expression of sexual conflict in orangutans, and especially prolonged, male-maintained associations may qualify as an indirect form of sexual coercion. 

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2. Female‐directed aggression by adolescent male chimpanzees primarily constitutes dominance striving, not sexual coercion

   https://doi.org/10.1002/ajpa.24296  

   Enigk, Drew K.; Emery Thompson, Melissa; Machanda, Zarin P.; Wrangham, Richard W.; Muller, Martin N. (May 2021, American Journal of Physical Anthropology)

   Abstract ObjectivesChimpanzees (Pan troglodytes) are notable for exhibiting high levels of male‐to‐female aggression. Much of this aggression from adult males serves sexually coercive functions. Despite being smaller and lower‐ranking than adult males, adolescent males also engage in regular aggression against adult females. Here, we test whether the primary function of this aggression is sexual coercion, as in adult males, or, alternatively, whether adolescent males use aggression to establish social dominance over females. Materials and MethodsWe analyzed 1771 copulations and 1812 instances of male‐initiated aggression between adolescent males (aged nine through 14 years) and adult females across 21 years of observation of the Kanyawara chimpanzee community in Kibale National Park, Uganda. ResultsOur test of the sexual coercion hypothesis revealed that adolescent males did not selectively target cycling females for aggression, nor did aggression against cycling females predict rates of copulation with those females. Our test of the social dominance hypothesis showed that males succeeded in dominating all adult females before, or soon after, dominating their first adult male. Additionally, we found that adolescent males dominated females approximately in the order of the females' own ranks, from the bottom to the top of the female hierarchy. DiscussionOur data illustrate that the establishment of social dominance was more important than sexual coercion in explaining patterns of adolescent male aggression toward females. In comparison, evidence for sexual coercion was clear and compelling in adult males. These findings highlight that the primary function of male‐to‐female aggression differs between adolescent and adult males. 

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3. Testosterone Coordinates Gene Expression Across Different Tissues to Produce Carotenoid-Based Red Ornamentation

   https://doi.org/10.1093/molbev/msad056  

   Khalil, Sarah; Enbody, Erik D; Frankl-Vilches, Carolina; Welklin, Joseph F; Koch, Rebecca E; Toomey, Matthew B; Sin, Simon Yung; Edwards, Scott V; Gahr, Manfred; Schwabl, Hubert; et al (April 2023, Molecular Biology and Evolution)

   Parsch, John (Ed.)

   Abstract Carotenoid pigments underlie most of the red, orange, and yellow visual signals used in mate choice in vertebrates. However, many of the underlying processes surrounding the production of carotenoid-based traits remain unclear due to the complex nature of carotenoid uptake, metabolism, and deposition across tissues. Here, we leverage the ability to experimentally induce the production of a carotenoid-based red plumage patch in the red-backed fairywren (Malurus melanocephalus), a songbird in which red plumage is an important male sexual signal. We experimentally elevated testosterone in unornamented males lacking red plumage to induce the production of ornamentation and compared gene expression in both the liver and feather follicles between unornamented control males, testosterone-implanted males, and naturally ornamented males. We show that testosterone upregulates the expression of CYP2J19, a gene known to be involved in ketocarotenoid metabolism, and a putative carotenoid processing gene (ELOVL6) in the liver, and also regulates the expression of putative carotenoid transporter genes in red feather follicles on the back, including ABCG1. In black feathers, carotenoid-related genes are downregulated and melanin genes upregulated, but we find that carotenoids are still present in the feathers. This may be due to the activity of the carotenoid-cleaving enzyme BCO2 in black feathers. Our study provides a first working model of a pathway for carotenoid-based trait production in free-living birds, implicates testosterone as a key regulator of carotenoid-associated gene expression, and suggests hormones may coordinate the many processes that underlie the production of these traits across multiple tissues. 

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4. Ejaculate Allocation and Sperm Characteristics Differ among Alternative Male Types in a Species of Fish with Cooperation and Competition among Unrelated Males

   https://doi.org/10.3390/cells10102612  

   Alonzo, Suzanne H.; Stiver, Kelly A.; Kindsvater, Holly K.; Marsh-Rollo, Susan E.; Nugent, Bridget; Kazancıoğlu, Erem (October 2021, Cells)

   Sexual selection arising from sperm competition has driven the evolution of immense variation in ejaculate allocation and sperm characteristics not only among species, but also among males within a species. One question that has received little attention is how cooperation among males affects these patterns. Here we ask how male alternative reproductive types differ in testes size, ejaculate production, and sperm morphology in the ocellated wrasse, a marine fish in which unrelated males cooperate and compete during reproduction. Nesting males build nests, court females and provide care. Sneaker males only “sneak” spawn, while satellite males sneak, but also help by chasing away sneakers. We found that satellite males have larger absolute testes than either sneakers or nesting males, despite their cooperative role. Nesting males invested relatively less in testes than either sneakers or satellites. Though sneakers produced smaller ejaculates than either satellite or nesting males, we found no difference among male types in either sperm cell concentration or sperm number, implying sneakers may produce less seminal fluid. Sperm tail length did not differ significantly among male types, but sneaker sperm cells had significantly larger heads than either satellite or nesting male sperm, consistent with past research showing sneakers produce slower sperm. Our results highlight that social interactions among males can influence sperm and ejaculate production. 

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5. A Conserved Somatic Sex Determination Cascade Instructs Trait‐Specific Sexual Dimorphism in Horned Dung Beetles

   https://doi.org/10.1111/ede.70004  

   Mitchell, London C; Moczek, Armin P; Nadolski, Erica M (March 2025, Evolution & Development)

   ABSTRACT Sex‐specific trait expression represents a striking dimension of morphological variation within and across species. The mechanisms instructing sex‐specific organ development have been well studied in a small number of insect model systems, suggesting striking conservation in some parts of the somatic sex determination pathway while hinting at possible evolutionary lability in others. However, further resolution of this phenomenon necessitates additional taxon sampling, particularly in groups in which sexual dimorphisms have undergone significant elaboration and diversification. Here, we functionally investigate the somatic sex determination pathway in the gazelle dung beetleDigitonthophagus gazella, an emerging model system in the study of the development and evolution of sexual dimorphisms. We find that RNA interference (RNAi) targetingtransformer (tra)caused chromosomal females to develop morphological traits largely indistinguishable from those normally only observed in males, and thattra^RNAiis sufficient to induce splicing of the normally male‐specific isoform ofdoublesexin chromosomal females, while leaving males unaffected. Further,intersex^RNAiwas found to phenocopy previously described RNAi phenotypes ofdoublesexin female but not male beetles. These findings match predictions derived from models of the sex determination cascade as developed largely through studies inDrosophila melanogaster. In contrast, efforts to targettransformer2via RNAi resulted in high juvenile mortality but did not appear to affectdoublesexsplicing, whereas RNAi targetingSex‐lethaland two putative orthologs ofhermaphroditeyielded no obvious phenotypic modifications in either males or females, raising the possibility that the function of a subset of sex determination genes may be derived in select Diptera and thus nonrepresentative of their roles in other holometabolous orders. Our results help illuminate how the differential evolutionary lability of the somatic sex determination pathway has contributed to the extraordinary morphological diversification of sex‐specific trait expression found in nature. 

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