Hapalochlaena maculosa in this study appeared capable of utilising chemotactile cues to discriminate among socially and biologically relevant traits in the conspecifics they physical contacted. The likelihood of an individual to retreat after contacting a conspecific was associated with the sex (Fig. 2), size, and shared mating history (Fig. 4) of the conspecific being contacted. This likelihood also varied by the sex and size of individual initiating the physical contact. Males fled from female conspecifics more often than they did male conspecifics, demonstrating chemotactile sex recognition. They also appeared able to discriminate females with whom they had previously mated from those they had not. By contrast, females showed no indication of using chemotactile information to determine whether to retreat. Intriguingly, a prior study of this species indicated that females are able to identify the sex and possibly the identity of conspecifics from chemical signatures in the water, however males have not been observed to be able to do this (Morse et al. 2017). The novelty or familiarity of conspecifics did not influence the probability of an individual to retreat after contact in this study. Therefore, shared mating history rather than familiarity itself was more likely to influence a male’s decision whether to retreat from a conspecific female. Together these results suggest octopuses, particularly male H. maculosa, may use chemotactile information gathered by chemosensory cells on the arms to recognise socially-relevant categories of conspecifics.
Male H. maculosa were able to assess the prior mating history of females by touch, but without necessarily inserting the hectocotylus. Male mate recognition may occur in cephalopods that adjust copulation time based on prior mating history of females (Cigliano 1995; Wada et al. 2010; Morse et al. 2015). In all of these cases known previously, males had already initiated copulation, presumably allowing the hectocotylus to assess the presence and potentially the identity of existing sperm. By contrast, H. maculosa males in this study appeared able to assess whether they had already mated with a female prior to mounting her – presumably via tactile chemosense, though mechanosense cannot be discounted. It is unknown whether males remember the scent of their former mates, impart their chemical signature onto females’ skin while mounting to mate (i.e., scent marking), or whether females emit the chemical signature of their stored sperm. While all of these possibilities merit further exploration, scent-marking, despite being widespread among terrestrial animals (Hurst 2005), is not known to occur in marine systems and therefore seems unlikely. It is more probable in this case that males can remember characteristics about the phenotype or possibly the identity of the females with which they have mated, as occurs broadly across social animals (Mateo 2004). The use of chemotactile cues in social recognition may be widespread among male cephalopods. In numerous reported observations of squids, cuttlefishes, and octopuses, including H. maculosa (Morse et al. 2018a), males make physical contact with females prior to mating (e.g. O. vulgaris, Wells and Wells 1972; O. digueti, Voight 1991; Sepia pharoanis, Nabhitabhata and Nilaphat 1999; Sepioteuthis lessoniana, Wada et al. 2005; and O. tehuelchus, Berrueta et al. 2020). This behaviour is performed by both guarding (S. officinalis, Allen et al. 2017) and sneaker males (Doryteuthis opalescens, Zeidberg 2009). Tactile behaviour has long been suspected to play a role in sex recognition in O. vulgaris (Wells and Wells 1972), a possibility that is supported here for this and the other species listed above.
Female H. maculosa did not appear to use utilise chemotactile cues in this study. This finding is consistent with previous observations that females of this species can already achieve some degree of social recognition from a distance via odours cues in the water (Morse et al. 2017). Instead, physical contact initiated by female H. maculosa may align with the suspected use of chemotactile behaviour in other female cephalopods: communicating mate choice to males prior to mating (O. vulgaris, Wells and Wells 1972; H. lunulata, Cheng and Caldwell 2000; Idiosepius spp., Nabhitabhata and Suwanamala 2008; and the larger Pacific striped octopus, Caldwell et al. 2015). Copulation by H. maculosa is often preceded by a tactile phase, which is sometimes initiated by females (Morse et al. 2015; Morse et al. 2018a). However, only nine of the 129 female initiated chemotactile interactions observed in the current study resulted in copulation (Fig. 1), and females retreated from 42% of the males they contacted (Fig. 2), suggesting that female chemotactile behaviour likely serves more functions in this species than just signalling honest mate choice.
Hapalochlaena maculosa joins the long list of invertebrates that use multiple sensory inputs to facilitate social recognition (e.g. annelids, Lorenzi et al. 2015; and crayfish, Patullo and Macmillan 2015). Multi-modal social recognition may be particularly important for nocturnal (e.g. H. maculosa) or eyeless animals (e.g. annelids) for which visual recognition may be difficult or impossible (Aquiloni and Tricarico 2015). Data collection for this study occurred at night, and visual cues were assumed to be limited. Octopuses’ sucker-lined arms sense both textures (Wells and Wells 1957) and chemical signatures (Wells et al. 1965; van Giesen et al. 2020); either of those inputs, or both, may have been used here. It is suspected in this case that dermal hormones, which can vary by both sex and reproductive status (e.g. O. bimaculoides, Chancellor et al. 2021), may be the most likely chemicals assessed and utilised in social recognition here. These chemotactile cues would have complemented other cues (e.g. chemical or possibly visual) sensed from a distance. A prior study of combined visual and tactile learning in O. vulgaris found these modalities operated largely without interaction (Allen et al. 1986), and the neural underpinnings of visual and tactile learning are reported to take place separately within the octopus brain (Wells 1961). Octopus vulgaris uses multiple sensory cues to recognize and discriminate between prey types (Maselli et al. 2020). Individuals offered simultaneous access to both visual and chemical recognition cues in prey discrimination tests performed only slightly, or no better, than those offered either chemical cues only, or conflicting sensory inputs (ibid). In these experiments, individual O. vulgaris were given up to an hour to make those decisions. By contrast, H. maculosa in this study only took seconds to sense, process, and prioritise multiple information channels, and make decisions about retreat based on social recognition. While all individuals in this study had access to the same sensory channels, males (this study) and females (Morse et al. 2017) appeared to use different cues in social recognition, suggesting sex differences in this prioritisation of sensory information in decision-making.
In addition to revealing social recognition, patterns of retreat following touch in H. maculosa align with previous assessments of its mating system, and may provide insights into prosocial mechanisms that maintain it. Larger females were less likely than smaller females to retreat following contact with conspecifics, regardless of conspecific sex or size. Larger females are also more receptive to mating attempts (Morse et al. 2015). Both of these findings suggest larger female H. maculosa are more likely than smaller females to tolerate the presence of conspecifics. However, cannibalism is an extreme form of social intolerance (Chiara et al. 2019), and sexual cannibalism occurs in the Hapalochlaena genus (Cheng and Caldwell 2000). Where this behaviour has been observed in other octopuses in the wild, it has been expressed by large females (Huffard and Bartick 2015). In order to reproduce successfully, large female octopuses must tolerate males in their presence, and suppress any aggressive motivation to cannibalise before spermatophores have been transferred to the oviduct. It might be that females would be more likely to cannibalise males with which they have already mated, because they have already stored their sperm. If so then this tendency could induce strong selection for males to avoid females that are former mates. Serotonergic neurotransmission may play a role in suppressing aggressive asocial or antisocial tendencies in O. vulgaris, thereby releasing conserved prosocial behaviours (Edsinger and Dölen 2018). If this mechanism is present in H. maculosa as well, then results here suggest it may develop as females grow in size.
The evolution of chemotactile social recognition by male H. maculosa may have been influenced by the risk of sexual cannibalism, and mate preference for novel females in the context of significant biological and ecological constraints. In stomatopods, the species with the best social recognition capabilities are also the most aggressive (Vetter and Caldwell 2015). Given the prevalence of cannibalism among octopuses (Ibáñez and Keyl 2010), males have likely evolved the means to recognise those individuals most likely to initiate sexual cannibalism (e.g. large females). This recognition may be especially acute for males that also prefer to mate with large females (e.g. A. aculeatus, Huffard et al. 2008). Male H. maculosa do not share this preference (Morse et al. 2015), but nonetheless appeared more likely to retreat after contacting larger females. It is curious that sex recognition requires male H. maculosa to physically contact potentially cannibalistic females, especially when distance olfactory sex identification can be achieved by females (Morse et al. 2017). Furthermore, males do not appear to use sex identification information to reduce male-male mating attempts. Do males instead require uniquely chemotactile cues to evaluate mate quality? Male H. maculosa appear to prefer novel females (Morse et al. 2015), and efforts to maximise mating opportunities with those females yield paternity benefits (Morse et al. 2018a). This choosiness has evolved in the context of limited mating capacity of both males and females (i.e. ~50 spermatophores and ~ 50 eggs: Morse et al. 2015), likely significant energy expenditures associated with mating (Franklin et al. 2012), and demersal young with limited dispersal capabilities and gene flow (Morse et al. 2018b). Matings between siblings and half-siblings are less successful than those between more distantly related individual (Morse et al. 2018a). Therefore, it may pay for males to maximise the total number of mates and therefore the genetic combinations of their young.
The preference of male H. maculosa for novel females, supported by chemotactile social recognition, likely benefits both sexes. Females will be freed to mate with more males if repeat pairings are avoided. Polyandry is associated with faster egg production and larger hatchlings in another cephalopod (Euprymna tasmanica, Squires et al. 2012). Any risk of male fitness lost to cannibalism while obtaining information about female novelty must be lower than the fitness risk of falsely identifying novel mates (i.e. accidentally mating with the same female twice rather than leaving to seek new females). The fitness risks of a “false positive” in sex identification by males (i.e. seeking a novel female but encountering and attempting copulation with a male) also appear to be low. Male-male chemotactile interactions and mating attempts in H. maculosa are common, and rarely lead to aggression or injury (Morse et al. 2015). Contrastingly, females may experience entirely different risks associated with the act of mating. During copulation, male Hapalochlaena spp. mounts and wraps his arms tightly around the female’s mantle (Cheng and Caldwell 2000), possibly interfering with ventilation and oxygenation of the gills. Females sometimes expend significant effort to terminate mating attempts by males (Cheng and Caldwell 2000; Morse et al. 2015). Therefore, individual females that can identify males via olfactory cues from a distance (Morse et al. 2017), and can preclude mating attempts by unpreferred males by avoiding them altogether, may gain a selective advantage.