Bad tenants: female sheet-web spiders, Cambridgea foliata (Araneae: Desidae), lose feeding opportunities when cohabiting with males.
However, while it is advantageous for male spiders to mate multiply over their life time, brief periods of cohabitation, a form of mate-guarding, are not uncommon. In spiders, mate-guarding males will share nesting chambers (e.g., jumping spider Bavia aericeps Simon. 1877) or webs (e.g., Inula subtilis Davies, 1982), or remain on secondary structures of the females' webs (e.g., Araneus anatipes (Keyserling. 1887) reviewed in Jackson 1986). Male spiders predominantly cohabit with subadult females with records for 161 species in which adult male araneomorphs cohabit with conspecific juvenile females (Jackson 1986). In these cases, males often mature earlier than females, find a female one or two molts from maturity and remain with her until she matures.
This predominance of males cohabiting with subadult females is logical, provided that the first male to mate with a female receives some advantage over subsequent males. If that male alone mates with the female and she oviposits shortly after or becomes unreceptive. then he will secure 100% paternity for her clutch. However, while cohabitation with subadults is more common, post-copulatory cohabitation with adults does occur, even at the cost of additional mating opportunities (Parker 1974). Alcock (1994) identifies several alternative hypotheses to explain the evolution of prolonged male-female associations following copulation. Mate-guarding may prevent females from accepting additional copulations (Thornhill 1984) or favoring the ejaculates of subsequent males (Eberhard 1991). In species that exhibit "last sperm precedence" in which subsequent mates receive a greater share of paternity, it is valuable for males to guard their mates (Austad 1984) in some cases until oviposition (e.g., cellar spiders, Pholcus phalangioides (Fuesslin, 1775), Schaefer & Uhl 2003) or until females enter a refractory period during which they are unreceptive to further mating (e.g., marbled cellar spiders, Holocnemus pluchei (Scopoli, 1763). Calbacho-Rosa et al. 2010).
Males may also associate for longer in order to mate multiply and increase their share of paternity (Simmons 2001). For example, male leaf-curling spiders (Phonognatha graeffei (Keyserling. 1865)) improve fertilization success through multiple, prolonged copulations and for this reason cohabit with adult females for several days (Fahey & Elgar 1997). While only a short period of pre-copulatory cohabitation may be needed to ensure a single copulation, longer periods of cohabitation may maximize fertilization success for the male.
Nevertheless, cohabitation should be costly to males as they forgo seeking additional mates (Birkhead & Moller 1992; Fryer et al 1999; Harts & Kokko 2013). However, these "costs" assume not only that there are a large number of receptive females available elsewhere but also that, if they should locate additional mates, males would not meet any resistance from other guarding males (Harts & Kokko 2013). Furthermore, remaining in a female's web may provide additional benefits not directly related to reproduction, such as safety. Austad (1984) remarks that post-copulatory mate-guarding should only be advantageous if moving between webs, and finding a newly molted female is risky. This risk of travelling has been demonstrated in several spider species in which less than a quarter of males observed in a female's web are likely to be found again in the web of another female (e.g., golden orb web spider, Nephila clavipes (Linnaeus, 1767). Christenson & Goist 1979; bowl and doily spider, Frontinella pyramitela, Austad 1984; redback spider Latrodectus hasselti, Andrade 2003). Predation by other invertebrates is likely to be greatest source of mortality (Andrade 2003). Consequently, it can be advantageous for males to maximize their reproductive output with a resident female before rather than risk searching for another.
Cohabitation may also provide males with greater feeding opportunities. Suter & Walberer (1989) found that male bowl and doily spiders (Frontinella pyramitela) feed on prey caught in female's webs and suggest that this could be why cohabitation can go on for several days, far longer than is necessary for only courtship and copulation. They suggest that this may compel the female to eventually expel her tenant. Furthermore, Erez et al. (2005) demonstrated clear benefits of cohabitation for males at the expense of resident female desert spiders. Stegodyphus lineatus. When they provided food to webs containing cohabiting pairs, the males gained in condition while females did not. In species that depend on capture webs to feed, foraging in a resident female's web is an ideal way for a male to prepare for searching for his next mate (Austad 1983). Apart from these studies, there are no other examples that we are aware of which test whether males gain feeding benefits while cohabiting on female webs.
Cambridgea foliata (L. Koch, 1872) (Desidae) are arboreal spiders distributed throughout the North Island of New Zealand. They are nocturnal and build three-dimensional sheet-webs in native forest which males and females can be seen sharing. The webs consist of a non-sticky, thick horizontal mainsheet anchored from below, with many knock-down threads above the mainsheet. The spiders run along the underside of the mainsheet. The rear of the web tapers into a silken tunnel or "retreat" which may extend under the bark of trees or into vacated burrows; the spider resides in the retreat during the day. During the summer, males mature in their natal webs before leaving to wander in search of female webs. Once a male finds a female's web, he may cohabit with her. McCambridge (2017) found that C. foliata will cohabit for between one day to about 14 days, with a small number of males cohabiting with subadult females for more than 20 days. Based on the author's data and our own observations, we expect that one to two days is the most common duration for cohabitation. During this time, the cohabiting male defends the female's web by engaging in ritualized fights with intruding males (pers. obs). When males light in a female's web, we have often observed the resident female departing the mainsheet and sitting either on one of the anchoring threads or off the web entirely. This suggests that one male's presence in females' webs, or a succession of cohabiting males, may entail some foraging cost to females. However, we have not observed any instances of sexual cannibalism or fighting between males and females, which are similar in size.
Courtship and mating behaviors have not been observed in this species. However, as most spiders take only a few minutes to copulate (Fahey & Elgar 1997), it is reasonable, barring the use of mating plugs or genital mutilation (reviewed in Huber 2005), to expect that cohabiting pairs have the opportunity to mate more than once. Males often cohabit with juvenile females--which is consistent with the predictions of Austad (1984) for cases of first sperm precedence--but males have also been observed cohabiting with adults. As C. foliata are entelegyne spiders with "conduit" or "one-way" type copulatory ducts, we may expect them to exhibit first sperm precedence. However invertebrate copulatory and fertilization duct morphology is highly variable, which can generate significant variation in the strength of first or last sperm precedence (Uhl 2000). For this reason, we do not assume whether this species exhibits first or last sperm precedence.
Given that male C. foliata cohabit with females for longer than required for a single copulation, in the present study we aim to assess the impact of males cohabiting with females by (1) comparing positions of male and female spiders on their webs when alone and when cohabiting, and (2) comparing responses of males and females to simulated prey. If we assume that the probability of prey landing is approximately equal at all points on the mainsheet, then the center of the mainsheet (which we call the "hub") should be the optimal location for the spider to sit, as it is approximately equidistant to all edges of the web and to the retreat where these spiders hide if disturbed. For this reason, we predict males and females will most commonly position themselves in the hub when alone. When in pairs, we expect that both sexes will spend less time in the hub compared to when alone but that males will spend more time in the hub than females, as the mainsheet serves as the arena on which males fight each other. Due to males and females sharing the web when in pairs, we expect that both will be less likely to approach a simulated prey item compared to when alone. Another objective is to describe courtship interactions on females' webs, which have not been described before and may be useful for understanding mate-guarding in this species.
Courtship observations.--In addition to making observations of male-female interactions in the field, we collected juvenile C. foliata for a lab population which included females with a known reproductive history. Each spider was housed in a 30 x 30 x 60.1 cm mesh enclosure. A wooden retreat was attached to the upper rear corner of the enclosure. We modelled these on weta enclosures devised by Bowie et al. (2006). They consist of a (45 x 45 x 150 mm) block of untreated wood with a furrow (18 x 18 mm) cut two-thirds the length of the block, which was placed against the mesh wall. A black polyethylene flap was attached to the outside of the cage, covering the retreat. We fed the spiders every two days on a mixture of meal worms (Tenebrio moliter), blow flies (Lucilia sericata) and locusts (Locusta migratoria). Spiders were misted with water three times a day.
By the time females reached maturation their webs were dense enough to support multiple spiders. To observe courtship interactions, we introduced a random unmated male to a random female's web and recorded subsequent behaviors for up to 3 hours.
Cohabitation surveying.--We surveyed C. foliata webs at night in Matuku Forest and Bird reserve in West Auckland (36[degrees] 51' 48.3"S 174[degrees] 28' 47.7"E). We observed webs on either side of the walking track across the 2015/2016 and 2016/2017 summer seasons. We could ensure that we did not observe the same individuals across the separate seasons as in 2015/2016 the spiders were subsequently collected for other research. Meanwhile in the 2016/2017 season, we ensured that we did not repeat observations of the same spiders by not visiting the same trees. We do not have data on the reproductive history of observed males and females.
When we found an adult spider in a web, we made a note of sex, location in the web and whether the spider was solitary or in a pair. The "locations" we included were: "retreat," meaning at the opening of the retreat or on the mainsheet immediately outside the retreat; "hub." referring to the approximate center of the mainsheet; "web," referring to all remaining parts of the mainsheet; and "off," indicating that the spider was either on the knock-down threads, on the guying threads or just off the mainsheet on the substrate within 10 cm of the web.
To examine whether spiders changed how they responded to a prey stimulus when in a pair compared to when alone, we provided an artificial stimulus simulating prey movement in the web. All surveys were conducted between 1 and 3 hours after sunset. In order to standardize the stimuli, we used a Winner tuning fork with a pitch A440 (440 Hz). We had previously observed that C. foliata females consistently approach a vibrating tuning fork when it is touched to the mainsheet of the web, and will attempt to bite it before realizing that it is not a prey item. In the current study, we would strike the tuning fork and touch the tip lightly approximately 20 cm from spider's location. This controlled for differences in web size, which varied considerably (mean area = 3169.48 [cm.sup.2], SD = 1220.63).
We recorded whether the spider(s) approached, moved away from or did not respond to the stimulus. As most individuals either responded within 5 seconds of the stimulus being applied or did not respond for more than 30s or with additional applications of the stimulus, we did not record latency.
We surveyed the webs of 62 solitary females, 34 solitary males and 47 adult pairs. We collected response data for 35 solitary females, 22 solitary males and 23 pairs. When analyzing spider locations and responses, we excluded 11/62 of the webs with solitary females. 3/34 webs with solitary males and 5/47 webs with pairs because spiders were feeding. We kept these data separate to our other location data as a prey item falling into the web would draw one or both spiders away from their original location and therefore would not be representative of the "preferred" location.
Statistics.--We conducted a [chi square] test for homogeneity of variance on contingency tables for female locations depending on whether they were alone or in a pair. When tabulating responses to the stimulus, our expected count values for our contingency tables violated the assumptions for a yj test of homogeneity. In the vast majority of responses, the spider either approached or did not respond; very few moved away from the stimulus. Thus, we pooled negative responses and no response together. The resulting contingency table for whether females approached the stimulus (with binary responses of yes or no) did not violate the assumptions for a [chi square] test.
We calculated odds ratios for the probability that females would take up certain positions when in pairs compared to when alone, and for the probability that they would approach a stimulus as opposed to move away or not respond. We conducted these same tests for males. All analyses were conducted in R version 3.2.0. (R core team 2015).
Courtship.--We observed 10 instances of courtship by the male. Of these observations, 6 males entered a copulatory posture. When males first enter webs occupied by females, they may shake the web using their whole body. The female orients towards him and. soon after, approaches him. She touches him with her forelegs and then may retreat a short distance in the direction of the retreat. The male may continue to interact with the female or move past her and enter the retreat. The interactions we observed lasted for an average of 93.8s (SD = 168.3s). In one case, a male courted a female for 615s.
Often at the entrance of the retreat, the male shakes the web and dorso-ventrally flexes his abdomen. If the female approaches, he will drum on the web with his first and second pairs of legs. The female may angle her cephalothorax away from the web allowing the male to approach and place one palp over her epigyne (Fig. 1). The pairs which we observed remained in this copulatory posture for 382.8s (SD = 212.5). While the male's palp was placed over her epigyne at all times, the male's palp was never charged with sperm, meaning that sperm transfer could not have been occurring. Males also only ever used one palp. When the male moved out of this posture. he would either remain on the web a short distance from the female or return to the retreat.
Cohabitation.--We observed males cohabiting with both adult and subadult females and. in one case, we found an adult male sharing a web with a subadult male. We never observed copulation. When surveying for solitary males, we sometimes found individuals on webs produced by heterospecifics (e.g., Badumna sp. Thorell, 1890).
In addition to sharing webs during the night, males and females can frequently be found sharing retreats during the day. As adult males wander, we expect that these webs and retreats belong to the female. We observed that in the morning when the pair return to the retreat, the male will usually enter the retreat first with the female following (pers. obs.). Similarly, if the pair are disturbed during the night, the male will enter the retreat first regardless of whether he or the female is closer to it.
Beyond occasional observations from other research of marked males appearing in different females' webs over sequential nights, we do not have data to show how many females a C. foliata male may encounter in his lifetime. However. McCambridge (2017, pers. comm.) marked and recaptured 3/26 adult male Cambridgea plagiata Forster & Wilton. 1973 after 29, 43 and 49 days. While we cannot extrapolate these findings to the survival rate of males in general, these males should have encountered at least two females" webs.
Preference for location.--When alone, males and females demonstrated very similar preferences for different locations in the web. Solitary females were most often found in the hub of the web or near the retreat (Tables 1. 2). Similarly, solitary males were most often found in the hub or at the retreat (Fig. 2). Indeed, the hub was the most frequented location for spiders alone in the web, regardless of sex. The second most common location for the spiders was off center from the hub but more than 10cm from the edge of the web. When we observed spiders feeding (both in pairs and alone), they were similarly found in or near the hub.
Male distribution did not change significantly when in a pair compared to when alone ([chi square]= 1.708, df =2, P = 0.426). Males in pairs were still most often found at the hub, although they were more likely to be found at the retreat compared to solitary males (odds ratio. OR = 1.77). By contrast, we found that females in pairs placed themselves differently on the web compared to lone females ([chi square]= 40.109, df = 2. P < 0.0001). Females in pairs were less likely to be in the hub or elsewhere on the mainsheet (OR = 0.346, 0.320 respectively). They were 3.95 times more likely to be at the retreat. However, the largest share of females in pairs (38.1%) were found off the web entirely. They were sometimes found on the knock down threads, but most frequently on the guying threads. They were never seen off the web when alone, making it impossible to calculate a meaningful odds ratio.
Responses to stimulus.--When alone, the majority of males and females approached a vibratory stimulus (Fig. 3. Tables 3, 4). However, while solitary females almost always approached, males sometimes moved away or did not respond. For cohabiting females, we found significant evidence that the presence of a male in the web affected the likelihood of approaching a stimulus ([chi square] = 42.437, df = 1. P= <0.0001). Females in pairs were 31.96 times more likely not to respond or to move away from the vibratory stimulus, with 20/23 females in pairs not responding to the stimulus at all compared to only 1/35 of solitary females. Of the 20 paired females which did not respond, 10/20 were on the mainsheet (retreat, mainsheet or hub) while 10/20 were off the web (guying threads, knockdown threads, off entirely). This suggests that even females who are still in contact with the main web (and able to feel the vibration) did not respond to the stimulus.
There was also no evidence that males were more likely to approach a stimulus when sharing a web compared to when alone (OR = 0.957; [chi square] = 0, df = 1, P = 1).
Feeding behavior.--We observed 19 instances of spiders feeding. In total, spiders were most often observed feeding in the hub closely followed by elsewhere on the mainsheet (Table 5). We observed one solitary female feeding on the knockdown threads. Within the cohabiting pairs we observed, we only saw males feeding.
While C. foliata of both sexes most frequently placed themselves in the hub of their web and approached prey when alone, male and female behavior diverged significantly when cohabiting. When in a pair, males were still found most frequently in the hub and still approached a stimulus mimicking prey. By contrast, females were often off the web entirely and almost never approached our stimulus. Cumulatively. this suggests that cohabitation is disadvantageous for females but advantageous for males.
The C. foliata mating system is best described as "prolonged searching polygyny" (Herberstein et al. 2017). It is advantageous for males to find unmated females rapidly and males depart webs in search of other webs. However, at each female's web. males may invest significant time into cohabiting. The hub of the web is equidistant from all edges of the sheet and the retreat, suggesting that it is probably the location which minimizes distance to both prey and to safety. It is unsurprising, then, that we predominantly found solitary females and males in the hub of the web.
However, while males in pairs largely continued to place themselves in the hub when cohabiting, a large proportion of females in pairs were found off the mainsheet entirely. In this way, it is unsurprising that so few females responded to vibrational signals given that they were less likely to detect the vibrations or be in a position to rapidly take prey. However, even females that remained on the mainsheet in the presence of a male were less likely to approach the stimulus compared to when alone. In this way, females that cohabit with males for an extended period or with a series of males almost certainly forgo most, if not all, foraging opportunities. Given that when we observed pairs, we only saw males feeding, it is reasonable to expect that if cohabitation were prolonged, male condition would improve at the expense of female condition, as has been found in desert spiders (Stegodyphus lineatus; Erez et al. 2005).
An extended period of cohabitation would also be particularly advantageous for males if travelling between females' webs were risky (Austad 1984). In other spider species, there is a high mortality rate for males travelling in search of females, most likely as a result of predation (e.g., Christenson & Goist 1979; Andrade 2003; Berger-Tal & Lubin 2011). It would be worth investigating whether this is also true for C. foliata.
At least, the value of webs as a source of food is apparent in how males take advantage of abandoned webs which, naturally, provide no reproductive opportunities. We sometimes observed males in webs previously inhabited by females who had disappeared after storms. We even observed one male living on a web built by a house spider (Badumna sp.) suggesting that males can be truly opportunistic about finding refuges while roaming. We expect that the majority of solitary males we found were inhabiting webs built by spiders other han themselves, given that the adult males we kept in the lab did not build webs. This cumulatively suggests that the most important advantages of cohabiting may be directed towards male survival.
Our primary limitation to fully understanding the benefits of cohabiting is that we do not yet know how often cohabiting pairs copulate. While we observed males placing only one palp over the female's epigyne during courtship, our males never charged their palps with sperm prior to this and we never observed copulation in the cohabiting pairs we found in the field. Based on our behavioral observations, we expect that courtship and copulation should occur at least once more, after the male has charged his palps with sperm, if not on the web during the night then in the retreat during the day. If a single copulation does not ensure paternity then guarding is essential to ensure male fertilization success which would make sperm competition the primary benefit of guarding rather than safety and/or foraging opportunities.
Furthermore, as we do not know the sperm precedence patterns for this species, it is difficult to estimate to what extent cohabitation reduces sperm competition. It is possible that if females re-mate, subsequent males may gain a proportion of paternity, making mate-guarding an adaptive strategy. Cambridge foliata are entelegyne spiders, generally thought to have first sperm priority, which should lessen the value of post-copulatory cohabitation (Austad 1984). However, it has been demonstrated in other entelegyne spiders that the second male may gain between a 5% share of paternity (Frontinella pyramitela; Austad 1982) to near parity (46% in Nephila plumipes; Schneider & Elgar 2001) or even the majority (66% in Nephila edulis; Schneider et al. 2000). Regardless, even if re-mating by the female merely reduces the first male's share of paternity, a male will benefit from mate-guarding, particularly if there are additional benefits to remaining with the same mate.
Extended cohabitation with adult females is a relatively uncommon form of mate- guarding in spiders, and cohabitation beyond the time required for copulation seems particularly uncommon. Furthermore, in New Zealand sheet-web spiders, cohabitation seems particularly costly to the female, who frequently leaves the mainsheet of her web when in pairs and most likely loses foraging opportunities as a consequence. By contrast, cohabitation has the potential to be highly beneficial for males. First, it helps males to control mating by allowing multiple copulations and preventing re-mating by the female. Second, it provides males with feeding opportunities and likely keeps males safe from heterospecific predators which they may meet when searching. Determining when copulation occurs, how frequently it occurs and how sperm competition manifests when females mate multiply is critical to understanding the value of cohabitation in comparison to searching for additional mating opportunities in this species. However, male and female behavior on webs when in pairs compared to when alone provides an explanation as to why males frequently cohabit following copulation rather than continue searching for additional mates.
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Manuscript received 27 September 2017, revised 21 June 201S.
Lcilani A. Walker and Gregory I. Holwell: (1) School of Biological Sciences, University of Auckland. 3A Symonds St, Auckland Central 1010, New Zealand. E-mail: Leilani, walker (5 auckland.ac.nz
Table 1.--Positions of female C. foliata on webs. Location Solitary Pair Total Hub 28 8 36 Retreat 4 13 17 Web 19 5 24 Off 0 16 16 Total 51 42 93 Table 2.--Positions of male C.foliata on webs Location Solitary Pair Total Hub 16 20 36 Retreat 5 12 17 Web 10 10 20 Off 0 0 0 Total 31 42 73 Table 3.--Female responses to simulated prey. Response Solitary Pan Total Approach 34 2 36 Move away 0 1 1 No response 1 20 21 35 23 58 Table 4.--Male responses to simulated prey. Response Solitary Pair Total Approach 12 12 24 Move away 1 7 8 No response 9 4 13 22 23 45 Table 5.--Number of spiders observed feeding in different web localities. Solitary Paired Location Male Female Male Female Total Web 2 4 1 0 7 Hub 1 5 2 0 8 Retreat 0 1 2 0 3 On- 0 1 0 0 1
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|Author:||Walker, Leilani A.; Holwell, Gregory I.|
|Publication:||The Journal of Arachnology|
|Date:||Sep 1, 2018|
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