Both theoretical and empirical studies have treated mate-guarding in aquatic Crustacea purely as a male decision problem. However, male and female interests are rarely identical, as implied by observations of female resistance against guarding attempts. We tested experimentally the occurrence of sexual conflict over guarding duration in three crustacean species: Idotea baltica, Asellus aquaticus (Isopoda), and Gammarus zaddachi (Amphipoda). Specifically, we manipulated, by osmotic stress or a neuromuscular blocking agent, the female's ability to resist guarding attempts. Female manipulation, by both methods, roughly doubled precopula duration in I. baltica (Figs. 1 and 2) showing that female resistance effectively diminishes guarding duration. However, in A. aquaticus and G. zaddachi female manipulation had no effect on guarding duration, which also was longer than in I. baltica (Fig. 2). This implies either that male and female interests are equal or that the conflict is resolved according to the male interest in these species. The lack of female resistance in such species allows long precopulatory guarding. In I. baltica we also manipulated, by osmotic stress and by clipping nails, male ability to hold the female. These treatments had no effect on guarding duration (Figs. 1 and 2). Male size tended to correlate positively with guarding duration in control groups, but not in female manipulation groups (Fig. 3). Thus, conflict is mainly resolved according to the female interest in I. baltica. Results in this species also suggest that female resistance selects for large male size. Consequently, mechanisms of sexual selection may differ considerably between species with otherwise comparable mating patterns.
Sexual selection in mate-guarding Crustacea may involve several processes: male choice, male-male competition, and female choice. To evaluate the relative importance of the different processes in mate choice of the aquatic isopod I. baltica we studied 1) the mate-choice criteria of males, 2) effects of sex ratio on the outcome of the mating contest, and 3) the role of size in male-male interactions. When given a choice between a small and a large female, males most often chose the one that matured earlier for parturial ecdysis. Maturity was a more important choice criterion than female size, but these also correlated positively. Large males had a mating advantage in both male-and female-biased sex ratios; pairing was size-assortative only in the male-biased ratio where guarding was also longer. If an extra male was placed with a precopulatory pair, 30 % take-overs occurred, large males surpassing. Present and earlier work suggests that male size is an asset in both intra- and intersexual interactions. There is little or no direct phenotypic sexual selection on female size: sexual selection for large males presumably contributes to the evolution of sexual size dimorphism in I. baltica
Precopulatory mate guarding in crustaceans is a common male mating strategy when female receptivity for copulation is short. However, start of guarding is not only a male decision making problem; it is commonly found that females resist males’ guarding attempts. Furthermore, experimental data shows that males aim to longer guarding duration than what females allow. Short guarding duration for females may be favored by a number of potential costs of guarding. Precopulatory guarding, therefore, presents a model case of intersexual conflict where the fitness maximizing strategies of males and females differ. When interests of the sexes are in conflict, actual guarding duration may be a compromise between male and female optima, arising from the adjustment of contest behavior to fitness gains of winning and fighting abilities of the parties. Intersexual conflicts are also likely to generate sexual selection on male and female traits related to the outcome of contests.
We present a theoretical approach to the optimisation of crypsis in heterogeneous habitats. Our model habitat consists of two different microhabitats, and the optimal combination of crypsis in the microhabitats is supposed to maximise the probability of escaping detection by a predator. The probability of escaping detection for a prey is a function of its (i) degree of crypsis, (ii) probability of occurrence in the microhabitats and (iii) probability of encountering a predator in the microhabitats. Because crypsis is background-specific there is a trade-off between crypsis in two visually different microhabitats. Depending on the nature of the trade-off, the optimal coloration is either a compromise between the requirements of the differing microhabitats or entirely adapted to only one of them. An increased risk of predation in one of the microhabitats favours an increased crypsis in that microhabitat. Because the trade-off constrains possible optimal solutions, it is impossible to predict the optimal coloration only from the factors (i) - (iii). However, habitat choice may fundamentally change the situation. If habitat choice minimising predation risk does not incur any costs, the prey should exclusively prefer the microhabitat where it has a lower probability to encounter a predator and better crypsis. The implications of these results to variation in cryptic colorations and polymorphism are discussed.
In species with time-limited opportunities for insemination,precopulatory mate guarding is expected to coevolve with the duration of female reproductive cycles. Despite this adaptation to female characteristics, it may also be advantageous for males to adjust guarding duration with respect to sex ratio because the benefits of guarding are dependent on the availability of females. If female fitness is reduced due to guarding, male guarding behavior leads to intersexual conflict. We studied these aspects of male mate guarding behavior in two closely related, thermal spring isopods ( Thermosphaeroma ). First, guarding duration showed species specificity which was related to the duration of reproductive cycle; cycle length for females and guarding duration by males in T. milleri were twice as long as in T. thermophilum . Second, males in both species adjusted their guarding duration with sex ratio, guarding longer when a competing male was present. Third, in T. thermophilum, ovarian development began immediately after the birth of the previous brood and continued through guarding, sexual molt and postmolt periods until oviposition, whereas in T. milleri, ovarian development was largely postponed until the postmolt period. Because guarding during ovary provisioning periods may be costly for females we tested the existence of intersexual conflict over guarding duration in T. thermophilum. We compared the guarding duration of control pairs with those of pairs in which either male guarding ability or female ability to resist guarding was reduced experimentally. Guarding durations for manipulated and control males were equal, but manipulated females were guarded longer, suggesting that conflict exists and that females can effectively shorten guarding duration by their behavior. Moreover, we suggest that selection in the context of intersexual conflict may play an important role in the evolution of delayed oviposition and sperm-storage organs in mate-guarding crustaceans.
We examine the reproductive anatomy of both sexes of the isopod Thermosphaeroma thermophilum using SEM and light microscopy, and study the occurrence of multiple paternity using allozyme electrophoresis. Female reproductive openings are found ventrally, under the cuticular plate covering the body between the fifth and seventh pereonites. Receptivity for copulation is short during the sexual moult. Sperm from copulation are stored within the oviducts, close to the ovary, for up to two weeks before fertilisation and oviposition into ventral brood pouches. Sperm are not stored between broods. Male genitalia consist of penes with erectile extensions, and curved, channelled appendices masculinae. We suggest that appendices masculinae either channel sperm from the penes to oviduct openings located beneath the cuticular plate, or are used to push aside the cuticular plate for penile intromission. Paternity analyses suggest that multiple mating is uncommon and that precopulatory guarding is an effective mate monopolisation strategy for males. We discuss the evolution of reproductive anatomy in the context of intersexual conflict, and suggest that conflict resolution may play a prominent role in the evolution of temporally restricted receptivity, male mate guarding, sperm storage, and delayed oviposition.
Intraspecific microhabitat segregation is expected to arise when age or sex-specific differences in predation risk exist. The degree to which conspecific predation (cannibalism) may generate this risk, however, is poorly known. In this paper, we examine microhabitat use, cannibalism and individual responses to the presence of conspecifics in Thermosphaeroma thermophilum, an endangered species of isopod crustacean that is endemic to a single, thermal spring in Socorro, NM, USA. In samples from the natural habitat, juveniles (mancas) were found mainly on vegetation, whereas adults were found mainly on bottom sediments; females were found on vegetation more often than males. In laboratory containers without refugia, males cannibalized females, both males and females cannibalized mancas, and mancas cannibalized each other, even in the presence of alternative food. When placed in containers provided with refugia, mancas actively avoided adults. We suggest, therefore, that cannibalism in T. thermophilum generates age-, size- and sex-specific predation risks which are responsible for microhabitat segregation between mancas and adults, and between males and females. Since interspecific predation in the spring is negligible, we suggest that cannibalism plays a significant role in population regulation and behavioral evolution in this species. We recommend, given this species' current "endangered" status, that microhabitat heterogeneity be maintained in this species native spring because it provides refugia from cannibalism, and may support a larger and more viable population in nature.
A9.
Härdling, R., Jormalainen, V. & Tuomi,
J. (1999): Fighting costs stabilize aggressive behavior in intersexual conflicts.
–Evolutionary Ecology 13: 245-265.
We analyse the evolution of aggressive behaviour in intersexual conflicts,
with a special reference to mate guarding behaviour in crustaceans. An
analysis of a discrete-strategy game shows that an ESS with only one of
the sexes being aggressive prevail if fighting costs or fitness
values of winning are asymmetric. Non-aggressiveness of both sexes is stable
if fighting behaviour is very costly for females and if the cost is at least
partly paid independent of the strategy of the opponent. Most interestingly,
the solutions of both sexes being aggressive prevails only if both sexes
have some probability of winning, and if fighting costs are small. Second,
we solve for the expected levels of aggressiveness in a game with continuous
strategies. The stability of the solution is largely determined by the form
of the fighting cost function. When fighting cost increases linearly with
aggressiveness, mutual aggressiveness fluctuates cyclically instead of
stabilising at an ESS. However, if there is an asymmetry in fitness payoffs,
a solution with only the sex having most to lose being aggressive alone is
possible. With quadratically increasing fighting costs an ES combination of
mutual aggressiveness may exist. It is predicted that fights between the sexes
should be hardest when payoffs are symmetric, and that an overt behavioural
conflict will always take place as long as there is a fitness loss to each
of the sexes if losing the conflict and both sexes have a chance to win.
We discuss the models in the context of fights preceding precopulatory guarding,
but the models offer a general frame for analysing any intersexual conflict.
Merilaita, S. & Jormalainen, V. (2000): Different roles of feeding and protection in diel microhabitat choice of sexes in Idotea baltica. –Oecologia 122: 445-451.
In Idotea baltica, a marine isopod living and feeding on the brown alga, Fucus vesiculosus, microhabitat choice differs between sexes so that males more often than females are found on the light-coloured and exposed apical parts of the alga. We have studied how the need to avoid visual predators and to feed are related with diurnal and life cycle stage dependent microhabitat choice in males and females. Faced with a choice between an apical and a basal piece of the alga, females spent more time than males on the basal piece, but this difference was not due to food choice. Faced with a choice between a dark concealing and a light exposing background, the preference for dark background was stronger at day than at night, and stronger in females than in males. This suggests that a sex difference in the importance of avoiding visual predators can explain the sex difference in microhabitat choice. Further, the preference for a dark background and night feeding both increased with age, suggesting that feeding is increasingly subordinated to the need of avoidance of visual predators. We also found that the use of the protective microhabitat was increased in the presence of an individual of the opposite sex. Our results support the hypothesis that sexes differently trade feeding against predation risk, presumably because growth is more important to males than to females, which have more to gain by protection and therefore spend more time on the lower parts of the alga.
The evolutionary hypotheses on plant-herbivore interaction assume that plant secondary compounds, such as the phlorotannins of brown algae, function as feeding deterrents for herbivores. We studied the effect of seaweed quality on the feeding preferences and performance of the isopod Idotea baltica. We offered I. baltica 6 species of algae, abundant in the Fucus vesiculosus belts where this mesograzer lives, in simultaneous preference tests. The tests were conducted both with natural algae and with artificial food made of freeze-dried and powdered algae of the same species. We found clear feeding preferences among the natural algae: the order of decreasing preference was F. vesiculosus > Dictyosiphon foeniculaceus > Elachista fucicola > Pilayella littoralis > Enteromorpha intestinalis > Ceramium tenuicorne . The preferences in the test with artificial food, however, did not parallel those with natural algae, suggesting that the chemical quality of algae is not the major determinant of feeding preferences. Furthermore, performance of isopods when reared on a diet of single algal species did not match the feeding preferences of natural algae: the most preferred brown alga provided poor growth rate. Surprisingly, the more phlorotannin a seaweed species contained, the more it was preferred by I. baltica. Moreover, the assimilation efficiency of soluble sugars was generally high when isopods fed on brown algae, and in the 2 species richest in phlorotannins it was not correlated with the phlorotannin concentration of the algal individual. In contrast to the conventional assumption of the defensive function of phlorotannins, this study shows that phlorotannins in seaweeds do not function as feeding deterrents to I. baltica. Instead, this herbivore readily feeds on phenolic-rich host plants, which, however, carries a cost in terms of decreased growth rate. We suggest that feeding preferences and habitat choice behavior evolve together; habitat structure, in terms of predation avoidance, and the spatiotemporal stability of the host algae are more important factors selecting for feeding preferences in mesoherbivores than the chemical composition of algae.
Jormalainen, V., Merilaita, S. & Härdling, R. (2000): Dynamics
of intersexual conflict over precopulatory mate-guarding in two populations
of the isopod Idotea baltica. –Animal Behaviour 60: 85-93
Investment into aggressiveness during intersexual conflicts is predicted to depend on its costs, the value of winning and the power asymmetry of the contestants, all of which may vary between populations. In the marine isopod Idotea baltica (Pallas) a conflict occurs as females resist the attempts by males to start precopulatory mate guarding. With the approaching reproductive moult, the potential costs of guarding for the female diminish while her value for the male increases. We analysed contest dynamics with respect to female maturity stage, that is, to time left to reproductive moult and the effects of male size and male experience of other individuals on contests and pair formation. We did this in two populations that differ in seasonality and synchrony of reproduction, sex ratio and the degree of sexual dimorphism. Contests occur in both populations, but with varying intensity and dynamics: in the more size-dimorphic population, females being the smaller sex, females resisted less by forceful flexing but more by hooking their body than in the other population. Male aggressiveness stayed at a constant level with respect to female maturity. Female resistance by flexing in the less size-dimorphic population was more intense than where size-dimorphism was greater and it decreased, while male persistence increased, with the approaching reproductive moult. Contests were more intense with small than with large males. These results fit well with the predictions from models of conflict behaviour. Assessment of the payoffs of winning versus contest costs, co-adaptation of the level of aggressiveness to the other traits affecting contest outcome, and counter-adaptations by the sexes to each others largely explain the dynamics and between population differences of these contests.