UC Berkeley

ABSTRACT

The Evolution of Social Monogamy and Biparental Care in Stomatopod Crustaceans

By Mary Louisa Wright

Doctor of Philosophy in Integrative Biology

University of California, Berkeley

Professor Roy L. Caldwell, Chair

Although social monogamy and biparental care have been extensively studied in birds, mammals, and fish, the evolutionary origins and maintenance of these phenomena are not well-understood, particularly in invertebrate taxa. The evolution of social monogamy is of interest because current theory predicts that both males and females will usually gain more fitness from mating with multiple partners. Furthermore, biparental care should only occur when males and females both gain more fitness benefits from providing parental care than from investing time and energy into mate searching. Given these expectations, under what environmental and social conditions will social monogamy and biparental care arise and do the same conditions maintain monogamy and biparental care on an evolutionary time scale? Long-term social monogamy, which occurs when a male and female pair for longer than a single breeding cycle, has been reported in eight genera of Lysiosquilloid stomatopods. Furthermore, the Lysiosquilloidea also contains the only marine crustacean genus (Pullosquilla) in which biparental care has been systematically studied. This dissertation examines the evolutionary maintenance and origins of both biparental care and long-term social monogamy in the Lysiosquilloidea, using experimental manipulations, ecological surveys, and comparative, phylogenetically-based methods.

Chapter 1: The maintenance of biparental care

I examined the fitness costs and benefits of biparental care in the stomatopod Pullosquilla thomassini using an experimental manipulation of the number and sex of care providers. In the absence of any care, egg clutch survival and growth decreased. However, neither the number, nor the sex of the care providers had a significant effect on changes in egg clutch mass. Parental care treatment did not affect ovary size, the total number of eggs in a clutch, or egg size. Thus, while parental care increases production of offspring, uniparental care by either sex is sufficient to achieve this goal. Males providing uniparental care lost more weight than those providing biparental care or no care. This may lead to sexual conflict over female desertion. These results suggest that biparental care is not evolutionarily maintained in P. thomassini by increasing the number of offspring hatching in an egg clutch. Instead, I hypothesize that biparental care may be evolutionarily maintained in P. thomassini by increasing the rate of egg clutch production and facilitating double-clutching.

Chapter 2: The effects of environmental and demographic variation on pairing behaviors

I examined the effects of environmental and demographic variation on pairing behaviors, egg clutch production, and burrow distribution in two sympatric stomatopod crustaceans, P. litoralis and P. thomassini . These small (<16mm) stomatopods are found as heterosexual pairs in U-shaped burrows in coral patch reef ecosystems. Coral patch reef ecosystems consist of coral heads of varying sizes separated by sand flats; in this heterogeneous environment, the coral heads host high levels of invertebrate and fish diversity and abundance in comparison to the sand flats. I hypothesized that environmental heterogeneity in the coral back reef environment would affect pairing behaviors, egg clutch production, and burrow distribution of Pullosquilla species due to gradients in food abundance or predation by fish. I tested this hypothesis on demographic and environmental data collected from a survey of Pullosquilla species collected in a patch reef in Moorea, French Polynesia. My findings indicate that proximity to coral heads is an important factor in structuring the demography and pairing behaviors of P. litoralis, but not its congener, P. thomassini. The directionality of the relationships between proximity to coral heads and several demographic traits suggests that gradients in fish predation are responsible for these patterns in P. litoralis. This suggests that selective pressures from fish predation may play an important role in the maintenance of pairing behaviors in P. litoralis. Determining the causes of differences in demographic patterns and pairing behaviors of P. litoralis and P. thomassini may yield a better understanding of the evolutionary maintenance of social monogamy in stomatopod crustaceans.

Chapter 3: The evolutionary origins of long-term social monogamy in stomatopods

I examined two hypotheses for the evolutionary origins of long-term social monogamy in stomatopod crustaceans using comparative, phylogenetically-based methods. One of the most commonly posited explanations for the evolution of social monogamy is that biparental care is required to successfully raise offspring. A prediction of this hypothesis is that biparental care should evolve in a clade before or at the same time as social monogamy. I tested this prediction by reconstructing ancestral states of social monogamy and biparental care on a Maximum Likelihood tree of 66 stomatopod species and found that long-term social monogamy evolved before biparental care in the Lysiosquilloid stomatopods. This indicates that a need for biparental care did not lead to the origin of social monogamy in this clade. Based on my finding that predation influences pairing behaviors in P. litoralis (Chapter 2) and the observation that all known socially monogamous stomatopods are sit-and-wait predators, I propose an alternative hypothesis for the origin of social monogamy in stomatopods. Sit-and-wait predation evolves as a strategy to maximize energy intake while minimizing predation risk when a lineage lives in an environment where both prey items and potential predators are abundant. I therefore hypothesized that a suite of behaviors, including burrowing, sit-and-wait predation, and social monogamy, that allowed stomatopods to escape high levels of predation evolved in the Lysiosquilloidea. I tested two predictions of this hypothesis: 1) social monogamy should evolve more often in burrow-dwellers living in soft-bottom substrates and 2) the evolution of long-term social monogamy should be correlated with the evolution of sit-and-wait predation. I tested this hypothesis on a Maximum Likelihood phylogeny of 66 stomatopod species using ancestral state reconstructions and Pagel's (1994) test of correlated trait evolution and found that burrowing, sit-and-wait predation, and social monogamy evolved sequentially in the Stomatopoda. Long-term social monogamy may have evolved as a way of further maximizing the fitness benefits of the sedentary lifestyle associated with sit-and-wait predation. This novel evolutionary route to long-term social monogamy may be associated with the shallow benthic marine environments that most Lysiosquilloids inhabit.

Conclusions

The findings of my thesis emphasize the importance of studying a diversity of taxa and environments when trying to understand the evolution of important behavioral traits. For example, it is often assumed that when biparental care is widespread in a species, it increases the viability of the current brood of offspring. However, in P. thomassini there is no evidence that biparental care increases either the survival or development of embryos (Chapter 1). Additionally, the evolution of social monogamy in many animals is attributed to a need for biparental care. The Lysiosquilloid stomatopods appear to provide a counter-example in which social monogamy likely facilitated the evolution of biparental care and other form of paternal effort (Chapter 3). Instead, my findings support the hypothesis that long-term social monogamy and sit-and-wait predation may have evolved to decrease mortality from predation during foraging and mate searching. The role of the risk of predation during mate searching in the evolution of social monogamy has received relatively little attention in the large body of literature on mating system evolution, but it appears that predation plays an important role in determining pairing behaviors and burrowing distributions in P. litoralis (Chapter 2). Taken as a whole, these findings provide compelling justification for studying the evolution of behaviors in a wide diversity of taxa.