behavioral ecology = natural history + evolutionary theory
Areas of Research:
Ongoing Projects:
The evolution of cooperation between non-relatives
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Cooperation between non-kin is a long-standing puzzle in evolutionary biology. Since cooperation typically involves some short-term fitness cost, it should be easily undermined by cheating and competition between group members. Our long-running research project in Panama focuses on the Greater Ani, a Neotropical cuckoo with an unusual breeding system. Whereas many cuckoos are specialized parasites which lay their eggs in the nests of other species, anis nest communally. Up to eight unrelated individuals construct a single nest in which all of the females lay their eggs, and all group members participate in incubation, defense, and food delivery to the shared clutch. However, reproductive competition within the group is intense: before laying her first egg, each female ejects any eggs that have already been laid in the communal nest. Each female stops ejecting eggs once she has laid her first egg, presumably to avoid accidentally removing her own. As a result, the first female to begin laying in the communal nest invariably loses at least one egg — sometimes several — whereas the last female to enter the laying sequence loses none. A main goal of this research is to quantify the reproductive costs and benefits to females of nesting communally.
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Four unrelated Greater Anis (two males and two females) simultaneously arrive at their communal nest bearing food for the mixed clutch of nestlings. Adults cannot recognize their own eggs or nestlings, so parental care is provided indiscriminately to the shared brood. © 2016 Christina Riehl.
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Evolutionary dynamics of brood parasitism and host defenses
Striped Cuckoos parasitize white egg-laying hosts in South America and blue egg-laying hosts in Central America. Different populations of Striped Cuckoos have evolved polymorphic egg colors to mimic the color of the host egg and fool the hosts into accepting the parasitic egg. © 2016 Christina Riehl.
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Brood-parasitic cuckoos are notorious cheaters: instead of raising their own young, they lay their eggs into other species’ nests and trick the unwitting “hosts” into raising the parasitic cuckoo nestling. Some cuckoos even lay mimetic eggs that resemble the hosts’ own, fooling the host into accepting the cuckoo’s egg. We are currently studying the evolution of this fascinating adaptation in the Striped Cuckoo, a widespread but little-studied bird of the New World tropics. In South America, Striped Cuckoos lay white eggs that are identical to those of the hosts that they parasitize. In Central America, however, they lay blue eggs which mimic a different set of host species. In areas where these two host groups overlap, cuckoos in the same population lay either white or blue eggs. What is the genetic basis of egg color– do these two populations represent distinct species, or is the adaptation maternally inherited? Where did blue eggs evolve from the ancestral white condition, and did they enable the Striped Cuckoo to spread northwards from South America and exploit new host species? This project is an international collaboration between researchers in the United States and Latin America, providing a rare opportunity to study a breeding system that is unique among Neotropical birds.
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Collective cognition: group decision-making in the wild
Honeybees, which dance to communicate information on potential hive sites, provide a spectacular example of how many individuals can arrive at a consensus to choose a new home. But how do other animal groups, such as cooperatively breeding birds and mammals, come to collective decisions? Despite a large theoretical literature, experimental studies on wild animals are rare - and they have mainly focused on animals with linear dominance hierarchies and despotic decision-makers. This project proposes a comprehensive analysis of group decision-making in a cooperative species with egalitarian social relationships: the Greater Ani (Crotophaga major), a tropical cuckoo that breeds in groups that are stable over many years. Each group is composed of 2-4 adult pairs plus non-breeding helpers (up to ten individuals), which build a shared nest in which all of the group's females lay their eggs. The choice of a nest site is the single most important determinant of the reproductive success of the group - but how do so many individuals agree on where to build the nest? Preliminary data show that different pairs within the group often begin to build separate nests within the group's territory, and that communal calling displays, or rallies, play a role in determining which of these sites the group eventually chooses. How do individuals "vote" in these collective forums? How do they overcome conflicts of opinion when group members disagree? And how do their kin relationships, social bonds, knowledge, and experience influence the group's decision?