Evolving from Promiscuity to Monogamy

New research has revealed how the process of evolution can result in sexually promiscuous animals undergoing adaptation for monogamy.


Reproduction is an expensive endeavor. Tremendous time and resources are invested in seeking-out healthy partners, in consummating relationships, and in rearing the resultant offspring. Luckily, evolution has resulted in life being programmed to strive for resource efficiency; to work towards maximum reproductive benefit at minimum personal expense. In regards to pair-bonding between the sexes, this biological imperative for reproductive economy has made promiscuity the rule and monogamy the exception. However, despite the fact that promiscuous mating systems are the prevailing strategy in nature, environmental factors can push typically promiscuous species towards monogamy.

As a case in point, a report published in the April issue of The American Naturalist details how the ‘mimic poison dart frog’ (Ranitomeya imitator) parted ways with promiscuity to adapt a lifestyle as the first scientifically recognized genetically monogamous amphibian.

Like other frog species, poison dart frogs incur a certain amount of risk by laying their eggs in water. Although water is a biological prerequisite for frog survival, ponds, lakes and puddles also house predatory fish and other animals that prey on vulnerable eggs and tadpoles. During its evolutionary past, the menace of predation pushed the mimic poison dart frogs away from larger, riskier ponds to the considerably smaller, but safer, pools held by leaves of large bromeliad plants. Unfortunately, although the tree-top bromeliads decreased the rate of frog young predation, the movement from the big ponds raised a separate issue – nutrient limitation.

The big ponds definitely had more predators; but, they also had substantially more food. In fact, the ponds had so much food that a single frog-parent (in this case the male) was able to handle the tadpoles all by himself – a single parent family arrangement was all that was necessary to raise the next generation. In contrast, the waters held by the bromeliads averaged only about 24 milliliters in volume, far too little to hold ample provisions for a startup tadpole. In order to maintain their newly acquired safe housing, the mimic dart frogs had to adapt a new tactic.

Male mimic dart frogs had previously evolved the capacity to both transport and guard young tadpoles, but having moved to the suburbs, the females needed to help-out with feeding; rearing had become too difficult a task for the males to handle on their own. If they were to ensure the survival of their young, the days of leaving dad to care for the kids were over – monogamy was the best option. Unlike males, female mimic darts have the ability to produce eggs. To do their part, mom frogs adopted a strategy called trophic egg feeding, a practice in which they lay unfertilized eggs in the bromeliad pools for the tadpoles to eat.

An absolutely amazing video of this monogamous behavior was recorded by the BBC during the dissertation work of Jason Brown. Jason was the lead author of the cited paper, and the mimic dart footage was included in the David Attenborough narrated documentary “Life in Cold Blood.”

This is awesome footage:





Brown, J., Morales, V., & Summers, K. (2010). A Key Ecological Trait Drove the Evolution of Biparental Care and Monogamy in an Amphibian The American Naturalist, 175 (4), 436-446 DOI: 10.1086/650727

Frog Ears and Ultrasonic Playback

The first amphibian known to call exclusively in the ultrasonic is identified!

"You look at the frog and can see it is vocalizing — you can tell because their vocal pouches pulsate — but you don't hear any sound. It's amazing. Then you look down at the recording equipment and see the lights flashing, indicating sound. I have never seen that before. In a frog, it's unprecedented to have purely ultrasonic vocalizations." Victoria Arch, UCLA grad student.

The “hole-in-the- head” frog (Huia cavitympanum) - so called because of its recessed ear drums- resides in hillside forest ecosystems of Borneo and Southeast Asia at elevations between 250 and 1000 meters. It is unique among the Ranidae for its ability to vocalize and hear ultrasound calls – well outside of the human range of hearing.

According to Victoria Arch, one of the authors of a recent paper published at PLoS One describing the communication of Huia cavitympanum, "Our hypothesis is that these frogs have shifted to use higher frequencies in their communication to avoid the interference of sound produced by rushing water in the lower-frequency range. However, high-frequency sounds do not transmit as far. By producing some lower-frequency calls, they can transmit calls over a greater distance, so they can communicate with frogs that are farther away. But by also producing some calls that are high-frequency — all ultrasonic — they may be able to communicate better over that background noise. Producing both types of calls might be a bet-hedging strategy to be heard," Arch said. "This is how males attract females, communicate with one another and establish territorial boundaries."

H. cavitympanum relies on fast moving streams for reproduction; these sometimes torrential waters may have necessitated enhanced communication tactics in order to affectively stake out territories and attract potential mates. The tadpoles of the species have also adapted to streamside existence - they’ve developed huge abdominal suckers in order to suction-cup themselves to the sides of rocks.

Abstract (Full Article Here)

Huia cavitympanum, an endemic Bornean frog, is the first amphibian species known to emit exclusively ultrasonic (i.e., >20 kHz) vocal signals. To test the hypothesis that these frogs use purely ultrasonic vocalizations for intraspecific communication, we performed playback experiments with male frogs in their natural calling sites. We found that the frogs respond with increased calling to broadcasts of conspecific calls containing only ultrasound. The field study was complemented by electrophysiological recordings from the auditory midbrain and by laser Doppler vibrometer measurements of the tympanic membrane's response to acoustic stimulation. These measurements revealed that the frog's auditory system is broadly tuned over high frequencies, with peak sensitivity occurring within the ultrasonic frequency range. Our results demonstrate that H. cavitympanum is the first non-mammalian vertebrate described to communicate with purely ultrasonic acoustic signals. These data suggest that further examination of the similarities and differences in the high-frequency/ultrasonic communication systems of H. cavitympanum and Odorrana tormota, an unrelated frog species that produces and detects ultrasound but does not emit exclusively ultrasonic calls, will afford new insights into the mechanisms underlying vertebrate high-frequency communication.

Photographic Credits: Frogs of Borneo

Arch, V., Grafe, T., Gridi-Papp, M., & Narins, P. (2009). Pure Ultrasonic Communication in an Endemic Bornean Frog PLoS ONE, 4 (4) DOI: 10.1371/journal.pone.0005413

University of California, Los Angeles (2009, May 9). Ultrasonic Communication Among Frogs. ScienceDaily. Retrieved May 9, 2009, from http://www.sciencedaily.com/releases/2009/05/090508192231.htm
Posted by Johnny at 5:45 PM 0 comments

Andean to Amazon, an Anuran Account

The tropical rainforests of the Americas are a hotspot of biodiversity and boasts the largest reservoir of amphibian diversity on the planet; but how did such variety arise in our slippery skinned friends?

If I would have been asked this question yesterday, my reply would have a bit different than it would be today – this after reading an article by Juan C. Santos of the Integrative Biology and Texas Natural Science Center recently published at PLoS.



“The unstable coexistence of lineages within a large community for extended periods of time has been hypothesized as a cause of Neotropical diversity. However, our results suggest that such a model is incomplete; rather, the complex pattern of diversification is strongly intertwined with paleogeographic events. Our inferences about the past history of the poison frogs using ancestral area reconstructions and diversification analyses provide new insights on speciation and extinction patterns in the Neotropics. Three species richness patterns are potential explanations for the extant diversity differences among regions of the Neotropics:

(1) high immigration into one region after suitable geoclimatic conditions are established;

(2) gradual in situ diversification of old endemic clades, regardless of the geoclimatic conditions, promoting species accumulation; or

(3) rapid in situ diversification of endemic clades after favorable geoclimatic conditions are established. We found that all three patterns might apply to different areas depending on historical context.

All extant Amazonian species descended from 14 lineages that dispersed into the Amazon Basin, mostly after the Miocene floodbasin system receded. The recurrent immigrations that originated mostly in the adjacent Andes, combined with an increased rate of diversification, explain the high α–diversity of Amazonia. Later, from the Miocene-Pliocene boundary to the present, a rapid in situ diversification gave rise to the extant Amazonian endemic biota. Therefore, most species in Amazonia originated in the last 10 MY. Moreover, lineages immigrating into Amazonia at <8.0>

The diversity in the Chocoan-Central American super-region derived from scattered immigrations from Andes to the early Chocoan rainforest during the late Miocene. However, starting at the Miocene-Pliocene boundary, significant orogenic events gave rise to the Central American archipelago followed by sea level fluctuations, which provided the conditions for repeated dispersal and vicariance events in pre-PLB islands. Evidence of rapid in situ diversification is supported by the high genetic diversity observed among poison frogs and other lineages especially between Western and Eastern Panamá. Interestingly, our results might explain the high β–diversity of other endemic clades within the Chocó-Central America super-region as originating initially from long-distance dispersals between disconnected islands, with diversification later during isolation by high sea levels.

The Andes have undergone extended in situ diversification since the late Eocene. However, our analyses also provided evidence of decline in the diversification rate since the middle Oligocene, which has important implications for history and conservation of the endemic Andean fauna. First, the Andes uplift at the Miocene–Pliocene boundary caused significant changes in the rate of diversification in the lowland transition zone. We found that several poison frog lineages distributed on one or both sides of the Andes had dispersed repeatedly before the Miocene uplift (i.e., five cross-Andean and five Northern to Central Andes migrations). Paleogeological evidence supports introgression of shallow seas across the northern Andes during the Miocene, suggesting a historical connection between the Amazon Basin and the Chocó. Second, the Pliocene Andean uplift (>2,000 m above sea level) formed a significant barrier to dispersal, because no other cross-Andean dispersals were found. The uplift also was associated with dramatic ecological changes and a decrease in diversification rates. These results suggest a role for niche conservatism, in that some lineages may have gone extinct because of failure to adapt. Alternatively, despite greater sampling effort in the Andes region than in other areas, we failed to find some previously common Andean species (e.g., Hyloxalus jacobuspetersi and the Ecuadorian H. lehmanni). Consequently, it is difficult to separate a natural decrease in diversification rates from the current trend of amphibian species extinctions at high altitudes due to anthropogenic habitat alteration, increased UV radiation, climate change, or pandemic infection. In contrast, the montane transition zones of the Andes and adjacent lowlands (Chocó and Amazonia) have become centers of rapid cladogenesis, and species richness in these transition zones might be underestimated because many Neotropical lineages have been shown to contain several cryptic species. Therefore, dispersals within or across the Andes diminished during the Pliocene, but diversification has intensified in the Andes-lowlands interface.

Although some of the oldest lineages of poison frogs originated in the Guiana Shield and the Venezuelan Highlands (>30 species), our results suggest extended in situ diversification followed by a decline in the rate of diversification of endemic clades in both areas since the early Miocene. Along the same lines, the Guiana Shield has high poison frog endemism, which is mostly restricted to the summits of the sandstone tepuis, while recent Amazonian poison frog immigrants occupy lowlands adjacent to the tepuis. Our results suggest that the decline of endemic Guianan diversity might be associated with ecological changes in habitat due to the collapse of the ancient tepuis and repeated dispersals from Amazonian lineages since the Pliocene. However, the diversity of poison frogs in the Guiana Shield is only beginning to be revealed. In contrast, diversification in the Venezuelan region most likely reflects the oldest vicariant event in Dendrobatidae, at 40.9 MYA. The costal ranges of Mérida, Cordillera de la Costa, and Paria peninsula are species rich but their total area is less than 5% of that of the Amazon Basin. No lineage of this endemic fauna has dispersed out to other regions since the early radiation of the poison frog family in the late Eocene. However, Eocene floristic paleoecological reconstruction of the Venezuelan Highlands area showed that it was more diverse than at present, suggesting that the ancestral habitat of the first poison frogs might have been lowland. The depauperate dendrobatid fauna of the Venezuelan llanos and Brazilian Shield plateau is puzzling, but might be related to Holocene aridity.

The recurring dispersals to Amazonia suggests that a large part of dendrobatid diversity results from repeated immigration waves at <10.0>

Santos, J., Coloma, L., Summers, K., Caldwell, J., Ree, R., & Cannatella, D. (2009). Amazonian Amphibian Diversity Is Primarily Derived from Late Miocene Andean Lineages PLoS Biology, 7 (3) DOI: 10.1371/journal.pbio.1000056