Continuing my trend of “catching up,” an article in the November issue of Natural History magazine, talks about a new study in the Quarterly Review of Biology, that finds group nesting to be very common place among extant reptiles.
That study would be:
Doody, J.S., Freedberg, S., Keogh, J.S.? 2009. Communal Egg-Laying in Reptiles and Amphibians: Evolutionary Patterns and Hypotheses. Quart. Rev. Biol. Vol.84(3):229-252.
In the paper, Doody et al (no laughing) did a massive search through the herpetological literature (both technical journals, and hobbyist magazines) to look at instances of communal egg laying in reptiles and amphibians (herps). I’m not being hyperbolic here either, as the paper states:
In total, our assembled database was gathered from 290 different sources, including 176 different scientific journals, 72 books or book chapters, 29 unpublished reports, and 13 unpublished theses. We also have included a number of reliable personal communications from herpetologists.
What the authors found was that group gatherings of herps are vastly more common than previously believed. Group egg laying was found to be present in 345 reptile species. Now you might be thinking 345 really isn’t all that much for a group composed of some 8700 species.
Well then aren’t you a Debbie Downer?
Seriously though, the authors address this by mentioning:
Although the difficulty in locating nests hampers our ability to determine the actual frequency of communal egg-laying among species, we can better estimate this proportion by dividing the number of known communally egg-laying species by the total number of species, excluding those for which eggs have not been found. We conducted such a calculation for the three families of Australian lizards known to include multiple communally egg-laying species—Gekkonidae, Pygopodidae, and Scincidae—as gleaned from the Encyclopedia of Australian Reptiles database (Greer 2004). Proportions of these lizard families known to lay communally were 4–9%, but, when we exclude species for which nests are not known, these values rise dramatically to 73–100%
The biggest take home message to get from Doody et al’s review, is just how much we don’t know about extant reptiles.
…the present review highlights our inadequate knowledge of the nests and/or eggs of reptiles. For instance, the eggs or nests are known in only 7% of Australian lizards of the three families that commonly lay communally (N = 411 oviparous spp.) (Greer 2004).The extent of this knowledge for Australian lizards is probably similar to that for reptile eggs on other continents, particularly South America, Africa, and Asia, where the reproductive habits of reptiles are poorly known. This is in stark contrast to other vertebrates such as birds, for which complete field guides to the eggs and nests are available for several continents
Indeed, just by doing the brief research run needed to compile this blog post, it was apparent that communalism is much more common in reptiles than anyone ever thought. However, because so many of these reports are either anecdotal, or buried in obscure journals, it is easy to miss all the many cases where it is known.
This discovery lead the authors to the inevitable follow up question of: “why?” What benefit do mothers gain by nesting communally?
Numerous hypotheses for why animals nest communally, have been proposed.
- Saturated habitat (only so many suitable nest sites)
- Sexual selection (choice of males that live in a particular area)
- Artifact of grouping for other reasons
- Attack abatement (easier to hide a bunch of eggs in one site, than in multiple sites. Less chance that your eggs will be the ones that are eaten).
- Maternal Benefits (save time and energy finding a suitable nest site by “freeloading”)
- Reproductive success (if the nest site worked once before…)
- Egg insulation
The authors reviewed all of these possible reasons for communal egg laying in herps. In the end, they found evidence for both the maternal benefits hypothesis, and the reproductive success hypothesis, though they felt a mixed model better explained things.
Sadly, though the authors mentioned how a lack of information on the natural history of most reptiles is largely responsible for this sudden revelation about their nesting behavior, they nevertheless make repeated mentions of how “social interactions are generally less complex in reptiles and amphibians than in other tetrapods” or how herp sociality forms “relatively simple systems“.
The reality is that the old view of simplistic “loner” reptiles that only come together to mate, is not accurate. This is especially true for parental care in reptiles.
The popular view (among the public, and the scientific community) is that reptiles are? “lay’em and leave’em” types when it comes to reproduction. Despite all the herpetological knowledge to the contrary that has been acquired in the past 50 years, it is still popular to spout the party line about reptiles being “uncaring parents.”
Zoologist Louis Somma took issue with this view of reptilian (in particular, chelonian and lepidosaurian) parenting. He conducted a literature search to see how often mentions of parental care in reptiles are recorded. In the end he wound up finding 1400 references to parental care in reptiles (Somma 2003)!
Somma’s survey covered various aspects of parental care. He found reported evidence of nest building and / or guarding in tortoises like Manouria emys (McKeown 1999), Gopherus agassizii (Barrett & Humphreys 1986) and 4 other species of chelonian.
Turning to lepidosaurs, Somma found parental behaviour to be present in 133 species of lizards and 102 species of snakes. Even a species of tuatara (Sphenodon punctatus) is known to guard its nests (Refsnider et al. 2009). Though these numbers appear small compared to the total amount of species that have been described; much like the Doody et al. paper, this is just based off of species whose nesting behaviours we do know. That these taxa all span a wide phylogenetic range, suggests that parental care is more commonplace than initially thought.
Nest guarding is usually a maternal trait, but some squamates exhibit nest guarding behaviour in both parents, such as some cobra and crotaline snakes (Manthey and Grossman 1997) , as well as tokay geckos (Zaworski 1987).
Not only active guarding of the nest, but actual brooding of the eggs is also commonly reported in squamates such as various python species (Harlow & Grigg 1984, Lourdais et al. 2007), and skinks (Hasegawa 1985, Somma & Fawcett 1989). Some species are even known to groom their newly hatched young (Somma 1987).
More interesting still are various reports and observations of parental feeding in some reptile species, such as the skink Eumeces obsoletus (Evans 1959), and the cordylid lizard Cordylus cataphractus (Branch 1998). Not to mention recent evidence of parental feeding in captive crocodylians.
This leads me to the only reptile group where parental care is well publicized: that of the 23 extant crocodylian species. I could, at this point, list references for parental care in crocodylians. However because this behaviour is so well documented for this group, it would seem unnecessary. It is? better to shed light on the many (MANY) examples of parental care in other reptile species. I also didn’t include related examples like placental evolution in the skink genus Mabuya, or instances of egg binding in captive reptile mothers; due to a lack of appropriate substrate to lay their eggs.
In the end, the paper by Doody et al. adds to a growing body of evidence which suggests that the “lay’em and leave’em” reptile species of the world, are the exceptions? and not the rule.
Next time: Biomechanics of running suggest “warm-blooded” dinosaurs. Or: why the aerobic capacity model needs to die already.