It's the common image associated with theropod dinosaurs: they are either chasing something down to eat it, or they are eating it. Every lazy bit of sad science storytelling depicts theropods consumed with one objective: devouring poor innocent plant-eating prey. From Lex asking "Where's the goat?" in Jurassic Park to Littlefoot's mom being killed by Sharptooth, theropods (and all carnivores, really) get painted with the "evil" brush and brushed off as mindless killers.
|It's still too soon for me to post Littlefoot's mom's death.|
We know that theropods built nests and incubated their young: research by Tanaka et al. (2015) demonstrated the different nesting strategies that dinosaurs used based on egg shell porosity. Egg shell is not solid: it is full of tiny pores that allow for moisture and gas exchange to happen between the egg and its environment. Based on modern nesting crocodiles and birds, the more porous eggshell is, the more likely it was that the eggs would be completely buried in a nest mound. Less porous eggs would only be partially buried with the upper surfaces of the eggs exposed. Maniraptoran theropods (dromaeosaurs, oviraptors, troodons, and our modern birds) have low porosity eggs, which would be partially exposed in the nest.
Maniraptoran theropods are well-known for another tender-loving trait: incubating eggs. Several fossil nests have been recovered with a maniraptoran caught in the act of brooding. The spectacular specimen of Citipati, an oviraptorosaur, on top of a nest of eggs is on display at the American Museum of Natural History. This is a good example of parental care in theropods.
We know that theropods (at least the maniraptorans) engage(d) in nest building and egg brooding behavior...but what about the pre-nesting activities, like courtship? Our modern theropods are famous for their courtship behaviors. Check out the mating dance of the Flame Bowerbird...
...and now imagine Oviraptor doing this dance.
"Hold up! There should be evidence of male theropods having some skeletal differences that can be used to support possible mating dances, right? Right!?!"
Using the skeleton alone, the best way to tell if a theropod skeleton was male or female is to look for a structure called medullary bone: it's a special deposit in the hollow portion of theropod bones that acts as a calcium reserve for adding shells to eggs. Medullary bone is only going to occur in egg-laying (female) theropods. However, medullary bone is an internal structure: you can't tell by looking at the exterior of a bone whether it contains medullary bone.
While there have been a few - quite a few - papers published that purportedly contain evidence of skeletal sexual dimorphism (anatomical differences in the skeleton) in dinosaurs (the most recent one uses a small sample size of tails of oviraptorosaurs) the numbers simply do not support that the differences seen are the result of sexual differences, as opposed to good ol' natural variation. An excellent study by Dr. Jordan Mallon was recently published that rigorously tests the statistics of all of the proposed cases of sexual dimorphism that involve visual differences in bones...and no evidence of sexual dimorphism was found in any of the cases. Internal eggs, embryos, and medullary bone are still the only way to confidently identify the sex of a dinosaur.
So, are there any fossils that possibly support courtship activities in theropods? We may have fossils in the form of trace fossils...ichnology to the rescue! In 2016 we published on these enigmatic traces from the Early Cretaceous of Colorado. They are paired scrape marks made by the feet of large theropods (likely an allosaur.) No tracks led up to any of the scrape marks, showing us that the theropods dug down through the layer they were walking on.
|Figure 1 from Lockley et al. (2016) showing the scrape marks.|
|Figure 3 from Lockley et al. (2016).|
These marks were a puzzle at first. We initially thought that the trackmakers were digging for water, but the geology of the area showed that water was active and abundant. Next we considered that they were digging for food, but the sandy layer the theropods were digging down to was devoid of traces of most burrowing animals. Next we considered nest bowls and/or dust bathing. Both activities, like rooting around for food, tend to wipe out the marks made by digging (based on what we've seen with dust bowls made by Spruce and Ruffed grouse in our area.)
|Ruffed Grouse dust bathing, Jose Schell.|
This led us to consider the different reasons a theropod would make a visible territory mark...and then we came across the nest scrape ceremony.
Check out this video of the Piping Plover nest scrape ceremony (and because I love plovers, check out the Great Lakes Piping Plover Recovery Project.)
Also see this video of a Killdeer nest scrape ceremony. I know where Killdeer were nesting locally last year, so I'm hoping to get some of my own footage this spring.
To start the nest scrape ceremony, the male will define and defend their territory. They vocalize to nearby females, and demonstrate to them how good they are at digging out nests. A male may perform and create several nest scrapes during the pre-mating ceremony. If the female is satisfied with his performance, she allows the male to mate. One of the nest scrapes becomes the nest bowl.
There is a good chance that our large theropods were engaging in a courtship ceremony that involved scraping at the ground. It is not uncommon to have multiple males displaying in one location: game birds are a great example with their display arenas or leks, like the Greater Prairie Chicken.
Of course, we had to come up with an Early Cretaceous version of a theropod lek...
|Figure 6 of Lockley et al. (2016). Yes, those theropods in the background are doing exactly what you think they are doing.|
Recently a paper by Carr et al. (2017) published on the facial scales of the tyrannosaurid Daspletosaurus horneri (a new species) that has skeletal evidence of not just scales on its face, but very sensitive facial scales. How sensitive? These scales were likely more sensitive to touch than human fingertips. Why would a tyrannosaur have such a sensitive snout? I'll let the authors speak to that:
"ISOs [integumentary sensory organs] would have aided adult tyrannosaurids in harmlessly picking up eggs and nestlings and, in courtship, tyrannosaurids might have rubbed their sensitive faces together as a vital part of pre-copulatory play." (Carr et al., 2017)
We now have more than enough evidence to abandon the tired cliche of the one-dimensional killing machine image of theropod dinosaurs. Extinct theropods were just as multifaceted and complex as any of our modern theropods or animals that we see today. A carnivorous animal is not simply a vicious slaughterhouse on legs and wings: they attract mates and care for the young that they produce. The fact that they eat meat to support these tender activities should be free of judgement on our part. We should learn to appreciate all aspects of a carnivore's life and pass that appreciation on to the next generations.
While you are here, I highly recommend peeking into the tender lives of our modern theropods by watching live nest cams! Here are links to the nest cam I frequent. Most are nest cams of birds of prey, so you will see prey either in the nest or being brought to the nest.
Barred Owl: http://cams.allaboutbirds.org/channel/43/Barred_Owls/
Savannah Osprey: http://cams.allaboutbirds.org/channel/54/Savannah_Ospreys/
Laysan Albatross: http://cams.allaboutbirds.org/channel/41/Laysan_Albatross/
Peregrine Falcon: http://explore.org/live-cams/player/peregrine-falcon-cam
Bald Eagle: http://explore.org/live-cams/player/decorah-eagles-north-nest
Great Blue Heron: http://explore.org/live-cams/player/great-blue-herons-chesapeake-conservancy
Hummingbirds (there are babies in the nest right now!) http://explore.org/live-cams/player/rosie-hummingbird-nest
Carr TD, Varricchio DJ, Sedlmayer JC, Roberts EM, Moore JR (2017) A new tyrannosaur with evidence for anagenesis and crocodile-like facial sensory system. Scientific Reports 7, Article number: 44942 (2017) doi:10.1038/srep44942
Lockley MG*, McCrea RT, Buckley LG, Lim JD, Matthews NA, Breithaupt BH, Houck KJ, Gierlinski GD, Surmik D, Kim KS, Xing L, Kong D-Y, Cart K, Martin J, Hadden G. 2016. Theropod courtship: large scale physical evidence of display arenas and avian-like scrape ceremony behavior by Cretaceous dinosaurs. Scientific Reports 6:1–10
Mallon JC. 2017. Recognizing sexual dimorphism in the fossil record: lessons from nonavian dinosaurs. Paleobiology, doi: 10.1017/pab.2016.51
Persons SW IV, Funston GF, Currie PJ, Norell MA (2015) A possible instance of sexual dimorphism in the tails of two oviraptorid dinosaurs. Scientific Reports 5, Article number: 9472 (2015) doi:10.1038/srep09472
Tanaka K, Zelenitsky DK, Therrien F (2015) Eggshell Porosity Provides Insight on Evolution of Nesting in Dinosaurs. PLoS ONE 10(11): e0142829. doi:10.1371/journal.pone.0142829